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Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
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10
Setbacks and Buffer Zones

Regulations governing the use of private land within a specified distance of a watercourse, lake, wetland, or tidal shoreline have been in effect in many states and localities since the early 1960s. Such ''setbacks" or "buffer strips" serve diverse purposes, for example, protection of surface waters from pollution, protection of structures from flooding or erosion, and preservation of riparian habitat and shoreline amenities. One of the most prevalent features of the Memorandum of Agreement (MOA) Watershed Rules and Regulations is the use of setback distances for separating waterbodies from potentially polluting activities. Depending on the activity, 25–1,000 ft of land must separate the activity from nearby waterbodies. Greater distances are required for setbacks around reservoirs, reservoir stems, and controlled lakes than for those around wetlands and watercourses, which encompasses all perennial streams and in some cases intermittent streams.

Although the use of setbacks is quite common in watershed regulations across the country, little research has been done regarding the effectiveness of setbacks per se in preventing contamination of waterbodies from nonpoint source pollution. Rather, research has focused on the use of buffer zones for nonpoint source pollutant removal. Buffer zones are natural or managed areas used to protect an ecosystem or critical area from adjacent land uses or sources of pollution. They are an increasingly used best management practice (BMP) for many activities. Effective buffers along rivers, reservoirs, and lakes (riparian buffers) either retain or transform nonpoint source pollutants or produce a more favorable environment for aquatic ecosystem processes.

Setbacks, in contrast to buffer zones, are simply prescribed distances between pollutant sources and a resource or aquatic ecosystem that needs protection. Only

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

if a setback is subject to management or natural preservation can it be considered a "buffer" that reliably insulates ecosystems and resources from nonpoint source pollution. Because of the lack of information regarding unmanaged setbacks, this review focuses on management of buffer zones for achieving pollutant removal. In the absence of management, it is virtually impossible to predict what effect the setback distances in the MOA will have on the water quality of the New York City watershed. However, if the management practices reviewed and recommended in the following sections are used, then the setbacks may approach the pollutant-removal capabilities predicted for buffer zones.

The next section enumerates and explains key functions and characteristics of riparian buffer zones. It should be noted that waterbodies have a substantial effect on the characteristics of the surrounding buffer zones. That is, depending on whether they border wetlands, reservoirs, or streams, buffer zones will function differently. These differences are discussed when appropriate. Another important consideration is that buffer zones may not be permanent pollutant sinks, but rather may act as temporary storage areas that can be both sources and sinks of pollution. This is especially true for sediment and phosphorus, for which no degradation processes exist in the buffer (nitrogen can be removed via denitrification). Factors that enhance the long-term storage potential of riparian buffer zones, such as harvesting of vegetation, are important in evaluating their long-term effectiveness.

STRUCTURE AND FUNCTION OF RIPARIAN BUFFER ZONES

Riparian buffer zones refer to lands directly adjacent to waterbodies such as lakes, reservoirs, rivers, streams, and wetlands. These land areas have a significant impact on controlling nonpoint source pollution and on the associated water quality in nearby waterbodies. As a result, they are widely used in water resource protection programs and are the topic of intense investigation, especially in agriculture and forestry. Unfortunately, as noted in a recent symposium on buffer zones, policy-driven initiatives that have accelerated the debate on buffer zones have, at the same time, stretched scientifically based management to the limits of knowledge on this issue (Haycock et al., 1997). This is the case in the New York City watersheds and most other regions of the country.

Hydrology

Evaluating the effectiveness of riparian buffers to remove diffuse pollution from runoff requires a basic understanding of their hydrologic structure and function. Because of their proximity to waterbodies, riparian buffers are sometimes flooded by stream overflow. Riparian buffer zones are also strongly influenced by water from upslope areas, which is generally divided into three

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

categories: (1) overland or surface flow, (2) shallow subsurface flow, and (3) groundwater flow (Figure 10-1).

Overland flow across buffer zones can occur via two pathways. Infiltration-excess overland flow is generated when rainfall intensity or snowmelt rate exceeds the rate at which water moves through the soil surface (the infiltration process). Infiltration-excess overland flow typically occurs when the soil surface is frozen, is compacted, or is otherwise unable to transmit water to the root zone. Extreme rainfall events may deliver water rapidly enough to generate infiltration-excess across a wide range of soil types and watershed locations. As in most predominantly forested areas, this mechanism of overland flow is rare in the Catskill/Delaware region.

Saturation-excess overland flow occurs when soil water storage capacity is exceeded by precipitation volume combined with lateral inflow from upslope areas. When total inflow exceeds total outflow, saturation from below is the obligate result. Once the zone of saturation reaches the soil surface, any new input (rain or snowmelt) is immediately converted to overland flow. As shown in Figure 10-1, saturation-excess overland flow typically occurs at the transitions from the uplands to the riparian zone. Saturation-excess overland flow is usually less damaging to water quality than is infiltration-excess overland flow and, though still uncommon, it is more likely in the Catskill/Delaware region.

Both infiltration-excess and saturation-excess overland flow occur during rain or snowmelt events and constitute the bulk of stormflow. Because this water

FIGURE 10-1 Hydrologic pathways surrounding riparian buffer zones. Source: Burt (1997).

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

travels overland, it can accumulate high levels of particulate matter such as sediment, bacteria, and particulate-phase phosphorus. Depending on its velocity and on the soil water status of the riparian zone, overland flow can either infiltrate into buffer zones (generally desirable) or flow across buffer zones and discharge directly into neighboring waterbodies.

Shallow subsurface flow travels laterally through the root zone below the land surface (see Figure 10-1). It may be caused by an abrupt decrease in soil permeability or simply because a shallow soil is underlain by slowly permeable or impermeable bedrock. The latter is the most common case in the Catskill/Delaware region. The response of subsurface flow to storm events is more attenuated than that of overland flow, although increases in subsurface flow do generally occur during and for a period of time following precipitation or snowmelt. Filtering and biogeochemical transformations in the soil limit shallow subsurface transport of suspended particulate matter. By contrast, the concentration of dissolved solids may increase in proportion to residence (travel) time (Burt, 1997).

Groundwater flow occurs when vertical flow extends beyond the root zone into lower strata. This may occur in deeper unconsolidated material (e.g., glacial or lacustrine sands and gravels) and/or through bedrock fractures. Travel time increases in proportion to the length of the flow path and in relation to hydraulic limitations imposed by the media. Hence, groundwater can have high dissolved solids but transports little, if any, suspended solids. Shallow subsurface flow and groundwater flow combine to generate baseflow, the water entering streams, wetlands, lakes, and reservoirs during dry periods. Shallow subsurface flow is much more likely to interact with riparian buffer zones than groundwater flow because it passes laterally through the root zone. In some instances, shallow subsurface flow in upstream areas can become saturation-excess overland flow by the time a buffer zone is reached (exfiltration or seepage).

Pollutant Removal and Other Functions

The structure and function of riparian buffer zones are determined by (1) the soil, vegetation, and hydrologic characteristics of the buffer and (2) the interactions with upslope and downslope water. For management purposes and for conceptualization of the various functions, the U.S. Department of Agriculture (USDA) guidelines suggest riparian buffers can be divided into three zones, each of which has certain physical characteristics and pollutant-removing abilities (Figure 10-2) (NRCS, 1995; Welsch, 1991). Zone 1 is the area immediately adjacent to the waterbody; Zone 2 is an intermediate zone upslope from Zone 1 where most active woody BMPs are used; and Zone 3 is the vegetated areas upslope from Zone 2. Although this conceptualization has not been universally adopted, it is particularly useful in this report for describing how riparian buffer zone functioning varies with distance from nearby waters.

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

FIGURE 10-2 USDA's three-zone schematic of a riparian buffer zone. Source: EPA (1995).

Zone 3

The pollutant-removal abilities of riparian buffers are maximized when overland flow infiltrates into buffer zones rather than discharging directly into adjacent waterbodies. For this reason, the most important function of Zone 3 is to alter the hydraulic properties of rainfall runoff such that the overland flow interacting with Zones 2 and 1 is sheet flow rather than channelized flow. Depending on the characteristics of nearby land, overland flow reaching Zone 3 may be predominantly channelized flow. By design, when channelized flow reaches Zone 3, it is usually converted to sheet flow and subsurface flow by the hydraulically rough surface and the enhanced infiltration of the buffer zone.

Coarse sediment removal from stormwater is also predominantly accomplished in Zone 3, although it can also occur in Zones 2 and 1. When the hydraulic of stormwater change from channelized flow to sheet flow, infiltration of the water is enhanced. Sediment and other materials entrained in overland flow (such as particulate phosphorus) are deposited on the surface as water infiltrates into the soil. If Zone 3 land is properly managed, sediment removal can reach 80 percent (Sheridan et al., 1999).

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×
Zone 2

Zone 2 is designed to remove, sequester, and transform nonpoint sources of pollution in stormwater (Lowrance et al., 1997; Welsch, 1991). Nutrients such as nitrogen and phosphorus, microbes, and sediment can all be altered during passage through a Zone 2 riparian buffer. Zone 2 can also reduce pesticide transport (Lowrance et al., 1997) and may trap other pollutants, including metals and hydrocarbons. In general, the ability of a Zone 2 riparian buffer to remove pollutants depends on (1) whether sheet flow has been established in Zone 3, (2) the type of vegetation present, and (3) the length of the buffer zone. The first criterion is determined by the condition of the Zone 3 riparian buffer upslope from Zone 2. The other two criteria are characteristics of Zone 2, some of which can be altered or managed to maximize pollutant removal. A literature review found later in this chapter discusses the extent of pollutant removal that can be achieved when rainfall runoff travels through riparian buffers. This review focuses on the pollutants of greatest concern in the New York City watersheds, including phosphorus, microbial pathogens, and sediment.

Zone 1

Zone 1 is the area of the riparian buffer closest to the waterbody. In forest ecosystems, it is characterized by a canopy of trees and shrubs that provide shade to near-shore areas of lakes, larger streams, and rivers during a portion of each day. The cumulative effects of the canopy on the energy balance can have a substantial (10–15°C) effect on water temperature. Because dissolved oxygen concentration is inversely proportional to water temperature, increases in temperature caused by the removal of riparian vegetation can impose chronic or acute stress on invertebrate and fish populations. Riparian vegetation has the greatest influence along headwater streams where vegetation can cover the entire width of the stream. The microclimate effect decreases downstream as the width of the stream, river, or lake increases relative to the height of the riparian vegetation.

Leaves, needles, and wood supply energy—as carbon—to headwater streams. Like microclimate effects, the relative importance of carbon inputs from riparian vegetation decreases as the receiving water becomes larger. However, the inflow of dissolved and particulate carbon from headwater areas remains an important supplement to in situ primary production by algae and other aquatic plants in rivers and lakes. The contribution of vegetated riparian zones to the total dissolved carbon load at the point of water withdrawal is an important issue in watershed management. Because of the concern over the role of dissolved organic carbon in producing disinfection byproducts, there are possible drawbacks to increasing the dissolved organic carbon levels in streams.

In addition to being a persistent source of carbon, woody debris ranging from

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

small twigs to branches, boles, and entire trees is a critical structural feature for stream ecosystems. As woody debris lodges and jams along streambanks, it forms a matrix that captures leaves and other small organic matter as they drift downstream. The interlocking roots of riparian vegetation anchor streambank, floodplain, and lakeshore soils and substantially increase their resistance to erosion and slumping.

Finally, the zone nearest the waterbody can be responsible for unique aquatic habitats. Woody debris and leaf packs at the land/water interface increase the variation of flow velocity in headwater streams. Quiet water and eddies behind leaf packs and larger debris jams lead to the formation of alternating pool and riffle sections in headwater streams. Diverse hydraulic conditions provide a continuum of spawning, rearing, feeding, hiding, and overwinter habitat for fish, amphibians, and invertebrates.

All Zones

In all zones of a riparian buffer, vegetation helps to reduce soil erosion. All zones are also capable of increasing the thickness of the unsaturated zone through evapotranspiration of water from the soil profile. As a consequence, available storage for rain, snowmelt, and upland inflow is maximized.

Riparian buffer zones exhibit soil physical and hydraulic properties that further enhance the ability of the land to attenuate stormwater. Decomposition of vegetation in the riparian zone leads to reduced bulk density, thereby increasing porosity (storage capacity), infiltration capacity (rate of water movement into the soil), and permeability (rate of water movement through the soil). Water retention characteristics also are enhanced by the addition of organic matter. The growth, senescence, and death of roots, along with the actions of invertebrates and small mammals, produce a complex system of macropores that augment the permeability of the soil matrix. Collectively, these soil properties maximize the likelihood that rain, snowmelt, or overland flow from adjacent uplands will pass beneath the soil surface and travel as subsurface flow through the riparian zone. In addition, small-scale variations in slope, woody debris, herbaceous plants, and leaf litter on the forest floor present additional barriers to overland flow.

ACTIVE MANAGEMENT OF BUFFER ZONES

The most important management practice for influencing functioning of buffer zones is to stabilize the hydraulic properties of stormwater so that channelized flow does not reach nearby streams. Channelized flow can form very quickly during rainfall. In urban areas, stormwater concentrates into channelized flow within as few as 75 ft of its source (Schueler, 1996; Whipple, 1993). Given the typical land uses found on the East Coast, only about 10 percent to 15 percent of a watershed area produces sheet flow during precipitation

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

(Schueler, 1995). The remaining runoff is usually delivered to streams in open channels or storm drains, the flow from which can be extremely difficult, if not impossible, to dechannelize.

Hence, converting channelized flow to sheet flow or to multiple smaller channels is a critical aspect of buffer zone management. Most regulations involving setbacks and buffer zones have been written and enacted with no consideration of this important issue. Converting channelized flow to sheet flow may require the installation of a structural BMP in Zone 3. For example, at sites with significant overland flow parallel to the buffer, water bars should be constructed perpendicular to the buffer at 45-to 90-ft intervals to intercept runoff and force it to flow through the buffer before it can concentrate further. Low berms or vertical barriers, known as level-lip spreaders, have been used successfully to spread concentrated flow before entering a forest buffer (Franklin et al., 1992). Buffers should not be used for field roadways because vehicles and farm equipment will damage the buffers and may cause concentrated flows (Dillaha and Inamdar, 1997). Specific suggestions for dechannelizing urban stormwater are given later.

Buffers may accumulate significant amounts of sediment and nutrients over time. To promote vegetative growth and sediment trapping, herbaceous vegetated buffers should be mowed and the residue should be harvested two or three times a year (Dillaha et al., 1989a,b). Mowing and harvesting will increase vegetation density at ground level, reduce sediment transport, and remove nutrients from the system. Herbaceous vegetated buffers that have accumulated excessive sediment should be plowed out, disked and graded if necessary, and reseeded in order to reestablish shallow sheet flow conditions. Although natural herbaceous buffer zones are rare in the Catskills, those created during active management of setbacks should be harvested. The primary management for Zone 1 is to reestablish and maintain native woody vegetation.

Although it is known that vegetation type can greatly influence buffer zone functioning, field data are not available for most types of buffers. There are numerous aspects of vegetation management for which more information is needed. For instance, the rooting depth will influence nutrient uptake from shallow or deeper groundwater, and more must be known about the differences among root systems of various types of vegetation. Different types of vegetation also have different management requirements, with woody vegetation providing a natural longer-term sink for nutrients than does herbaceous vegetation. Some general conclusions can be drawn. First, riparian forest buffers require native woody vegetation near the waterbody. States make determinations as to the appropriateness of different species, with native hardwoods required in most states of the eastern United States. Second, in areas experiencing runoff high in sediment, herbaceous vegetation is recommended between a forest buffer and the runoff source because a well-managed grass buffer can be more effective at trapping sediment and associated contaminants. Combinations of vegetation may

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

prove most effective at removing a range of pollutants. For example, an outer grassed strip followed by an inner forested strip has been suggested for complete sediment, phosphorus, and nitrogen removal (Correll, 1991; Osborne and Kovavic, 1993). In all cases, the hardiness of riparian vegetation will determine how well it accomplishes pollutant removal and other functions. The New York City Department of Environmental Protection (NYC DEP) has recently noted that high densities of white-tailed deer may be preventing regrowth of forests around Kensico Reservoir (NYC DEP, 1997), which should be considered when determining vegetation requirements for buffer zones.

The preceding discussion applies only to those nonpoint source pollutants found in stormwater. Active management of buffer zones will have no effect on atmospheric deposition of pollutants directly over the surface of waterbodies, nor can it control in-stream increases in dissolved organic carbon derived from terrestrial vegetation. Additional suggestions for the active management of buffer zones are given below in relation to specific activities that produce nonpoint source pollution.

Agriculture

In general, agricultural land uses tend to increase surface runoff and decrease infiltration and groundwater recharge in comparison to perennial vegetation such as forest or grassland. Grazing animals can cause compaction of soils, especially under wet soil conditions. Tillage may increase subsurface compaction and lead to crust formation at the soil surface. The severity of these effects depends on soil properties and climate. In some watersheds, increased surface runoff, often combined with ditches and drainage enhancements, can change a groundwater-flow-dominated system to a surface-runoff-dominated system (Schultz et al., 1994). Increases in surface runoff cause increases in the stormflow/baseflow ratio and in the amount of sediment and chemical transport.

Many of the effects of agriculture on hydrologic and transport processes can be mitigated through the use of properly managed buffer zones. The USDA launched a National Conservation Buffer Initiative in 1997 to increase the adoption of conservation buffers and the integration of conservation buffers into farm plans. Conservation buffers include many practices designed to impede and retain surface flows and pollutants such as vegetated filter strips, contour filter strips, and riparian forest buffers. The general guidance given above on establishing and maintaining buffer zones is largely derived from studies in agricultural areas and is of primary importance. Hydrologic enhancement (conversion of channelized flow to sheet flow) can be accomplished through grading of soils, removal of berms or channels, and creation of shallow overland flow paths. Vegetation establishment may involve fertilizer and lime application, seeding, or other planting. Active management may also involve harvesting of vegetation to remove nutrients.

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

Other important aspects of buffer zone management on agricultural lands include restrictions on grazing and pesticide application. Grazing of riparian buffer systems including riparian forest buffers and filter strips, is generally not allowed under programs such as the USDA Conservation Reserve Program. Riparian buffers should be combined with practices such as fencing and alternative water supplies to exclude domestic animals from the entire buffer zone. Certain pesticides have setback restrictions from watercourses for storage, mixing, and application that are part of the label restrictions issued for the chemical by the Environmental Protection Agency (EPA). For example, metolachlor, a common herbicide used in corn production, cannot be mixed or stored within 50 ft of lakes, streams, and intermittent streams. The herbicide atrazine may not be mixed or loaded within 50 ft of an intermittent or perennial stream, it may not be applied within 66 ft of where field runoff exits a field and enters a stream, and it may not be applied within 200 ft of natural or impounded lakes or reservoirs. Caution should be used when applying herbicides to adjacent fields to avoid damage to buffer zone vegetation.

Finally, it should be noted that artificial subsurface drainage (tile drains) may short-circuit the functioning of riparian buffers in agricultural settings. Although drain lines are not supposed to enter streams directly, they sometimes do, providing a direct conduit for pollutant movement to streams. Allowing tile drain water to flow through a spreading device before entering a riparian buffer is desirable.

Forestry

As noted in earlier chapters, the majority of nonpoint source pollution (primarily sediment) from timber-harvesting operations emanates from the road and skid trail network needed to remove sawtimber, pulpwood, or fuelwood from the forest. Overland flow is generated when the litter layer is scrapped away and the soil is compacted. This disturbance is usually limited to about ten percent of the harvest unit. The remainder of the site retains high infiltration capacity, with shallow subsurface flow as the predominant mechanism of streamflow generation. Therefore, it is usually unnecessary (and impractical) to construct stormwater control devices (e.g., level-lip spreaders) at the transition between the harvest unit and the riparian forest buffer. By contrast, a large proportion of agricultural fields or urban areas can generate overland flow and associated nonpoint source pollution because of changes in soil surface conditions.

Riparian forest buffers are subject to special operating restrictions, often specified by state forest practice acts, to minimize undesirable changes in site conditions. The most important restriction is the prohibition of direct access by heavy equipment. Selective harvesting of trees within Zones 2 and 3 can and should occur. However, logs can only be winched on a steel cable to a machine (skidder, specially equipped farm tractor, or small 4WD tractor) located outside of the buffer or removed by a mechanical harvester with a hydraulic boom. The

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

latter is capable of reaching up to 40 ft into the buffer. Restricting access by heavy equipment virtually eliminates the soil disturbance and compaction responsible for generating and conveying nonpoint source pollution. Except for the restriction on equipment access, the transition between the harvest unit and the riparian forest buffer should, by design, be gradual and indistinct.

Basic silvicultural practices to maintain or enhance the health, vigor, and growth rate of trees should be implemented in the riparian forest buffer. Typically, trees are marked and removed if they have been overtopped by their neighbors (low thinning), damaged by storms or careless logging (stand improvement), or severely damaged by insects and/or diseases (sanitation cut). Stand treatments reallocate the productive capacity of the site to larger, more vigorous trees, and they naturally regenerate the forest by fostering the establishment and growth of seedlings. Although tree planting is still used by forest products companies in parts of the United States and Canada to establish fast-growing, even-aged stands for pulpwood and sawtimber production, it is a costly and unnecessary practice when natural regeneration can be assured (as in the Catskills, where abandoned land will revert to forest after three or four years without mowing). Furthermore, one of the objectives of silviculture is to control the seed/sprout source and site conditions to ensure a mixed-species, uneven-aged stand will result from a series of carefully planned and implemented harvests.

Foresters and landowners should deliberately retain long-lived, commercially valuable species (e.g., northern red oak or yellow birch) and diversify the vertical structure, spatial arrangement, and species composition of the residual stand. An actively growing diverse forest maximizes resistance to and resilience from widespread natural (e.g., hurricanes, insect and disease outbreaks) and anthropogenic (e.g., atmospheric deposition) disturbance that may threaten source water quality (Barten et al., 1998; MDC, 1995).

Stormwater

As mentioned previously, stormwater runoff from impervious surfaces can become channelized quickly if not immediately, partly because of storm drains and pipes. In many cases, no amount of management will allow riparian buffers (in the absence of other BMPs) to convert this channelized flow to sheet flow. Thus, ''active management" of urban stream buffers must include maintenance of physical structures in addition to the buffer zone. Because the ability of urban stream buffers to remove many pollutants has not been tested in the field, the following design suggestions are based solely on engineering theory. If buffer zones are ever elevated to the status of other highly engineered stormwater BMPs (by accumulating the necessary field data), design improvements to help achieve sheet flow will become apparent.

An urban stream buffer is ideally comprised of three zones: a stormwater depression area that leads to a grassed filter strip that in turn leads to a forested

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

buffer. The stormwater depression should be designed to capture and store stormwater during most storm events. Stormwater detained by the depression can then be spread across a grass filter designed for sheet flow conditions, which in turn discharges into a wider forested buffer. The outer boundary of an urban stream buffer must be carefully engineered in order to satisfy these demanding hydrologic and hydraulic conditions. In particular, simple structures are needed to store, split, and spread surface runoff within a stormwater depression area at the boundary. Although past efforts to engineer urban stream buffers were plagued with hydraulic failures and maintenance problems, recent experience with similar bioretention areas has been much more positive (Claytor and Schueler, 1996). Consequently, it may be useful to consider elements of bioretention design for the outer boundary of an urban stream buffer.

Wetlands

Natural wetlands extend along all bodies of water to varying extents. In some cases, the saturated hydrosoils characteristic of wetlands may extend only a few feet; in other cases, wetlands can cover a number of acres beyond the watercourse or reservoir boundary. Because natural wetlands sequester certain nutrients, there may be a tendency to reduce the size of terrestrial buffers in the vicinity of natural wetlands. However, at certain times the hydrology of natural wetlands may be quite channelized, which can overwhelm and diminish the excellent sequestering of pollutants observed at low flows. Hence, it is essential that terrestrial buffers be maintained maximally in the vicinity of wetlands and that the waterbody boundary be delineated at the upgradient boundary of wetlands. Buffer management for wetlands should be no different than for reservoirs and streams.

SETBACKS IN THE CATSKILL/DELAWARE WATERSHED

Numerous setbacks are specified in the MOA regulations. The activities for which setback distances are proscribed are not all-inclusive. For example, agriculture, a contributor of nonpoint source pollution, is specifically excluded from setbacks. Table 10-1 is an inventory of the setback distances prescribed in the Watershed Rules and Regulations. Note that reservoir stems are defined to be the major tributaries within 500 ft of a reservoir.

In order to assess how effective setback distances in the Catskill/Delaware watershed are likely to be in protecting water quality, information on soil type, land use, and other factors is necessary. Presently, conditions on land within potential setbacks are not well known. New York City owns a substantial amount of land immediately surrounding each water supply reservoir, for which land cover is indicated in Table 10-2. Because the construction of residences is prohibited on these City-owned lands, and because there are few structures, this

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

TABLE 10-1 Inventory of Setback Distances in the MOA for Different Activities

Regulated Activity

Watercourse, Wetland

Reservoir, Reservoir Stem, or Controlled Lake

Storage of hazardous substances (new tanks at an existing facility or a new facility altogether)a

100

500

New aboveground and underground petroleum storage facilities with NYC DEP registrationb

100

500

New home heating oil tanks installed underground

100

500

New aboveground and underground petroleum storage tanks > 185 gallons without NYC DEP registrationb

25

300

Subsurface discharge from a wastewater treatment plant (WWTP)

100

500

Absorption field from new septic tanks

100

300

Raised septic systems

250(100)c

500(300)c

Impervious surfaces at (basic guidelines only)

100(50)d

300

New impervious surfaces at individual residencese

100

300

Siting or expansion of a solid-waste landfill or junkyard

250

1,000

Pesticide application without approval from NYC DEPf

250

1,000

a For storage facilities between 100 and 250 ft from a watercourse or wetland, additional forms and reports required.

b Expansion of pet. storage at existing facilities allowed within setbacks if business can prove it would otherwise fail.

c Setbacks in parentheses allowed if size or location of the land makes it impossible to abide by the current setbacks.

d Setbacks in parentheses allowed for intermittent streams. Many impervious surfaces can be built within the setbacks provided that a Stormwater Pollution Prevention Plan is drafted.

e Applies to surfaces constructed after October 16, 1995. If planned before October 16, 1995, impervious surfaces can be built within the 100-ft setback provided that an Individual Residential Stormwater Permit is drafted.

f Within the setbacks, NYC DEP approval must be gained annually. It should be noted that this setback has not yet been approved as part of the Watershed Rules and Regulations.

land is mostly forested. However, it is not necessarily representative of land use on setbacks throughout the watershed (D. Warne, NYC DEP, personal communication, 1998). On privately owned lands, there may be buildings on the banks of watercourses.

NYC DEP's Geographic Information System (GIS) was used to approximate land use on privately owned land that might fall within a 100-ft setback. As

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

TABLE 10-2 Land Cover in the New York City-Owned Land Surrounding Reservoirs in the Catskill/Delaware Watershed

Cover Type

Acres

Percent of Total

Deciduous Forest

19,898

61

Evergreen Forest

6,430

20

Mixed Forest

1,146

3.5

Grass/Shrub

462

1.4

Grass

4,587

14

Bare Soil

37

0.1

Impervious Surface

156

0.5

Data courtesy of the NYC DEP.

shown in Figure 10-3, land use varies substantially among the six reservoirs west of the Hudson River. Land use within setbacks surrounding reservoirs and reservoir stems only (additional data were not made available) for the six basins is as follows: (1) Ashokan is predominantly deciduous forest, (2) Cannonsville is predominantly grass, (3) Neversink has both deciduous and coniferous forest, (4) Pepacton has multiple land uses of approximately equal acreage, (5) Rondout is predominantly coniferous forest, and (6) Schoharie is split between deciduous forest and grass.

Land slope is another critical factor to take into consideration when evaluating the setbacks for their water protection capabilities. In fact, some suggest that lands with slopes greater than 15 percent should not be considered when assigning setback distances (Nieswand et al., 1990). Around reservoirs and reservoir stems in all six West-of-Hudson basins, slopes are predominantly between 0 and 6 percent (Figure 10-4). However, median and maximum land slopes are expected to increase higher in the watersheds. First-and second-order tributaries occur in steep, mountainous terrain throughout much of the Catskills region. Streams are incised into narrow (<100 ft) valleys with adjacent slopes that routinely exceed 15 percent. Finally, the soils within the setback areas of the reservoirs and reservoir stems are primarily 8,000-to 10,000-year old alluvial material.

EFFECTIVENESS OF THE MOA SETBACKS

Several approaches can be used to evaluate the effectiveness of the setback distances prescribed in the MOA, some of which were used in the 1993 environmental impact statement (EIS) prepared for the Watershed Rules and Regulations. The most straightforward approach is to monitor setback influents and effluents to demonstrate removal of pollutants. To our knowledge, this has not

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

FIGURE 10-3 Land use in the 100 feet surrounding West-of-Hudson reservoirs and reservoir stems only. Courtesy of the NYC DEP.

FIGURE 10-4 Land slope in the 100 feet surrounding West-of-Hudson reservoirs and reservoir stems only. Courtesy of the NYC DEP.

yet been accomplished anywhere in the New York City watersheds, primarily because of the newness of the Watershed Rules and Regulations and because it is not a stated goal of the NYC DEP's monitoring program. Monitoring activities are under way for specific projects involving the fate and transport of microbes through soil and groundwater (the Septic Siting Study). However, as of March 1999, no results are available.

In the absence of monitoring data, all other attempts to determine setback effectiveness are necessarily indirect. Experience from other localities that have imposed setback distances around waterbodies can provide an initial assessment

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

of the adequacy of New York City's setback distances, although the site-specific nature of setback effectiveness limits this type of evaluation. The discussion below considers setback distances in several states. A second alternative is to search the literature for field research conducted elsewhere. The removal efficiencies of other setbacks may have implications for setback effectiveness in the New York City watersheds if land conditions are similar and the pollutant loadings upstream of the setbacks are similar.

In lieu of literature and actual field data, a model can be used to simulate the functioning of setbacks. Several models, with varying levels of complexity, are available to examine this issue (Nieswand et al., 1990; Phillips, 1989b). The more detailed models require input data on pollutant concentrations in runoff entering setbacks; they generate pollutant concentrations leaving setbacks. If combined with models of reservoir water quality, setback models can potentially be used to determine the net effect of setbacks on reservoir health. However, no such data on pollutant concentrations in runoff for the Catskill/Delaware watershed are available. Additional data requirements for the more complex models effectively prevent their use. Thus, a simplified model of subsurface hydrology using slope and soil type data from the Catskill/Delaware watershed is used to draw some general conclusions about how effective a 100-ft setback may be (see below). This approach is similar to that found in the 1993 EIS, and comparisons are drawn between the results of each effort.

Setbacks Across the United States

The use of setbacks in watershed regulations throughout the United States is becoming increasingly common. Setbacks are either of fixed width, or they vary depending on such factors as slope, vegetation, and stream size. Fixed-width setbacks usually range between 50 and 200 ft (Robbins et al., 1994).

Some setback/buffer provisions simply require a permit for certain activities on private land within the stated regulatory area. This gives the permitting authority the opportunity to impose specific conditions on the way construction is designed in order to minimize impacts on the aquatic resource. Others entirely prohibit most alterations of the setback area. Some provisions take a middle position, requiring a permit for any land use change while forbidding certain uses such as underground storage tanks within the regulatory setback. The constitutionality of imposing setbacks on private land is considered in Box 10-1.

Massachusetts

Massachusetts has pioneered the use of setbacks bordering various aquatic resources. The Massachusetts Wetlands Protection Act (MGLA, Ch. 131, sec. 40) requires local permits for most activities within specified distances of aquatic resources. Prior to 1996, the law generally permitted work to proceed within

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

BOX 10-1
Constitutionality of Setbacks and Buffers

Public restrictions on the use of portions of private land are familiar and longstanding. Since the early days of land use zoning in the 1920s, residential construction has been subject to minimum street setbacks in the interest of providing front yards. Typically such space, while not literally accessible to the general public, is open and unfenced and is landscaped to provide mutual visual amenity and sense of spaciousness. Minimum side and rear yards are also commonly required in traditional zoning ordinances. The use of limited restrictions along waterbodies, wetlands, and streams to protect water quality and avoid flood damage is another common use of public setback regulations, as in the case of the New York City MOA.

Setback and buffer regulations, like other public land use regulations, typically do not involve compensation to the private landowner. As such, they sometimes are challenged as "takings" of the value of the land affected without compensation, in violation of the Fifth Amendment of the U.S. Constitution, which states, "Nor shall private property be taken for public use without just compensation." It has long been assumed that land may be "taken" by unfair or excessive regulation even if title to the property is not removed from the owner. Justice Oliver Wendell Holmes stated in the famous 1922 decision in Pennsylvania Coal Co. versus Mahon (260 U.S. 395), ''The general rule at least is, that while property may be regulated to a certain extent, if regulation goes too far, it will be recognized as a taking." Since that time, courts have struggled to define the limits of this regulatory power (i.e., how far is "too far") (Platt, 1996).

In resolving takings challenges, courts examine two primary factors: (1) do they reasonably serve a legitimate public purpose and (2) does the owner retain a reasonable (not necessarily optimal) economic return from the use of the total parcel of land. Two particular problems may lead to judicial overturn of setback regulations: (1) public access is allowed to the area covered by the setback for recreation or other purposes without compensation to the owner or (2) an owner's entire parcel is rendered valueless by the restriction. When a restriction is held to be an invalid taking, the court may order the public authority to compensate the owner, to remove the restriction, or both.

The most famous property rights case of the 1990s that involved a regulatory setback was Lucas versus South Carolina Coastal Council (112 S.Ct. 2886, 1992). The Lucas case arose from a takings challenge by the owner of two undeveloped lots on a coastal inlet in South Carolina. Because earlier developed adjoining lots had experienced severe erosion problems, the South Carolina Coastal Council denied Lucas approval

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

to develop his lots under authority of the 1988 South Carolina Beachfront Management Act (Platt, 1992).

The trial court ordered the state to pay Lucas $1.2 million in compensation for an invalid taking. In a 3-2 vote, the South Carolina Supreme Court (404 S.E.2d 895, 1991) reversed the trial court, holding the permit denial to be a reasonable restriction on the right of a private owner to build in an unsafe location. The state court was reversed by the U.S. Supreme Court in a 6-3 decision, which held that where a regulation "denies all economically beneficial or productive use of land" (112 S.Ct., at 2893), it is a "categorical" taking equivalent to a physical invasion of the property by governmental action. In dissent, Justice Harry Blackmun wrote, ''[S]ince no individual has a right to use his property so as to create a nuisance or otherwise harm others, the State has not 'taken' anything of value when it asserts its power to enjoin the nuisance-like activity…. It would make no sense under this theory to suggest that an owner has a constitutionally protected right to harm others, if only he makes the proper showing of economic loss" (112 S.Ct., at 2912).

The Lucas decision may be read narrowly to apply only to cases where regulation prevents an entire parcel of land from being developed, not just a portion of it. This may be viewed as a healthy counterbalance to the tendency of some regulatory agencies to act harshly toward private owners. On the other hand, it may have had a chilling effect on the ability of public agencies to impose and enforce necessary restrictions in areas of sensitive natural resources, such as along coasts and rivers.

In another taking issue case, Dolan versus City of Tigard (114 S.Ct. 2309, 1994), the U.S. Supreme Court cast further light on the bounds of public regulations along watercourses. The plaintiff, in applying to enlarge her hardware store, was required by the defendant to donate a portion of her land within a 100-year floodplain to the city, along with a 15-foot strip adjoining the floodplain to be used as part of the city's pedestrian and bikeway system. These exactions amounted to about 15 percent of her total land area. Upon a "takings" challenge by the owner, the city was upheld by the Oregon Supreme Court, which was in turn reversed by the U.S. Supreme Court. However, the majority opinion in this 5-4 decision was sympathetic with the need for land use planning, while holding the public access requirement to be too demanding: "Undoubtedly the prevention of flooding along Fanno Creek and the reduction of traffic congestion in the central business district qualify as the type of legitimate public purposes we have upheld…. It seems equally obvious that a nexus exists between preventing flooding along Fanno Creek and limiting development within the creek's 100-year floodplain" (114 S.Ct., at 2318). In dissent, Justice Stevens argued that "in our changing world, one thing

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

is certain: uncertainty will characterize predictions about the impact of new urban developments on the risks of floods, earthquakes, traffic congestion, or environmental harms. When there is doubt concerning the magnitude of those impacts, the public interest in averting them must outweigh the private interest of the commercial entrepreneur" (114 S.Ct., at 2329). With a shift of one justice, this could have been the majority view and thus the "law of the land" (Platt, 1994).

In conclusion, the ability of public authorities to impose setback and buffer restrictions on private property owners without violating the Fifth Amendment depends on the following factors:

  1. The objective of the restrictions must be clearly related to a valid public purpose (e.g., protection of public health by ensuring the purity of public drinking water supplies).

  2. The restriction should be no wider than necessary to accomplish the stated purpose.

  3. The achievement of secondary benefits (e.g., flood hazard reduction and protection of riparian habitat) is valid as long as public access to private property is not promoted without compensation to the owner (the Dolan problem).

  4. The economic impact on the entire parcel of land should be considered, namely all use of the setback/buffer area may be prohibited if the parcel is large enough to support reasonable economic use elsewhere. If the entire parcel lies within the setback/buffer (the Lucas problem), the owner must be allowed some limited economic use of that area.

  5. A provision for variance should be included to provide relief in cases of unnecessary hardship caused by literal enforcement of the restriction.

regulated areas subject to permits, land use zoning, and other applicable regulations. In 1996, the Act was amended to impose tighter limits on new construction within "riverfront areas," defined to extend 200 ft from a river's mean annual high-water line or 25 ft in certain urban areas. For proposed land use activities in riverfront areas, there can be no significant impact on the natural resources associated with the riverfront area, placing a higher level of protection (through closer scrutiny) on proposed work in such areas.

Massachusetts has imposed an even higher level of restriction for setback areas within the watersheds of the Quabbin and Wachusett reservoirs. As part of its program to qualify for a filtration avoidance determination, the 1992 Watershed Protection Act (Mass. Laws of 1992, Ch. 36) was adopted, establishing a "primary buffer zone" extending 400 ft from either reservoir and 200 ft from any

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

"tributary or surface water" within the source watersheds. Within the primary buffer, the act prohibits outright "any alteration, or the generation, storage, disposal, or discharge of pollutants." Secondary buffer zones were defined to include areas between 200 and 400 ft of tributaries or surface waters, floodplains, wetlands, and certain areas overlaying aquifers. Within secondary buffer zones, many activities—such as the outdoor storage of road salt, fertilizers, and manure and the operation of junkyards—are prohibited. Limits are imposed on impervious cover and on the density of residential construction in relation to septic systems.

North Carolina

The setbacks in Massachusetts are fixed-width rather than variable-width. Fixed-width setbacks are easier to administer than variable-width, but they may not be sufficiently flexible to protect all sensitive riparian areas. In North Carolina, watershed control programs generally advocate variable-width setbacks around surface water supplies. Beginning with a minimum width of 50 ft, the additional distance is generated by taking slope into account according to Equation 10-1 (a simple calculation if a GIS and digital elevation model are available):

Other watershed programs in North Carolina consider vegetation as well as slope when calculating setback distances. Depending on slope and on whether the land is forested or grassland, setbacks around University Lake in Orange County vary between 50 and 250 ft.

It should be noted that not all North Carolina setback requirements are variable-width. Fixed widths of 150 and 100 ft are required for perennial streams feeding Lake Michie and Falls Lake, respectively, which supply drinking water to Durham and Raleigh. In addition, the recently proposed Neuse River Rule delineates a 50-ft setback along every watercourse in the Neuse watershed, both perennial and intermittent. Land clearing for any purpose on the setback would be prohibited, regardless of whether the land is publicly or privately owned.

Other States

Setbacks in other eastern states provide further comparisons. Baltimore County, Maryland, requires from 75 to 150 ft around all streams, depending on slope and stream size. Several streams in Newport News, Virginia are protected by 100- and 200-ft setbacks for intermittent and perennial streams, respectively. The Chesapeake Bay Preservation Act of Virginia takes both land use and the existence of BMPs into account when determining appropriate distances. Setbacks around Bear Creek and Dog River in Douglas County, Georgia, measured

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

from the centerline of the streams rather than from the stream edge, vary between 100 and 300 ft. The same measurement technique is used around Town Lake and Lake Austin in Austin, Texas, where setbacks range from 100 to 300 ft.

Analysis

Compared to the setbacks described above, those found in the New York City Watershed Rules and Regulations are of similar extent. They are fixed-width setbacks whose distance depends on land use (like the Chesapeake Bay Preservation Act) and the type of nearby waterbody. Setbacks found in the MOA do not take slope, soil type, vegetation, and other factors into account, nor do they vary for particular land uses. Although this means that the provisions may be relatively easy to enforce, they may also be less protective of nearby waterbodies, especially in areas with steep slopes and where vegetation is less capable of dissipating rainfall runoff and sequestering pollutants.

Literature Review of Pollutant Removal in Buffer Zones

A limited body of research quantifies the effectiveness of buffer zones in removing pollutants such as nitrogen, phosphorus, sediment, pesticides, and some microbes from storm and drainage water. This literature is summarized below, and additional material is presented regarding other pollutants such as viruses from septic systems and landfill leachate. Because these research results were gathered from buffer zones with either native or managed vegetation, they represent best-case scenarios for pollutant removal in setbacks. Several attempts have been made to generalize on the required buffer widths necessary to accomplish pollutant removal (Castelle et al., 1994; Fennessy and Cronk, 1997; Nieswand et al., 1990). Extreme care should be taken when using such generalizations, because they may be based on particular pollutants (such as nitrogen) that are subject to higher removal efficiencies than are other pollutants.

Phosphorus

Phosphorus is found in both particulate and dissolved forms, with the majority of phosphorus in surface runoff being particulate phosphorus. Large inputs of dissolved phosphorus may originate from certain sources, such as when fertilizers are broadcast on the soil surface without incorporation (Sharpley et al., 1992). Dissolved phosphorus, usually orthophosphate, is thought to be generally bioavailable.1 Particulate phosphorus, on the other hand, is only partially bioavailable, depending on the specific form of phosphorus involved.

1  

Bioavailability refers to the physical state of the pollutant with respect to its uptake by microorganisms and other potential human or ecological "receptors" of the pollutant.

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

Particulate phosphorus, which is almost always associated with sediment and soil particles, is removed when suspended particles (especially fine particles) settle out of overland flow in Zones 2 and 3. Dissolved phosphorus can be removed via adsorption to leaf litter and soil particles associated with the litter, but it may also be produced from the desorption, dissolution, and extraction of phosphorus into solution as runoff moves across the surface (Sharpley, 1985; Uusi-Kamppa et al., 1997). Thus, the removal of dissolved phosphorus from surface runoff in buffer zones appears to be less effective than the removal of particulate phosphorus.

Phosphorus removal from surface runoff in buffer strips has received limited study (Table 10-3). Reports of the percentage of phosphorus removal in buffer zones are highly variable, ranging from zero percent to 95 percent, depending on the buffer length and other factors. A literature review conducted by Castelle et al. (1994) claims that buffer widths from 30 to 300 ft are necessary to accomplish nutrient removal. In general, studies of phosphorus removal in buffer zones have indicated that removal is most effective in the first several feet of the buffer, that dissolved phosphorus removal was less evident than particulate phosphorus removal, and that infiltration and deposition of fine particles was necessary for effective phosphorus removal.

These findings suggest that even narrow buffers might be important for phosphorus removal. Perennial vegetation that sequesters phosphorus in biomass is important, but should be removed as harvested material. Also, although total load of phosphorus may be reduced by the buffer because of the deposition of phosphorus-laden sediment, the proportion of bioavailable phosphorus may increase because of the release of dissolved phosphorus from the vegetation comprising the buffer zone. For example, both grass buffers and native vegetation have been found to increase dissolved phosphorus output in a number of studies (Correll and Weller, 1989; Dillaha et al., 1988; Jordan et al., 1993; Uusi-Kamppa et al., 1997).

Phosphorus can originate from many of the activities and structures prohibited on setbacks in the Catskill/Delaware watershed, particularly impervious surfaces, septic systems, and wastewater treatment plant (WWTP) discharges. The 100- and 300-ft setbacks in the Catskill/Delaware watershed for septic systems and impervious surfaces, and the 100- and 500-ft setbacks for WWTPs with subsurface discharge, fall in the range of widths encompassed by the literature. However, the wide range of reported removal efficiencies makes it impossible to predict the adequacy of these setback distances. The only conclusion that can be drawn is that if managed or maintained as buffer zones, the MOA setbacks may approach the removal efficiencies found in Table 10-3. Regarding soluble phosphorus, several studies show that buffer zones can be sources of this pollutant. Thus, the setbacks should not be expected to reduce this pollutant in stormwater runoff entering receiving waters.

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

TABLE 10-3 Phosphorus Removal in Buffer Zones

Reference

Location

Buffer Vegetation

Buffer Widths (ft)

Top-P retention (%)

Ortho-P retention (%)

Comment

Dillaha et al. (1989a)

Virginia

Grass

15

30

49–85

65–93

69–83

48–81

Less effective after initial events, simulated rainfall

Magette et al. (1989)

Maryland

Grass

15

30

41

53

 

Less effective after initial events, simulated rainfall

Syversen (1995)

Norway

Grass

16

33

49

45–56

56–85

73

2–77

0–88

10

Natural rainfall, slope of 12–17%, and strips with native grass

Uusi-Kamppa and Ylaranta (1996)

S. Finland Grass

Grass

33

20–36

0–62

Natural rainfall, increase of ortho-P

Schwer and Clausen (1989)

Vermont

Grass

85

89

92

Wastewater inputs to "overloaded" buffer; greatest removal in growing season

Vought et al. (1994)

Sweden

Grass

26

52

66

95

Greatest removal in first several feet

Peterjohn and Correll (1984)

Maryland

Native hardwood forest

164

81

Based on surface flow and ground water input-output budgets

Lowrance et al.(1983)

Georgia

Native hardwood forest

66–131

23

Based on subsurface flow budgets

Bacteria

Most research on microbial removal by buffer zones has concentrated on fecal coliform bacteria as indicators of human and animal contamination of natural waters. This research has most frequently been conducted in locations with high bacterial loads (such as feedlots and manured areas) that are affected by overland flow. Bacterial contamination of agricultural waters often exceeds the

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

primary contact standard of 200 fecal coliforms per 100 mL (Walker et al., 1990). Bacterial loss in runoff can be as high as 90 percent from soils with fresh, unincorporated manure applications (Crane et al., 1983). Heavy rain and rapid surface flow may keep fecal coliforms in suspension while denser soil particles are trapped, leading to preferential transport of bacteria compared to inorganic sediment. Because fecal coliform bacteria are 1–2 µm in diameter, they behave much like fine clay particles once they are entrained in overland flow (Coyne et al., 1995).

Coyne et al. (1995) found grass filter strips were much more effective at trapping soil in surface runoff than in trapping fecal coliforms. Approximately 99 percent of soil particles in surface runoff were removed in filter plots compared to 74 percent and 43 percent removal of fecal coliforms. Earlier studies also indicated that grass buffer strips were less effective for bacteria versus sediment trapping. Dickey and Vanderholm (1981) found grass buffers with short flow-lengths did not significantly reduce bacteria levels in surface runoff. Young et al. (1980) found that grass buffers reduced fecal coliforms about 70 percent in a 89-ft flow length. As expected, channelized flow further reduces the trap efficiency for fecal coliform bacteria (Coyne et al., 1995). Loading rate is also a very important factor; buffer strips recommended by the USDA for dairy barnyard runoff may be undersized, especially in cold regions. An overloaded filter 75 ft in length showed no significant filtration of either fecal coliform or fecal streptococcus bacteria in stormwater from a dairy barnyard (Schellinger and Clausen, 1992).

These studies all support the conclusion that bacteria in overland flow can move through buffer zones without significant reduction. In general, their removal is much less effective than that of sediment and particulate-phase phosphorus. The effectiveness of buffer zones in removing bacteria is reduced because bacteria are small compared to other particulate matter and have very slow settling rates. Other studies of bacterial transport through saturated porous media (shallow subsurface or groundwater flow) indicate that bacteria can travel up to 50 ft in as few as 8 days (Hagedorn et al., 1978).

Impervious surfaces, failing septic systems, and subsurface WWTP discharges can be significant sources of bacterial pathogens. Because bacterial loadings from these sources are likely to be different in concentration, duration, and mode of transport (i.e., overland flow vs. subsurface flow) than loadings from agricultural areas, it is not possible to say whether the associated 100-, 300-, and 500-ft setback distances will be protective of water quality with regard to such organisms. Active management of setbacks to improve bacterial removal should focus on increasing the residence time of runoff waters in buffer zones.

Other Microbial Pathogens

There are no published reports on removal of Cryptosporidium oocysts or

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

Giardia cysts in riparian buffer zones. Given their larger size, it is expected that they may be removed more efficiently than fecal coliforms. However, because they are more environmentally resistant than bacteria, whether buffer zones are effective sinks of these organisms depends heavily on prolonging their residence time in the buffer, which is related to the frequency of overland or channelized flow.

The transport and fate of viruses in porous media (groundwater) has been studied in relation to the siting of septic systems (Yates et al., 1986; Yates and Yates, 1989). The reports suggest that in order to reduce virus concentrations to acceptable levels, 50–300 ft of porous media are needed. Because the decay rate of viruses increases with longer residence times, greater than 300 ft of buffer zone may be needed to sufficiently reduce virus levels in overland flow. The lack of data on removal of nonbacterial microbes in buffer zones makes any evaluation of the MOA setbacks regarding these pollutants highly speculative.

Sediment

Soil erosion and sedimentation occur as a result of three separate but interrelated processes—soil detachment, sediment transport, and sediment deposition. Raindrop impact and stormwater can dislodge sediment particles when their shear forces exceed the critical sheer forces of the particles. Once detached, sediment particles are transported in overland flow until the total energy becomes less than the energy required for particle transport. When this occurs, deposition begins, with large-sized sediment depositing first. Thus, the sediment remaining in runoff has a higher proportion of fine particles compared to the original material.

Detailed studies of sediment retention in buffer systems, as well as field observations, have been used to draw conclusions about the ability of Zone 2 buffer zones to remove sediment (Dillaha and Inamdar, 1997; Dillaha et al., 1989a,b). First, high removal efficiencies have been observed for sediment traveling through grass buffers. These removal efficiencies are generally better than those observed for phosphorus and bacteria. For example, a 30-ft buffer was shown to remove 84 percent of suspended solids (Dillaha et al., 1989a) and 19- and 36-ft grass buffer strips were shown to remove 69 to 90 percent and 69 to 97 percent of suspended solids, respectively (Patty et al., 1995). The required buffer width for removing sediment has been proposed to range from 33 to 200 ft (Castelle et al., 1994). A second conclusion is that buffer zones of reasonable width are effective for sediment removal only if flow is shallow and uniform and if the buffers have not been previously inundated with sediment. Third, the effectiveness of herbaceous buffers for sediment removal appears to decrease with time as sediment accumulates in the buffer and encourages concentrated flow across the buffer. Finally, in some instances, herbaceous buffers may be ineffective for sediment removal if flow accumulates in channels, rills, or gullies before reaching the buffer zone.

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

The prohibited activity in the Catskill/Delaware watershed most likely to generate sediment is the construction of impervious surfaces. Because of the efficient sediment removal rates found in the literature for much shorter distances, the 100- and 300-ft setbacks required for impervious surfaces are likely to be sufficiently protective of pollution via sedimentation. However, as with phosphorus, prolonged sediment removal is dependent upon active management of the setback areas to maintain the vegetation and ensure sheet flow. Without such measures, sediment removal diminishes over time as buffers become saturated.

Pesticides

Because of the extensive use of buffer zones as an agricultural BMP, there have been numerous studies of pesticide removal in buffer zones. In general, managed buffer zones can be highly effective in reducing pesticide concentrations in agricultural drainage water. For example, Patty et al. (1997) found that 99 percent of isoproturon, 97 percent of diflufenican, from 72 to 100 percent of lindane, and from 44 to 100 percent of atrazine were removed in grassed buffer strips of 20, 39, and 59 ft under a range of soil and cropping conditions. In addition to these pesticides, grassed buffer strips are also efficient at reducing concentrations of 2,4-D, trifluralin, metolachlor, metribuzoin, and cyanazine in agricultural waters. The highly degradable structure of many pesticides and their rapid sorption onto organic matter and vegetation in grassed buffer strips contribute significantly to the effectiveness of these buffers (Patty et al., 1997). The 250- and 1,000-ft setback distances found in the MOA are expected to sufficiently protect nearby waterbodies from pesticide contamination. (It should be noted that these setbacks are not yet approved by the state for inclusion in the Watershed Rules and Regulations.)

Landfill Leachate

There have been no published studies on removal of pollutants from landfill leachate via buffer zones of any kind. However, by examining the typical constituents of landfill leachate, it may be possible to draw conclusions about the ability of buffer zones to remove some constituents. The chemicals originating from solid-waste landfills can be highly variable. Those landfills that contain municipal waste only are generally composed of high concentrations of total organic carbon (TOC), total dissolved solids, nitrogen compounds, and inorganic compounds (salts). Typical data on leachate components at solid-waste landfills are presented in Table 10-4.

Constituents of hazardous-waste landfills can vary greatly from municipal landfills and include chemicals such as methyl ethyl ketone, acetone, methyl isobutyl ketone, methylene chloride, phthalic acid, phenol, arsenic, and barium (Pavelka et al., 1994). Some landfills contain mixtures of hazardous waste and

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

TABLE 10-4 Typical Leachate Components of Solid-Waste Landfills, Both New (less than 2 years) and Mature (greater than 10 years)

Component

Typical concentration (mg/L)

New Landfills

Mature Landfills

pH (standard units)

6

6.6–7.5

Alkalinity as CaCO3

3,000

200–1,000

Total hardness as CaCO3

3,500

200–500

Total Suspended Solids

500

100–400

Biological Oxygen Demand

10,000

100–200

Chemical Oxygen Demand

18,000

100–500

Total Organic Carbon

6,000

80–160

Total phosphorus

30

5–10

Ortho phosphorus

20

4–8

Nitrate

25

5–10

Organic nitrogen

200

80–120

Ammonia nitrogen

200

20–40

Calcium

1,000

100–400

Magnesium

250

50–200

Potassium

300

50–400

Sodium

500

100–200

Chloride

500

100–400

Sulfate

300

20–50

Total Iron

60

20–200

 

Source: Reprinted, with permission, from Tchobanoglous et al., 1993. © by McGraw-Hill, Inc.

municipal waste. Because of the wide and unpredictable variability in landfill leachate composition, evaluating the 250- and 1,000-ft setbacks in the MOA is necessarily limited. Phosphorus and nitrogen compounds, for which buffer zone studies have been conducted, can be expected to be removed along the setback distances; however, it is impossible to draw conclusions about the ability of the setbacks to greatly reduce concentrations of other chemicals. The attenuation of dissolved landfill leachate components has been investigated in unconsolidated sandy/gravel groundwater aquifers (Christensen et al., 1994). Leachate components were detected within a 3,280-ft radius from the landfill. In addition, evidence from the Richardson landfill in the Cannonsville watershed suggests that landfill components dissolved in groundwater can migrate from their source to local drinking water wells (EPA, 1999). These studies suggest that the 250- and 1,000-ft setbacks are not sufficiently protective for many landfill leachate constituents.

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×
Petroleum Products and Hazardous Substances

There are no published studies on removal of petroleum substances and hazardous substances via setbacks or buffer zones. However, the transport and fate of these compounds through groundwater aquifers and unsaturated media are well studied. A recent study from the Lawrence Livermore National Laboratories and the University of California that included 271 petroleum-release sites in California suggests that most petroleum compounds do not migrate beyond 260 ft of their source in groundwater aquifers (Rice et al., 1995). This is attributed to natural microbial processes that break down petroleum compounds (a process known as natural attenuation). However, many hazardous-waste compounds are not amenable to biodegradation. In addition, petroleum storage tanks may contain hazardous substances that behave quite differently than petroleum compounds. The more highly mobile of these hazardous compounds are expected (and have been shown) to extend well beyond 260 ft of their source in certain circumstances. For example, the same studies on petroleum sites in California found that the fuel oxygenate MTBE migrated beyond the 260-ft mark at multiple sites. One Navy installation in California currently contains a groundwater plume of dissolved MTBE greater than 1,000 ft in extent (Department of the Navy, 1998). This research implies that the 100- and 500-ft setbacks separating hazardous-waste storage, petroleum storage, and heating oil storage are not sufficiently protective of water quality in nearby reservoirs.

Expert Panel Recommendations

To gain further insight into the MOA setbacks, 12 experts on buffer zone structure and functioning in the United States and Europe were polled for their opinions (see the Preface). Each expert was asked to judge the adequacy of the 22 setback provisions found in Table 10-1; six responses were received. The opinions were consistent with the conclusions drawn from the literature review.

Almost all respondents rated the setback requirements for hazardous-waste storage, petroleum storage, heating oil storage, and solid-waste landfills and junkyards as around inadequate, with one exception—the 1,000-ft setback for landfills and junkyards around reservoirs was thought to be adequate. Although each respondent did not provide details, one scientist noted that ''the risks associated with accidental contamination from these activities are too great." Almost all respondents thought that the setback requirements for pesticide application were adequate, which also supports the conclusion derived from the literature review.

The setbacks for septic systems and impervious surfaces were judged to be adequate by almost all the respondents. In this case, comparison with the literature review is not possible because the respondents were not asked to consider individual pollutants. The literature review supports the adequacy of these setbacks for sediment removal, but it is inconclusive or negative with respect to

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

phosphorus and microbial pathogens. In addition, evidence gathered as part of the septic siting study (see Box 11-1) suggests that 100 ft is not sufficient to retard movement of viruses or bacteria in the subsurface of the Catskill/Delaware region (M. Sobsey, University of North Carolina, personal communication, 1999). Finally, respondents' opinions were mixed regarding the adequacy of the setbacks for subsurface-discharging WWTPs. The larger 500-ft setback required around reservoirs, reservoir stems, and controlled lakes was viewed more favorably than the 100-ft setback required around watercourses and wetlands. The potentially significant risks associated with accidental contamination from WWTPs were responsible for this split decision.

General Comments

Perhaps the most important point made clear from the literature review and the survey of buffer zone experts is that determining setback or buffer zone distances should be done on a site-specific basis. Research on buffer zone effectiveness has revealed several important environmental parameters that should be taken into account when determining buffer zone width. Castelle et al. (1994) note that there are at least four criteria that should be used to size buffer zones: (1) resource functional value (e.g., removal of nonpoint source pollution or maintenance of aquatic habitat), (2) intensity of adjacent land use, (3) buffer characteristics, and (4) specific buffer functions required (such as nitrogen removal). Most buffers are sized taking only the first criterion into consideration. That is, buffers are given a fixed width based on one parameter, rather than having variable widths that optimize all four parameters.

Several buffer characteristics are critical to predicting its pollutant removal capabilities. The most frequently cited parameter is slope (Barling and Moore, 1994; Phillips, 1989a,b). In fact, some models of buffer zone efficiency take only slope into consideration (Barling and Moore, 1994; Nieswand et al., 1990). Researchers have even recommended that any land with a slope greater than 15 percent should be considered unacceptable as a buffer zone (Nieswand et al., 1990). Phillips (1989a,b) has shown that buffer zone roughness (which corresponds to the amount of vegetation present), soil hydraulic conductivity, and soil moisture can also significantly affect buffer removal efficiencies.

Quantitative Analysis of Setbacks

This final section examines the potential effectiveness of the 100-ft setback by estimating travel times of subsurface flow from available site-specific data. It revisits a similar analysis found within the 1993 EIS for the Watershed Rules and Regulations. The inferences drawn from our analysis and salient research in the mid-Atlantic and southeastern United States are tempered with a discussion of sources of scientific uncertainty.

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

The status of knowledge about the structure and function of riparian forest buffers is changing rapidly. At present, we have a well-rounded qualitative understanding of the multiple functions and importance of the riparian zone in relation to the quantity, quality, timing of water flow, and other ecosystem attributes. The effectiveness of riparian forest buffers has been quantified for some water quality constituents (e.g., nitrogen, phosphorus, pesticides, and sediment) on several experimental sites (Lowrance et al., 1997) but is currently unknown for many other contaminants of concern.

Analysis of Setbacks in the 1993 Environmental Impact Statement

In 1993, NYC DEP prepared an evaluation of setback regulations as part of the EIS for the proposed Watershed Rules and Regulations. Various modeling approaches, primarily Darcy's Law for subsurface flow and Manning's equation for surface flows, were used to estimate pollutant travel times or other measures given certain setback distances, slope, hydraulic conductivity, porosity, roughness, and rainfall intensity. Table 10-5 presents the travel-time results of the EIS analyses for each of the setback provisions considered. For most activities, the pollutants were assumed to behave like conservative tracers. In general, the EIS shows that requiring setbacks will increase travel times, and this increase is dependent on the values used for the parameters mentioned above. In particular, 250- to 1,000-ft subsurface flow paths (used for underground storage of hazardous wastes, septic systems, and junkyards) generate travel times on the order of days to years, while surface flow paths of the same length generate travel times on the order of hours. The travel-time information can be used to gauge how much time would be available for remedial measures if a pollutant discharge were to occur. It should be noted that there was no quantitative analysis of the effect of setback distances adjacent to new impervious surfaces.

For pesticides (nonagricultural uses) and on-site sewage treatment and disposal systems (OSTDS), the analysis was extended to estimate percent pollutant removal for variable setback distances or changes in pollutant loading, respectively. The analysis for OSTDS evaluated the effect of variable setback distances, slope, and hydraulic conductivity on pollutant removal in the subsurface. Using removal efficiencies from the literature2, physical data from the Catskill/Delaware watershed, and professional judgment, the analysis showed that for properly functioning OSTDS, setbacks between 100 and 550 ft would provide nearly 100 percent removal of biological oxygen demand, total suspended solids, coliforms, Giardia, and viruses. Removal efficiencies of setbacks around malfunctioning systems (those with less than 2 ft of unsaturated soil beneath the soil

2  

 It was assumed that Giardia and coliform bacteria would be filtered out of OSTDS effluent in a relatively short distance (<100 ft).

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

TABLE 10-5 Model Evaluation of Setback Distance

Prohibited Activity

Location

Equation

Variables

Values Used

Travel Times

Hazardous Substance and Waste Storage,

Above-ground

Manning's Equation

Roughness coefficient

24-hour rainfall

Slope

Setback length

0.011, 0.15, 0.24, 0.4

0.5 and 2.75 in

0.01 and 0.25 ft/ft

250 to 1,000 ft

0.5 to 2 hours

Generation, and Disposal

Underground

Darcy's Law

Hydraulic conductivity

Porosity

Hydraulic gradient

Setback length

0.352, 2.17, 15.7 in/hr

0.3

6% and 25%

250 to 1,000 ft

10 to 1,000 days

Petroleum Products

Above-ground

Manning's Equation

Roughness coefficient

24-hour rainfall

Slope

Setback length

0.011, 0.15, 0.24, 0.4 0.5 or 2.75 in

0.01 or 0.25 ft/ft

750 ft

0.04 to 6 hours

Septic Systems

Underground

Darcy's Law

Hydraulic conductivity

Porosity

Hydraulic gradient

Setback length

0.352 to 15.7 in/hr

0.3

8%, 15%, and 25%

100 to 550 ft

60 days

Landfills

Above-ground

Manning's Equation

Roughness coefficient

24-hour rainfall

Slope

Setback length

0.011, 0.15, 0.24, 0.4 0.5 or 2.75 in

0.01 or 0.15 ft/ft

250 and 1,000 ft

0.01 and 3 hours

Junkyards, Composting, Sludge, Transfer Stations

Underground

Darcy's Law

Hydraulic conductivity

Porosity

Hydraulic gradient

Setback length

0.352, 2.17, 15.7 in/hr

0.3 1%, 8%

250 to 1,000 ft

18 to 40,000 days

Pesticides

Above-ground

PESTRUNa and trap efficiencyb

Setback length

250 to 1,000 ft

Pollutant loading estimated

a PESTRUN generates pollutant loadings rather than travel times.

b The "trap efficiency" method of Wong and McCuen (1982) only applies to particulate-phase pollutants. Trap efficiency is similar to pollutant removal.

Source: NYC DEP (1993).

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

adsorption system) were slightly lower, especially under conditions of high slope and hydraulic conductivity.

Pesticide loading to water bodies was estimated for chemicals attached to sediments in surface runoff. Trap efficiencies were based on results reported in Wong and McCuen (1982), which provides a graphical representation of percent trapping of suspended solids based on buffer length, slope, and cover conditions. Pesticides were divided into groups based on their adsorption properties, and half-lives were estimated. In general, the setback distances of 250 and 1,000 ft showed large reductions (77 percent to 85 percent) for strongly adsorbed pesticides (group 1—glyphosate, diquat, and methoxychlor).

Many of the results in the EIS are directly influenced by the assumptions and limitations of the analysis. For instance, in determining trap efficiencies, it was assumed that the setback area was managed as a buffer zone and that channelized flow did not occur. Trap efficiencies were assumed to be zero in slopes greater than 15 percent, and dissolved pesticides were not considered. Although the study concluded that only a small fraction of the pesticides applied outside a 1,000-ft limiting distance would reach a reservoir, this conclusion is primarily based on best professional judgment and is only addressed for certain chemicals and conditions. Similarly, assumptions on pollutant removal downslope from OSTDS largely controlled the results generated in that analysis. Changing setback distances, slope, and hydraulic conductivity had minimal effect on pollutant removal.

Committee Analysis

Because the analysis in the EIS generates travel times rather than pollutant removals for almost all categories of setbacks considered, its use in predicting the effectiveness of the New York City setbacks is limited. However, given present modeling capabilities, the EIS analysis is still a relatively current approach. The types of data that could be used to refine the EIS analysis (e.g., pollutant concentrations emanating from different land uses and rates for degradation processes within a setback) are not available for the Catskill/Delaware watershed. The analysis that follows has three goals: (1) to generate new results on travel time, (2) to present the results in a format that highlights the interaction of the two most important physical parameters, and (3) to discuss assumptions that will substantially influence the predicted travel times.

Method. Unlike the EIS analysis, this analysis does not consider multiple land uses and their associated pollutant travel times through a setback. Rather, Darcy's Law (Hanks and Ashcroft 1980; Hillel, 1980) (Equation 10-2) is used to generate a single estimate that can be generically applied to all land uses and pollutants emanating from those land uses. The results are expressed as time of

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

travel through a 100-ft setback for multiple combinations of slope and saturated hydraulic conductivity.

In preparation for flux calculations using Darcy's Law, USDA soil surveys from the Catskill/Delaware region were reviewed to determine the observed range of saturated hydraulic conductivity (Ks) of soils—0.06 to >20 in/hr, or 0.04 to >12.2 m/day. Slopes ranging from 0 to 45 degrees were used to represent the potential range of hydraulic gradients (sin[slope angle]) for lateral flow through the riparian zone into adjacent streams. The flow velocity through the soil matrix was determined by dividing flux by total porosity (0.5 was used for this analysis). Dividing the riparian setback width (100 ft) by flow velocity (ft/day) yields the time required to travel 100 ft in relation to multiple combinations of hydraulic conductivity and gradient.

The contours (isochrones) in Figure 10-5 show estimated travel (residence) times for combinations of saturated hydraulic conductivity (1–12.5 m/day, or 1.6–20.0 in/hr) and slope angle (1–25 degrees, or ~2–50 percent). Larger values of conductivity or gradient generate travel times of only a few days. Locations where both slope and hydraulic conductivity are high are rare since high-conductivity sand and gravel deposits typically occur in valley bottoms with limited gradients. Conductivity and gradient values less than 1 m/day and 1 degree, respectively, yield estimated travel times of hundreds or thousands of days. In the Catskill and Delaware watersheds, low-conductivity soils or low-gradient sites are usually riparian wetlands. The travel times generated in the EIS falls within the range presented in Figure 10-5.

Not all combinations of slope and hydraulic conductivity occur simultaneously in the Catskill/Delaware watershed. A GIS could be used to map those areas of Figure 10-5 that represent conditions within the watershed. Data on terrain and soil layers could be used to generate spatial statistics for hydraulic conductivity and gradient in the riparian zone. The GIS also could be used to cross-tabulate soil and gradient data to characterize their association at the landscape scale, sometimes referred to as the drainage catena. For example, do clay and silt loam soils (low-conductivity) largely occur on gentle slopes while sandy loams and stony till soils (high-conductivity) are found on steeper slopes? A digital elevation model in the GIS could be used to quantify other key watershed characteristics such as contributing area, slope shape (concave, planar, convex), landform (divergent, planar, convergent), vegetative cover, and land use—all of

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

FIGURE 10-5 Estimated pollutant travel times for different combinations of soil hydraulic conductivity and slope.

which may influence the pathway and rate of water movement through the riparian zone (Bevin and Wood, 1983; O'Loughlin, 1986).

Assumptions, Field Conditions, Interpretation, and Inference. Several implicit and explicit assumptions influence the travel-time estimates. In essence, the data presented in Figure 10-5 are an idealized and simplified representation of field conditions. Saturated hydraulic conductivity data are derived from a laboratory determination on nominally undisturbed 3-inch (7.62-cm) diameter cores. Macropores (sometimes called preferential flow paths or soil pipes) formed by roots, organic matter, microbes, earthworms, and other invertebrates, and small mammals may increase in situ conductivity by several orders of magnitude (Mosley, 1982; Mullholland et al., 1990). Cobbles, gravel, and other coarse fractions (>2 mm) also may increase effective conductivity. Both sources of heterogeneity, which happen to be common in forest soils, decrease travel times for subsurface flow.

The analysis also assumes no overland flow in or through the riparian zone. Overland flow is uncommon but can occur when soil frost or compaction reduces

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

infiltration capacity. Saturation from below can occur when lateral inflow from upslope areas, combined with rainfall or snowmelt, exceeds the outflow—usually where subsurface flow converges at the base of concave slope. Impervious surfaces adjacent to the riparian zone can collect and concentrate water in rills or ephemeral channels that simply cut across the riparian zone. (The MOA mandates NYC DEP approval of new development and of modifications to existing infrastructure to prevent this circumstance. This, however, presents a substantial challenge for permitting, enforcement, and long-term maintenance.) Whatever the cause(s), overland flow short-circuits the subsurface flow path and decreases travel time.

The analysis assumes complete saturation of the soil mantle. This is a conservative assumption for at least two reasons. First, flow through the saturated zone is considerably faster than flow through the unsaturated zone. Flow through the unsaturated zone is both vertical and horizontal, while saturated flow is predominantly horizontal. In addition, unsaturated hydraulic conductivities may be as much as six to ten orders of magnitude lower than saturated hydraulic conductivities. Second, saturated conditions are uncommon in the Catskill/Delaware watershed. In this region, evaporation of soil water and transpiration by plants routinely produce unsaturated conditions. Rainfall is intercepted by and subsequently evaporated off leaf surfaces or enters the soil and is promptly sequestered by plants. During winter, precipitation is stored in a persistent snowpack. Deprived of new inflow, continuous subsurface flow steadily reduces soil water content. Consequently, there are only brief periods of the year—in October or November after prolonged rains and March or April during snowmelt—when soils approach a saturated condition. Hence, the direct and indirect effects of vegetation may override the influence of climate, soil hydraulic properties, terrain features, and other physical attributes and have the net effect of increasing travel time.

Summary

The foregoing discussion highlights multiple sources of uncertainty, their potential interaction and effect upon the interpretation of Figure 10-5, and the efficacy of 100-ft setbacks. Until research in the Catskill/Delaware region, or transferable work on similar sites, quantifies the performance of riparian setbacks and buffers, conservative interpretation and common-sense application of well-established basic principles, salient research, and operational experience should guide riparian zone protection efforts. Specifically, the width of riparian setbacks should be increased as slope steepness and/or soil permeability yields insufficient residence time to effectively assimilate pollutants. Conversely, zoning variances and setback reductions should be granted when in situ data and information consistently demonstrate residence times and biological activity sufficient to meet in-stream water quality standards. Although 100-ft setbacks have become the

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

default standard for watershed protection in the United States (Welsch, 1991), it is unlikely that a "one-size-fits-all" approach will be optimal in any particular system.

CONCLUSIONS AND RECOMMENDATIONS

  1. The setbacks prescribed in the MOA are not buffer zones. As described in the MOA, the setbacks are distances between activities and waterbodies. Setback descriptions do not discuss the characteristics of the setback land that are known to influence pollutant removal in buffer zones, such as slope, hydraulic conductivity, soil moisture, vegetation or surface roughness, and flow rates. Only setback width is defined.

  2. Active management for the setbacks is necessary to achieve the pollutant removal efficiencies attributed to buffer zones. NYC DEP should create incentives for managing setback areas to meet the following goals:

  • Setbacks function most effectively as buffer zones if runoff entering the setbacks is sheet flow rather than channelized flow. Thus, best management practices such as filter strips, bioretention areas, and level spreaders should be installed upslope from buffer zones to create sheet flow in areas prone to significant concentrated flow. Converting channelized flow to sheet flow can be extremely difficult in areas where storm drains and pipes convey runoff to nearby bodies of water.

  • Setbacks must be naturally regenerated or planted with the appropriate vegetation for retaining nutrients, sediment, and other pollutants. In most cases forested setbacks are the most effective buffers.

  • If setbacks are managed as buffers, they should be managed as described in the USDA three-zone buffer specification (Figure 10-2), and consideration should be given to periodic vegetation harvesting in Zones 2 and 3.

  • Compensation to private owners may be required to permit or conduct management activities in setback areas.

  1. Setback distances in the New York City watersheds are similar to or greater than those found in other locations. This suggests that they will be as effective as setback requirements found elsewhere.

  2. NYC DEP should set a slope threshold above which land cannot be included in setback considerations. The literature review suggests that areas with a slope of greater than 15 percent do not function as effective buffer zones. Fixed-width setbacks are desirable because of their easy demarcation and implementation compared to variable-width setbacks. However, fixed-width setbacks may pose unfair burdens on some landowners with primarily riparian properties,

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

and they may be underprotective in areas with increased travel times (such as steep regions).

  1. The existing state of knowledge on pollutant transport in buffer zones and the lack of site-specific information about pollutants that might be derived from activities behind a setback make a detailed quantitative analysis somewhat limited. Based on existing literature, an expert panel, and two analyses of travel times, the following setback requirements are judged most likely to be inadequate:

  • 100 and 500 ft for hazardous wastes

  • 100 and 500 ft for petroleum underground storage tanks

  • 100 and 500 ft for heating oil

  • 250 and 1,000 ft for landfills

  • 100 ft for septic systems, impervious surfaces, and WWTPs because of potential breakthrough of microbial pathogens.

  1. NYC DEP should undertake a program of field research to better justify and/or amend the current setbacks. In order to evaluate the effectiveness of particular setback distances, significantly more detailed data are needed on land use and pollutant transport through buffer areas to recipient streams and reservoirs. Performance monitoring of riparian buffer zones, in which shallow subsurface pollutant concentrations are measured above and below the buffer zone, is required. Integration of these data is possible by use of remote sensing and GIS techniques. Data of very fine resolution are needed prior to extensive application of predictive models. It should be noted that performance monitoring of buffer zones is much more difficult in the presence of channelized flow compared to sheet flow.

  2. Because of the uncertainty associated with evaluating specific setback distances, the present setback distances should not be reduced for any activities or land uses to which they currently apply. NYC DEP should consider requiring setbacks for new agricultural activities.

  3. Buffer zones should not be relied upon to provide the sole nonpoint source pollution control and are instead best used when integrated with appropriate source controls on pollutant releases. Although riparian buffers can ameliorate some nonpoint source of pollution, they are most effective when used as part of an overall pollution control or conservation plan.

  4. The MOA setbacks are likely to be found constitutional based on judicial precedents elsewhere, unless they preclude all economic use of a parcel of land.

Suggested Citation:"10 Setbacks and Buffer Zones." National Research Council. 2000. Watershed Management for Potable Water Supply: Assessing the New York City Strategy. Washington, DC: The National Academies Press. doi: 10.17226/9677.
×

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×

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In 1997, New York City adopted a mammoth watershed agreement to protect its drinking water and avoid filtration of its large upstate surface water supply. Shortly thereafter, the NRC began an analysis of the agreement's scientific validity.

The resulting book finds New York City's watershed agreement to be a good template for proactive watershed management that, if properly implemented, will maintain high water quality. However, it cautions that the agreement is not a guarantee of permanent filtration avoidance because of changing regulations, uncertainties regarding pollution sources, advances in treatment technologies, and natural variations in watershed conditions.

The book recommends that New York City place its highest priority on pathogenic microorganisms in the watershed and direct its resources toward improving methods for detecting pathogens, understanding pathogen transport and fate, and demonstrating that best management practices will remove pathogens. Other recommendations, which are broadly applicable to surface water supplies across the country, target buffer zones, stormwater management, water quality monitoring, and effluent trading.

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