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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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).
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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.
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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).
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy (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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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.
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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).
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 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 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. 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. 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. 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,
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy and they may be underprotective in areas with increased travel times (such as steep regions). 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. 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. 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. 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. 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.
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Representative terms from entire chapter: