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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico 2 Nutrient Inputs and Water Quality Effects Adequate monitoring and proper management of Mississippi River water quality, including its effects that extend in the northern Gulf of Mexico, represent important national water management challenges. The Mississippi River basin extends over 41 percent of the area of the conterminous 48 states. It is the world’s third-largest river basin (Milliman and Meade, 1983). The river basin includes all or parts of 31 U.S. states (Figure 1). Approximately 70 million people live in the basin, and water quality in the river and into the northern Gulf of Mexico is affected by urban and household activities, industry, agriculture, construction, forestry, and other sectors. Water quality across the river basin, in the mainstem Mississippi River, and into the northern Gulf of Mexico is affected by many different sources of nutrients (Figure 2) and the river experiences varying levels and types of degradation in different reaches. As noted in the previous 2008 NRC report on Mississippi River water quality and the Clean Water Act, at the scale of the river basin, nutrients and sediment are the primary water quality problems. This chapter discusses: 1) sources of nutrient inputs, 2) water quality impacts of nutrients, 3) scientific understanding of hypoxia and key management challenges. SOURCES OF NUTRIENT INPUTS Since its passage in 1972 (and subsequent amendments in 1977 and other years), the Clean Water Act has achieved many successes in helping address point source effluent into the Mississippi River. Today, the more challenging water quality problems across the river basin and in the northern Gulf of Mexico derive from inputs of nonpoint source pollutants, especially nutrients.1 Nonpoint source pollutants derive from a variety of unconfined and unchanneled sources of water pollution, such as runoff flowing across agricultural lands, forests, and urban lawns, streets, and other paved areas. In the Mississippi River basin, the majority of nonpoint source pollution comes from agricultural applications of nitrogen and phosphorus fertilizers, primarily 1 The specific forms of nutrient loads are crucial issues in gulf hypoxia. For nitrogen, nitrate is the prevalent form of nitrogen flux into the gulf. For phosphorus, dissolved phosphorus transported in water plays a larger role than does phosphorus that is transported with sediment.
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico FIGURE 1. Mississippi River basin, major tributaries, land uses, and typical summertime extent of northern Gulf of Mexico hypoxia (in red). The Mississippi River basin extends over 31 U.S. states and covers 41 percent of the conterminous U.S. The size of the river basin and the diversity of land types and uses magnify the challenges associated with improving water quality in the northern gulf. SOURCE: Reprinted, with permission, from Goolsby (2000). © by the American Geophysical Union.
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico FIGURE 2. Sources of Nutrients Delivered to the Gulf of Mexico. SOURCE: Reprinted, with permission, from Alexander et al. (2008). © by the American Chemical Society. to row crops such as corn and soybeans (Howarth et al., 1996; Bennett et al., 2001; Turner and Rabalais, 2003). The transport and delivery of nutrients across the Mississippi River basin have been affected substantially by changes in land use and related agricultural practices, including the installation of subsurface drainage systems, as well as nutrient inputs from fertilizers and manure (Baker et al., 2008). Nutrient discharges from point sources also contribute to nutrient loadings across the river basin. For example, a data file of point sources in the Mississippi River basin created by the EPA lists more than 33,300 discharge permits in the basin (USEPA, 2006), although many of these are minor contributors. Most discharge values for nitrogen in that file are based on secondary data sources, because effluent monitoring for nitrogen (and phosphorus) in the basin is minimal. Total discharge of nitrogen from these point sources into the basin is estimated to be in excess of 210,000 metric tons per year, but the source of this data file includes no estimate of what percentage of that load reaches the Gulf of Mexico. Of the estimated total amount of nutrients discharged into the basin from point sources, 64 percent is attributed to municipal wastewater treatment plants and at least 25 percent to a variety of industrial sources. The balance is from a large array of other point sources, such as commercial and public enterprises. Much debate has revolved around the relative importance of point sources to the total contributions of nutrients into the Gulf of Mexico. Although the estimated fraction of the current flux of nitrogen and phosphorous being delivered to the Gulf of Mexico from these point sources is roughly 10 percent
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico of the total (Figure 2), the relative importance and actual percentage are contested by some parties. Requiring monitoring and reporting as conditions in National Pollutant Discharge Elimination System (NPDES) discharge permits for large municipal and industrial sources could substantially reduce uncertainties in estimated point source nutrient discharges. These types of permit conditions are common in states containing watersheds that drain to the Chesapeake Bay, the Great Lakes, and Long Island Sound, and in North Carolina, Florida, and California, where hypoxia problems or nutrient-related nuisance conditions have been identified. Requiring the monitoring of nutrients of these point source discharges into receiving waters would improve knowledge of the effects of nutrient discharges and provide useful data regarding relative contributions of those discharges. Across the Mississippi River basin, farmers add large amounts of nutrients—in the form of nitrogen and phosphorus fertilizers—to supplement soil nutrients in order to increase crop yields. (Figure 3 shows the spatial distribution of phosphorus and nitrogen yields across the river basin.) Runoff and sub-surface flows end up in streams and groundwater systems. These processes result in increased nutrient loading in many streams and waterbodies across the river basin. This process of nutrient transport into the basin’s stream systems is exacerbated by subsurface tile drainage systems in some areas. These tile drainage systems are networks of below-ground pipes that allow subsurface water to move out from between soil particles and into the tile line. These systems underlie many areas of row-crop agriculture and are important conduits for nitrate entering surface waters across the Mississippi River basin. Runoff of nitrogen and phosphorus from agricultural land degrades water quality in many parts of the nation. This degradation is of particular concern across the Mississippi River basin because of the predominance of annual row-crop agriculture. Factors contributing to this region’s high productivity and high acreages in row-crop agriculture include naturally rich soil, adequate annual precipitation, relatively flat to gently rolling terrain, and a hydrologically-modified landscape from which excess water drains rapidly and easily. These factors also contribute to high nitrate levels found throughout the region’s streams and rivers. Numerous reports document the linkages to nutrient pollution from: prevalence of annual cropping patterns; augmentation of naturally occurring soil nutrients with nitrogen applied in commercial fertilizers and manure; animal manure from livestock operations and increasing runoff from urban development; and, nutrient leaching from failing septic systems (Kalkhoff et al., 2000; McMullen, 2001; Schilling and Spooner, 2006; Hatfield et al., 2008).
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico FIGURE 3. Total nutrient yield delivered to the Gulf of Mexico from sources in the Mississippi River basin. The map on the left shows total nitrogen yields, the map on the right shows total phosphorus yields. The large yields from agricultural areas are prominent on this map. These maps also show that large percent ages of total nutrient yields derive from a relatively small number of watersheds across the river basin. SOURCE: Reprinted, with permission, from Alexander et al. (2008). © by the American Chemical Society.
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico WATER QUALITY IMPACTS OF NUTRIENTS Just as terrestrial plants, such as corn, are synthesized from abiotic materials in the presence of sunlight, several different species of phytoplankton (some of which may be toxic to fish and humans) in waterbodies may be synthesized by similar processes. The primary abiotic building materials for phytoplankton are carbon, nitrogen, and phosphorous. Photosynthesis of phytoplankton is an oxygen-producing process. When phytoplankton die, they sink to lower levels of waterbodies, where microbiological oxidation of the organic matter depletes dissolved oxygen from the water column. Oxygen also is exchanged between upper layers of the waterbody and the atmosphere. So long as the rate of photosynthesis, atmospheric exchange, and decomposition are within proportional ranges, oxygen remains at levels sufficient to support a rich variety of species. Excess loadings of nitrogen and phosphorus, under the right conditions of sunlight and temperature, can lead to high rates of synthesis and decomposition, reducing oxygen levels in lower parts of the water body to levels that are not sufficient to support many type of fish and shellfish. If dissolved oxygen falls below about 2 milligrams per liter, that portion of the water body is said to be hypoxic and sometimes is referred to as a “dead zone.” The hypoxia zone is a seasonal but perennial feature of the coastal waters downstream from the Mississippi River discharge into the gulf and is most prevalent from late spring through late summer. Although hypoxia is mainly a bottom-water condition, oxygen-depleted waters often extend upward into the lower one-half to two-thirds of the water column. Gulf of Mexico waters are stratified for much of the year, primarily because of salinity differences. This stratification intensifies during the warmer summer months and is an important contributor to the hypoxia phenomenon (Rabalais and Turner, 2001; Rabalais et al., 2002). Since its mapping began in 1985, the hypoxia zone in the northern Gulf of Mexico has averaged an areal extent of 13,800 square kilometers (updated from Rabalais and Turner, 2006). The size of this hypoxia zone has varied from one year to the next, depending on levels of spring nitrate loading (Turner et al., 2006). Despite these year-to-year variations, the size and duration of the hypoxia area has increased during the second half of the twentieth century. For example, in 2007 the hypoxia area was estimated to cover 20,500 km2 (Figure 4), which represented the third largest hypoxia zone since measurements began in 1985 (LUMCON, 2007; USEPA, 2007). In 2008, the area of low oxygen measured over 20,720 km2. This made the size of the 2008 (summer) hypoxia zone the second-largest on record (the areal extent of 2001 was roughly equal to that of 2008; LUMCON, 2008).
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico FIGURE 4. Extent of bottom-water hypoxia in the Gulf of Mexico, July 21-27, 2007. Values are milligrams/liter (mg/l) of dissolved oxygen. SOURCE: N. Rabalais, Louisiana Universities Marine Consortium.
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico The dynamics of the hypoxia zone also apparently have experienced some recent and important shifts, as noted in the EPA SAB report: the Gulf of Mexico ecosystem appears to have gone through a regime shift with hypoxia such that today the system is more sensitive to inputs of nutrients than in the past, with nutrient inputs inducing a larger response in hypoxia as shown for other coastal marine ecosystems such as the Chesapeake Bay and Danish coastal waters (USEPA, 2007). Experience in other systems (e.g., the northwest shelf of the Black Sea; Mee, 2001), and a biological-physical model for the Louisiana shelf (Justić et al., 1997), both indicate that it may take several years or longer to detect a response of the marine system to changes in the nutrient load. Further, changes in land use, particularly with the growing demand for corn-based ethanol (and attendant increases in fertilizer applications) may further affect the hypoxic zone in yet unanticipated ways. In addition to hypoxia, excess nutrient loadings can result in local water quality problems within the drainage basin. Locally, excess nutrient inputs can impair freshwater systems by, for example, causing blooms of algae that can be dominated by toxic cyanobacteria, such as in a summer 2008 algae bloom in the Raccoon River that threatened the drinking water supply of the city of Des Moines, Iowa. Toxic cyanobacteria can cause human health problems and in some instances can lead to fatalities (Rabalais, 2005; Lopez et al., 2008). SCIENTIFIC UNDERSTANDING OF HYPOXIA AND IMPLICATIONS FOR MANAGEMENT ACTIONS The scientific and management challenges in addressing the hypoxia problem have been articulated well in several publications (see Box 1-1). Good scientific knowledge of the sources of nutrient pollutants across the basin, and downstream impacts on water quality, is fundamental to creating viable nutrient pollution management programs and strategies in the Mississippi River basin. Scientific understanding of the geographic sources of nitrogen and phosphorus inputs across the basin has improved greatly over the years. The attainment of significant reductions in nitrogen and phosphorus loadings represents a difficult goal. For example, there is only limited regulatory authority that the Clean Water Act grants to the federal government to regulate loadings from nonpoint sources of water pollutants. Many economic factors also will affect future nutrient loadings across the basin and discharges into the gulf, further complicating nutrient control measures. For example, current high commodity prices provide incentives for Midwestern farmers to maximize acreage devoted to grain production; on the other hand, higher prices of agricultural land, fertilizers, farm implements, may provide disincentives to
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico increased commodity production. Efforts to reduce nutrient loadings to the northern Gulf of Mexico, whether they be through improved management practices, construction of wetland areas to trap and filter pollutants, and other actions, will constitute significant management, economic, and public policy challenges. Part of this challenge relates to the long-term, temporal relations between upstream nutrient loadings and subsequent changes in downstream water quality and hypoxia. A difficulty in implementing successful nutrient management measures and programs is the long time required to determine the downstream impacts of changes in nutrient loading levels and patterns. This underscores the importance of evaluating local water quality impacts of nutrient control actions—these impacts will occur sooner and be easier to attribute to a specific course of action. According to presentations given to this committee by USGS scientists who have worked extensively on Chesapeake Bay water quality monitoring and modeling (Blomquist, 2008; Sanford, 2008), their experience in collecting water quality data and attempting to identify trends in bay suggests that a minimum 9-year period of data is necessary to determine a trend in water quality (see also Lindsey et al., 2003; Langland et al., 2006; Raffensperger and Langland, 2007; Sanford and Pope, 2007). If a 9-year period of trend data at a minimum is necessary to recognize whether changes in land use practices, or changes in fertilizer applications, or other nutrient management practices can affect water quality, it may require decades for nutrient control actions in the Mississippi River basin to be reflected in changes in the areal extent of northern Gulf of Mexico hypoxia. Box 1-1 summarizes Mississippi River water quality modeling efforts being conducted by USGS scientists. Studies being conducted within the USGS program on “SPAtially Referenced Regressions On Watershed attributes”, or SPARROW, present information on geographic sources of nitrogen and phosphorus loadings from across the river basin, and the relative proportions of land use categories of these sources. Figure 5, for example, comes from the SPARROW modeling team and shows the percentage of nutrient loads exported by different watersheds that are delivered to the gulf (Alexander et al., 2008). The nine Mississippi River states listed in Table 1 account for approximately three-fourths of the nitrogen and three-fourths of the phosphorus that reaches the Gulf of Mexico (Alexander et al., 2008). Although the remaining roughly one-quarter of nitrogen and phosphorus loads that reach the Gulf of Mexico is not insignificant, the SPARROW model results provide information that would be important in targeting nutrient control action to areas of higher nutrient yields (Alexander et al., 2008). Figure 6 shows several watersheds from the Corn Belt region and their total nitrogen yields. Watersheds in this figure are delineated as six-digit hydrologic accounting units (HACs) as defined by federal Hydrologic Unit Codes (HUCs). Each of those watersheds can be divided into eight-digit watersheds known in the federal coding system as hydrologic cataloging units (these smaller watersheds are not shown in order to avoid excessive detail).
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico TABLE 1. States that contribute approximately three-fourths of the total nitrogen and phosphorus delivered to the Gulf of Mexico. Arkansas Iowa Missouri Illinois Kentucky Ohio Indiana Mississippi Tennessee SOURCE: Reprinted, with permission, from Alexander et al. (2008). © by the American Chemical Society. Figure 6 shows HACs that have been overlaid with data on densities of loads (as estimated by the USGS). This figure shows that a large portion of the total Mississippi River basin nitrogen load delivered to the northern Gulf of Mexico comes from a relatively small number of HACs. There are approximately 125 HACs in the basin that drain directly to the Mississippi River mainstem (a few other HACs drain directly to the Gulf of Mexico). Of these 125, about 30 of them—which cover about 20 percent of the basin—account for substantial percentage of total nitrogen yield from the basin. This large concentration of the sources of nitrogen loadings in a relatively small number of hydrologic units/watersheds is important information for any program designed to reduce nutrient loadings to the Mississippi River and the northern Gulf of Mexico. Indeed, this information may identify opportunities for substantially reducing the areal extent of northern Gulf of Mexico hypoxia. Many groups have emphasized the importance of targeting nutrient control strategies at watersheds of higher nutrient yields. The previous 2008 NRC report, for instance, concluded that: Programs aimed at reducing nutrient and sediment inputs should include efforts at targeting areas of higher nutrient and sediment deliveries to surface water (NRC, 2008). The EPA Science Advisory Board Hypoxia Advisory Panel noted that the greatest concentration of nitrogen and phosphorus in runoff comes from the upper Mississippi and Ohio-Tennessee river subbasins, both of which have extensive tile drainage systems. The SAB report recommends “… targeting sub regions or watersheds that have a disproportionate effect on hypoxia and local water quality" (USEPA, 2007). In addition to targeting resources to priority watersheds, resources should be strategically targeted at specific geographic areas within a given watershed. There can be substantial variations in slope, land cover, soil type, and other features across a watershed, all of which affect runoff, erosion, and rates and levels of nutrient loadings into streams. This more exact targeting within an individual watershed promotes efficient expenditure of conservation program dollars and improvements in water quality (Sharpley et al., 2006). These more targeted actions can help identify and concentrate nutrient reduction efforts in priority watersheds on those areas where nutrient control efforts are more likely
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico FIGURE 5. Percentage of stream nutrient load delivered to the Gulf of Mexico. This figure illustrates the geographic differences within watersheds in percentages of nitrogen and phosphorus yields that reach the Gulf of Mexico. These maps suggest the need to focus nutrient reduction strategies on watersheds with both high total yields (as seen in Figure 3) and on high percentages of yields that reach the Gulf of Mexico. SOURCE: Reprinted, with permission, from Alexander et al. (2008). © by the American Chemical Society.
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico FIGURE 6. Hydrologic Accounting Units and spatial patterns of nitrogen yields in a portion of the Mississippi River basin. The figure clearly shows the high concentration of nitrogen yields from a relatively small number of watersheds. SOURCE: Reprinted, with permission, from Alexander et al. (2008). © by the American Chemical Society. Finding/recommendation 1: Realizing progress toward reducing the areal extent of northern Gulf of Mexico hypoxia will require an acknowledgement that there will be a considerable time lag—roughly a decade, at a minimum—between nutrient reduction actions across the river basin and ecological and water quality responses downstream in the gulf. Finding/recommendation 2: Purposeful targeting of nutrient control efforts toward areas of higher nutrient loadings will be essential to realize the greatest initial reductions in nutrient loadings. EPA and USDA should direct conservation programs and other nutrient management resources to priority Mississippi River basin watersheds with higher levels of nutrient loadings. In addition to targeting individual watersheds, those programs should identify specific areas within watersheds where expenditures and actions are more likely to produce initial, positive results. Finding/recommendation 3: To improve knowledge regarding point sources’ relative contributions of nutrient pollution, EPA should require major municipal and industrial
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Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico point source dischargers to monitor nutrient concentrations—nitrogen and phosphorus—in effluent at their discharge point as a condition of their National Pollutant Discharge Elimination System (NPDES) permits.
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