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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution 9 Source Reduction and Control KEY POINTS IN CHAPTER 9 This chapter reviews what is known about management options for reducing nutrient supply to coastal environments. It finds: Nutrient loads to coastal areas can be reduced by a variety of means, including improvements in agricultural practices, reductions in atmospheric sources of nitrogen, improvements in treatment of municipal wastewater (including tertiary treatment in some cases), and better control of diffuse urban nutrient sources such as runoff from streets and storm sewers (including both structural and passive controls). Regional stormwater control facilities, use of wetlands as nutrient sinks, better forest management to limit nitrogen export, enhancement of circulation in coastal waterways, and biological treatment also offer promise in some settings. Options to minimize nutrient export from agricultural areas include manure management strategies, careful estimation of native nutrient availability and crop requirements, and supplemental fertilizer application timed to meet crop demand. Watershed-scale implementation of best management practices needs to be targeted to ensure maximum reduction in nitrogen and phosphorus export. Post-implementation monitoring should be done to assess effectiveness. Lasting reductions in nutrient export from agriculture can be encouraged by focusing on consumer-driven programs and education, as well as on-farm production. Farmers’ decisions are often influenced by regional or even global economics. At these scales, farmers have little or no control over these economic pressures and the resulting changes in nutrient flows and distribution. New ways of using incentives to help farmers implement innovative source reduction and control are needed.
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution A positive side effect from regulatory initiatives to reduce NOx emissions, targeted to minimize ozone and acid rain, is a reduction in the atmospheric contribution to nutrient loading in estuaries. The need to minimize coastal eutrophication should be a component of air pollution control strategies. Unfortunately, current NOx emission efforts are aimed principally at control during the summer because of emphasis on ozone and smog formation; for eutrophication, year-round emission controls are necessary. A wide variety of methods, with variable effectiveness, are available to reduce urban point and nonpoint sourced nutrients. Natural options (enhancement of coastal wetlands) are one of a range of management tools. Many factors contribute to nutrient over-enrichment, and thus there are many avenues by which the associated loads might be reduced. The effectiveness of any method depends, in part, on how large a contribution the source in question makes: minor improvements to major sources can sometimes offer more overall improvement than eliminating some minor nutrient source. Accurate information about relative contributions is essential if policymakers are to prioritize control efforts. Again, the federal actions called for in Chapter 2 would greatly strengthen efforts by local, state, and regional decisionmakers to successfully prioritize control efforts. Nutrient over-enrichment in coastal waters is inextricably linked to human activities within estuarine areas as well as upstream, which in turn are tied to management and policy decisions. Conversely, physical, chemical, and biological impacts can be reduced by more effective control of anthropogenic inputs to the watershed, for instance by reducing loadings from agricultural, urban, or atmospheric sources. Figure 9-1 illustrates the significant effects that changes in tillage practices can have on nitrogen and phosphorus in a watershed. This chapter explores management strategies designed to reduce nutrient inputs. Because agricultural runoff is one of the greatest challenges in nutrient control, considerable attention is focused on control of agricultural sources, followed by control of atmospheric sources, urban sources, and control by other mechanisms. AGRICULTURAL SOURCES The goal of efforts to reduce nitrogen and phosphorus loss from agriculture to water is to increase nutrient use-efficiency. To do this, farmers attempt to balance the input of nutrients into a watershed from feed and fertilizer with outputs in crop and livestock produce, and also to manage the level of nitrogen and phosphorus in the soil. Reducing nutrient loss in agricultural runoff can be achieved by both source and transport control measures (Table 9-1). In general, there are reliable ways to reduce the transport of sediment-bound phosphorus from agricultural land by controlling erosion, and, to a lesser extent, there are methods to control nitro-
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution FIGURE 9-1 Annual nitrogen and phosphorus loss into the Little Washita River from a wheat-dominant and grass-dominant sub-watershed. Note the reduction in both nutrients after the eroding gullies in the grass watershed were treated in 1984 and conventional tillage was replaced with no-till in 1983 in the watershed growing wheat (modified from Sharpley and Smith 1994; Sharpley et al. 1996). gen leaching to groundwater and the transport of dissolved phosphorus in runoff. However, less attention has been directed toward source management of nutrients because controlling nutrients at the source typically requires significant extra labor and thus is an economic burden to the farmer.
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution TABLE 9-1 Practice Description Source Measures Feed additives Enzymes increase nutrient utilization by animals Crop genetics Low phytic-acid corn reduces phosphorus in manure Manure management Compost, lagoons, pond storage; barnyard runoff control; transport excess out of watershed Rate added Match crop needs Timing of application Avoid autumn and winter application Method of application Incorporated, banded, or injected in soil Crop rotation Sequence different rooting depths Manure amendment Aluminum reduces NH3 loss and phosphorus solubility Soil amendment Flyash, iron oxides, gypsum reduce phosphorus solubility Cover crop and residues If harvested can reduce residual soil nutrients Plowing stratified soils Redistribution of surface phosphorus through profile Transport Measures Cultivation timing Not having soil bare during winter Conservation tillage Reduced and no-till increases infiltration and reduces soil erosion Grazing management Stream exclusion, avoid overstocking Buffer, riparian, wetland areas, grassed waterways Removes sediment-bound nutrients, enhances denitrification Soil drainage Tiles and ditches enhance water removal and reduce erosion Strip cropping, contour plowing, terraces Reduces transport of sediment-bound nutrients Sediment delivery structures Stream bank protection and stabilization, sedimentation pond Critical source area treatment Target sources of nutrients in a watershed for remediation TABLE 9-1 Best management practices designed to control nonpoint sources of agricultural nutrients (unpublished table from A. Sharpley). Source Management Animal Feeding of Nitrogen and Phosphorus Both nitrogen and phosphorus are important dietary nutrients for animals and have a key role in various metabolic functions (NRC 1989). Most feedstuffs do not contain adequate nitrogen and phosphorus to meet the needs of growing animals; thus additional nutrient supplements are brought onto the farm. The nutritional goal is to feed adequate nitrogen
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution and phosphorus to meet the animal’s requirements while minimizing excretion. Recommended dietary requirements of nitrogen and phosphorus have been established by the National Research Council (NRC) and are routinely updated (e.g., NRC 1989). Although studies show these requirements to be accurate, many farms routinely over feed nitrogen and phosphorus (Shaver and Howard 1995; Wu and Satter 1998). Because about 70 percent of the nitrogen and phosphorus in feeds is excreted, routine overfeeding of nitrogen and phosphorus animals contributes to nutrient surpluses on farms (Isserman 1990; Morse et al. 1992; Wadman et al. 1987). It is common to supplement poultry and pig feed with mineral forms of phosphorus because of the low digestibility of phytin, the major phosphorus compound in grain. This supplementation contributes to phosphorus enrichment of poultry manures and litters. Enzyme additives for livestock feed that will increase the efficiency of uptake from grain during digestion are now being tested. Development of such enzymes would be cost-effective in terms of livestock weight gain and it is hoped that lowering mineral phosphorus supplementation of feed would reduce the phosphorus content of manure. One example is the use of phytase, an enzyme that allows the digestive systems of chickens and hogs to absorb phosphorus from grains. Ertl et al. (1998) showed a 23 percent reduction in excretion of phosphorus by poultry fed “low-phytic acid” grain compared to those fed normal corn grain. Another approach to balance farm phosphorus inputs and outputs is to increase the quantity of phosphorus in corn that is available to poultry and pigs. Corn can be genetically engineered to decrease unavailable phytate-phosphorus, which contributes as much as 85 percent of phosphorus in corn grain. Ertl et al. (1998) manipulated the genes controlling phytate formation in corn and showed that phytate-phosphorus concentrations in “low-phytic acid” corn grain were as much as 51 percent less than in normal grain. There was a 23 percent reduction in excretion of phosphorus by poultry fed the “low-phytic acid” grain compared to those fed the “wild type” corn grain. Thus, the use of low-phytate corn in poultry and pig feed can increase the assimilation of phosphorus and other phytate-bound minerals and proteins. Reducing Off-Farm Inputs of Nitrogen and Phosphorus The accumulation of nutrients on many animal feeding operations where on-farm crop production is supplemented by feed inputs is generally not as great as in other operations where the animals depend primarily on off-farm feed. The distinguishing feature among these animal operations is the breakdown between the amount of crops produced on a
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution farm (and the potential nutrient utilization by those crops) and the animal numbers on the farm, because the manure applied to crop fields will include both nutrients coming from those fields and from imports of off-farm feeds. The application of imported nutrients to crop fields can compensate for those lost in marketed products and manure handling operations and help to sustain the farm, but the additional nutrients can also be the source of excess nutrient loading. Nevertheless, restricting feed purchases to maintain the balance of nutrients can limit herd size and economic return if all manure from the herd must be applied to the farm cropland (Westphal et al. 1989). Improving Nutrient-Use Efficiency Management practices that improve nutrient-use efficiency are vital for minimizing losses to ground and surface waters. Specific best management practices (BMPs) for nitrogen and phosphorus vary from region to region due to large-scale differences such as climate, geology, depth to water, and irrigation or drainage practices, and also due to small-scale differences such as soils, cropping systems, and past field history. Therefore, BMPs for nitrogen and phosphorus will necessarily be site-specific and should be prescribed by a technical advisor who has a good knowledge of local nutrient cycles. This task often is assigned to state and federal extension agencies or soil conservation agencies. Nitrogen Nitrogen sources and reduction control strategies for Mississippi River Basin inputs to the Gulf of Mexico have been outlined by the National Oceanic and Atmospheric Administration (NOAA 1999b). Even though the major inputs of nitrogen and phosphorus to agricultural systems in this region are via fertilizer rather than manure, the principles of increasing nutrient-use efficiency are appropriate to other regions of the United States. Loadings to surface waters in the Upper Mississippi River and Ohio River sub-basins occur primarily by infiltration of water beyond the crop rooting zone into deeper soil layers, where it is collected by subsurface tile drains. In other basins, the primary pathway for nitrate loading to surface waters is groundwater seepage and irrigation return flow. Reduction of nitrate loading to surface waters in the Mississippi River basin can be achieved by reducing nitrate sources and controlling drainage (NOAA 1999b). Although the selection of BMPs for nitrogen must depend on the specific hydrologic setting, field, and source of nitrogen, there are some basic nitrogen management principles that apply if the goal is to mini-
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution mize nitrogen losses to ground or surface water. The most fundamental principle is to supply only the nitrogen needed to meet the needs of the next crop, and to apply it in synchrony with crop use. Steps in applying this principle include: estimating the nitrogen requirement of the next crop (expected yield); evaluating nitrogen available from native sources (soil nitrogen mineralization, residual soil nitrate, irrigation water, etc.); subtracting the available native nitrogen from the crop nitrogen requirement to estimate supplemental nitrogen needs; determining the most appropriate source of supplemental nitrogen (manure, fertilizer, crop residues); and determining the most efficient and practical management practice for the specific source of supplemental nitrogen (rate, time, and placement of the nitrogen). Crop nitrogen requirement Selection of the expected yield goal is one of the most critical BMPs for nitrogen, because most fertilizer and/or manure application rates are based directly on anticipated yield. Several studies have shown that farmers, or those advising them, often have unrealistic yield expectations and that resultant over-fertilization with nitrogen can be directly related to long-term increases in groundwater nitrate. The most direct way to integrate overall site-specific factors is to calculate the average yield of the specific soil-crop system over the past 3 to 5 years. One can then adjust the average yield for unusual conditions (eliminating unusually wet or dry years), for current conditions (stored soil moisture, planting date, tillage practices, etc.), or for new technologies (new varieties, new irrigation, etc.), and then calculate a final estimate of expected yield. In any case, it is important to base the estimated yield on “real world conditions” (i.e., actual field yields) to avoid excess nitrogen applications. Native nitrogen availability The second step is to evaluate nitrogen available from native sources (i.e., sources that are not directly manageable by the farmer). These sources include nitrogen present in the root-zone as inorganic nitrate, nitrogen released through organic matter decomposition (mineralized nitrogen), nitrogen contributed through water sources (irrigation), and nitrogen from atmospheric inputs. The most recent tools for including
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution native sources of nitrogen are the pre-sidedress nitrate test and the leaf chlorophyll meter (Magdoff et al. 1984; Meisinger et al. 1992; Schepers et al. 1992). The chlorophyll meter essentially measures the “greenness” of a specific leaf, from which the need (or lack of need) for more fertilizer nitrogen or potential for over fertilization can be estimated. Management of nitrogen source (rate, placement, and timing) The above steps produce an estimate of the appropriate nitrogen rate for a realistic yield of the next crop, which is the basic principle behind efficient nitrogen use. The final step is to manage the selected nitrogen source in a manner to supply nitrogen in phase or balance with crop demand. For fertilizer nitrogen, this is a relatively easy task because it can be applied just before the period of rapid crop nitrogen uptake. Applying nitrogen when needed does not itself ensure adequate control of losses. For instance, one study showed that losses from unfertilized cereal crops were not much less than losses where fertilizer had been applied at the economic optimum input, with both resulting in nitrate concentrations in excess of the European Community limit of 10 mg nitrate-nitrogen l−1 (Sharpley and Lord 1998). This is because nitrate leached during winter is largely derived from that mineralized by the soil during late summer and autumn, when crop uptake is small especially in arable systems. This mineralization is affected little by fertilizer inputs. It is only when inputs exceed crop uptake capacity (usually close to the economic optimum) that excess fertilizer nitrogen contributes directly to losses. The economic optimum application of nitrogen for most crops exceeds offtake, resulting in a small positive balance. Within grazing systems, nitrogen surpluses are often a large proportion of the total fertilizer input, because most of the nitrogen consumed in grazing is redeposited as urine and dung. This nitrogen is not efficiently recycled because some of it is not immediately available and it is deposited unevenly over the field. Thus for nitrogen, enforcing a balance of inputs against removals could seriously reduce productivity, cause significant economic penalties, and would not in itself solve the nitrate problem. Specific improvements in management may include: (1) reducing rates of nitrogen fertilizer by following fertilizer guidelines developed by land-grant universities, (2) switching from fall to spring or split applications, (3) changing the form of fertilizer nitrogen from anhydrous ammonia to slow-release urea fertilizers, (4) switching from broadcast to banded or incorporated application methods, (5) calibrating fertilizer application equipment, and (6) applying nitrification inhibitors (CENR 1997).
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution Phosphorus The long-term use of commercial fertilizers and manures has increased the phosphorus status of many agricultural soils to optimum or excessive levels. This, of course, was the intended goal of phosphorus fertilization, to remove soil phosphorus supply as a limitation to agricultural productivity. However, for many years actions taken to achieve this goal did not consider the environmental consequences of phosphorus loss from soil to water. The constraint on phosphorus buildup in soils from commercial fertilizer use was usually economic, with most farmers recognizing that soil tests for phosphorus were an accurate indicator of when to stop applying fertilizer phosphorus. Some “insurance” fertilization has always occurred, particularly in high value crops, such as vegetables, tobacco, and sugar cane. However, the use of commercial fertilizers alone would not be expected to grossly over fertilize soils because farmers would cease applying fertilizer phosphorus when it became unprofitable. Today’s concerns with phosphorus are caused by the realization that soils that are considered “optimum” in soil test phosphorus (or perhaps only slightly over fertilized) from a crop production perspective may still provide environmentally significant quantities of phosphorus in surface runoff and erosion. Basing manure application on estimates of soil phosphorus and crop removal of phosphorus can reduce the buildup of soil phosphorus but can present several technical and economic problems to many farmers. A soil test phosphorus-based strategy could eliminate much of the land area with a history of continual manure application from further manure additions, as several years are required for significant depletion of high soil phosphorus levels. This would force farmers to identify larger areas of land to use the generated manure, further exacerbating the problem of local land area limitations. In addition, farmers relying on manure to supply most of their crop nitrogen requirements may be forced to buy fertilizer nitrogen to supplement foregone manure nitrogen. As phosphorus is relatively immobile in soil compared to nitrogen, timing of application is less critical in BMP development for phosphorus than nitrogen. However, methods of phosphorus application are important. Rotational applications of phosphorus designed to streamline fertilizer operations may leave large amounts of available phosphorus in the surface, and should be avoided in areas of the landscape at risk of erosion or surface runoff. Efficient management of phosphorus amendments to soils susceptible to phosphorus loss involves the subsurface placement of fertilizer and manure away from the zone of removal in surface runoff, and the periodic plowing of no-till soils to redistribute surface phosphorus accumulations throughout the root zone. Both practices may indirectly
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution reduce the loss of nitrogen and phosphorus by increasing crop uptake and yield, which affords a greater vegetative protection of surface soil from erosion. However, these measures are often unrealistic for a farmer to implement. For example, subsurface injection or incorporation in rocky soils may be difficult, and without manure storage, farmers who contract out the cleaning of poultry houses will have little flexibility for when manure or litter is applied. Manure Management As discussed in Chapter 5, animal wastes are a major part of the nutrient over-enrichment problem, and management efforts are complicated by the long distances that feedstocks are transported. Managing nutrients from manure is often more difficult than from fertilizer, due to uncertainties in initial composition (e.g., ration, animal age, etc.), losses during storage or handling (e.g., ammonia volatilization), uncertainties of application rates (e.g., uncalibrated spreaders, uneven applications), difficulty in spreading manure to a growing crop without causing crop damage, greater gaseous nitrogen losses with manure after application, and time pressures producers face because of weather uncertainties. It is also important for manure management to know the approximate decomposition rate of the organic nitrogen, so as to minimize nitrogen loss in groundwater. This is generally estimated as a decay series for the particular type of manure. An example of a decay series for solid beef manure would be 40 percent mineralized the first year, 25 percent of the remaining nitrogen the next year, 6 percent the next year, and so on (Gilbertson et al. 1979). The last step is to calibrate the manure spreader. Obviously it does little good to know the crop nitrogen need, the manure composition, the likely ammonia loss, and the decomposition rate, if one cannot apply the calculated rate of manure accurately. Manure spreader calibration programs in Maryland and Pennsylvania frequently find that farmers are applying two to five times more manure than they originally estimate. Educational materials for spreader calibration can significantly improve manure nitrogen utilization, and further improvements could be obtained with monitoring or incentives. Farm advisors and resource planners now recommend testing manure for nitrogen and phosphorus, and soils for phosphorus, prior to land application of manure. However, nitrogen-based manure management plans are still based on crop needs. Without these determinations, farmers and their advisors can underestimate the fertilizer value of manure. Soil test results can also demonstrate the positive and negative long-term effects of manure use and the time required to build-up or deplete soil
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution nutrients. For instance, they can help a farmer identify the soils in need of fertilization, those where moderate manure applications may be made, and those fields already containing excess nitrogen and phosphorus where manure should not be applied. Commercially available manure amendments, such as slaked lime or alum, can help in manure management. Such amendments can decrease ammonia volatilization, which can significantly affect export to estuaries (Chapter 5), and at the same time lead to improved animal health and weight gains. Amendments can also decrease the solubility of phosphorus in poultry litter by several orders of magnitude and decrease dissolved phosphorus, metal, and hormone concentrations in surface runoff at least 10 fold (Moore and Miller 1994; Moore et al. 1995; Shreve et al. 1995; Nichols et al. 1997). Perhaps the most important benefit of manure amendments for both air and water quality would be an increase in the nitrogen:phosphorus ratio of manure, by reducing nitrogen loss because of ammonia volatilization. An increased nitrogen:phosphorus ratio of manure would more closely match crop nitrogen and phosphorus requirements. One approach to better manure management would be to establish a mechanism to facilitate movement of manures from surplus to deficit areas. At present, manures are rarely transported more than 10 miles from where they are produced. But mandatory transport of manure from farms with surplus nutrients to neighboring farms where nutrients are needed would face several significant obstacles. First, it must be shown that manure-rich farms are unsuitable for manure application, based on soil properties, crop nutrient requirements, hydrology, actual nutrient movement, and proximity of sensitive water resources. Second, it must be shown that the recipient farms are more suitable for manure application. The greatest success with re-distribution of manure nutrients is likely to occur when the general goals of nutrient management set by a national (or state) government are supported by consumers, local governments, the farm community, and the livestock industry involved. This may initially require incentives to facilitate subsequent transport of manures from one area to another. This may be a short-term alternative if nitrogen-based management is used to apply the transported manures. If this happens, soil phosphorus in areas receiving manures eventually may become “excessive.” To date, however, large-scale transportation of manure from producing to non-manure producing areas is not occurring. The main reasons for this are the high transportation costs and concern that avian diseases will be transferred from one farm (or region) to the next. Consequently, there is a need to develop a means to ensure the biosecurity of any manure transportation network that is developed, and in general to seek ways to over-
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution decrease (EPA 1983) (Table 9-8). Extended detention facilities dry out between storms and are not effective for removal of dissolved nutrients by biological mechanisms. However, such facilities can sometimes serve multi-purposes as recreational areas when dry. Design guidelines for storage and other passive treatment control devices are provided in references such as Schueler (1987), Camp et al. (1993), Urbonas and Stahre (1993), and American Society of Civil Engineers and Water Environment Federation (1998). Additional information may be found in Novotny and Olem (1994), EPA (1993b), Debo and Reese (1995), and Novotny (1995). Caution should be exercised when comparing urban BMPs on the basis of removal efficiencies. It is often found that while influent quality varies considerably, effluent quality exhibits a much smaller range (Strecker et al. 1999). Thus, high removal efficiencies may result purely from the fact that influent concentrations are high. Hence, BMPs might be better characterized simply by effluent quality. Alternatives for determining urban BMP performance effectiveness, based on a review of the most complete data set currently available, are provided by Strecker et al. (1999). Natural wetlands are protected waters by law in the United States and generally cannot be used for waste treatment, except sometimes for a “polishing” purpose. Nonetheless, they provide many functions that enhance water quality (NRC 1991b), including acting as a sink for phosphorus, and facilitating denitrification by converting nitrate to nitrogen gas. Their capacity for nutrient removal can be considerable (Mitsch and Gosselink 1986). Hence, protection of coastal wetlands and tidal exchange is an important water quality consideration for coastal waters. Many factors can act to impair the natural functions of wetlands, including the drainage of wetlands for additional cropland, overgrazing, construction of highways, channelization of an adjoining waterway, deposition of dredged material, and excavation for ports and marinas (EPA 1993a). TABLE 9-8 Type of Pond Total Suspended Sediments Nitrogen Phosphorus Extended detention basins 70-80 0 (dissolved) 20-30 (total) 0 (dissolved) 20-50 (total) Retention ponds 70-80 50-70 (dissolved) 30-40 (total) 50-70 (dissolved) 50-60 (total) TABLE 9-8 Comparison of nutrient removal percentages from well-designed extended detention basins and retention ponds (EPA 1983).
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution Measures for protection include: acquisition, protective zoning, application of water quality standards to wetlands, education and training, inclusion in comprehensive watershed planning, and restoration. Restoration measures include: maintenance of a natural hydrologic regime, restoration of native plant species, reduction of nonpoint source or other pollutant inflows, and maintenance of historic wetlands sites (EPA 1993a). Constructed wetlands function similarly to storage devices, and their nutrient removal effectiveness depends upon the characteristics of the inflow as well as hydraulic properties (e.g., avoidance of short-circuiting) and vegetation types (Schueler 1992; Strecker et al. 1992; Urbonas and Stahre 1993; Strecker 1996). Schueler (1992) indicates projected removal rates for total phosphorus and total nitrogen on the order of 40 to 60 percent and 20 to 30 percent, respectively. Actual monitoring of constructed wetland removal efficiencies yields a very large variability (e.g., from −4 percent to 62 percent for NH3 and −4 percent to 90 percent for total phosphorus) (Strecker et al. 1992; Urbonas and Stahre 1993; Strecker 1996). Nutrient removal efficiency depends strongly on the loading rate, percentage solid material, site conditions (such as soils), and hydraulic conditions that might lead to short-circuiting or scour—which might account for occasional negative removal efficiencies. Furthermore, Harper et al. (1988) point out that systems designed for removal of nutrients should avoid long detention times and stagnant conditions, both of which can decrease oxidation reduction potential and pH and reduce the efficiency of phosphorus removal. There is no clear advantage of constructed wetlands over storage ponds for nutrient control apart from the public appeal of wetland systems (Urbonas and Stahre 1993). Schueler (1992) presented extensive design guidelines for constructed wetlands. Maintenance is a critical concern for all stormwater management facilities. When maintenance is poor, both quantity and quality control effectiveness can be greatly diminished. It is important that operation and maintenance costs be included during the planning and design of BMPs. Robustness of a design is also a factor (ASCE and WEF 1998). High robustness implies that when all the design parameters are correctly defined and quantified, the design has a high probability of performing as intended. For instance, wet retention ponds have a high robustness for removal of particulates and solids, but only a low to moderate robustness for removal of dissolved constituents (ASCE and WEF 1998). The robustness of extended detention basins and wetlands is moderate to high for particulates. Wetlands have a low-moderate robustness for removal of dissolved constituents while extended detention has none to low. An additional number of structural controls exists for combined sewers, most of which are designed to store sewage during a storm for eventual treatment at the treatment plant or to divert only the cleanest
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution water to receiving waters during a storm event (WPCF 1989; Moffa 1990; EPA 1992, 1993b). The use of structural controls for combined sewer overflow management often involves significant costs when applied in dense, highly developed, older urban areas. Most urban BMPs are designed to control parameters other than just nutrients, such as heavy metals, solids, and oxygen demanding constituents. Hence, their design will also be predicated upon removal of these other constituents, as well as for management of stormwater quantity. A large number of nonstructural BMPs are available (ASCE and WEF 1998). In general, nonstructural BMPs emphasize source controls and “good housekeeping”. Many such options are routinely implemented as part of stormwater master plans in cities, although their effectiveness has not generally been quantified, especially for nutrient control. Examples include: public education; use of alternative products; vehicle use reduction; storm drain system signs (e.g., “dump no waste, drains to stream”); spillage control; control of illicit connections to storm sewers; street cleaning and catchbasin cleaning; general maintenance; control of leaking sanitary sewers; and land use controls. Overall, post-construction monitoring data is lacking so there is little concrete evidence of the effectiveness of urban BMPs. Management practices are often implemented under the assumption that they will be effective in reducing the load of targeted pollutants, without any follow up on how well they actually perform. Hence, good design information is lacking. In response to the need to assemble and evaluate available effectiveness data, the American Society of Civil Engineers is conducting such a study for EPA (Strecker et al. 1999), due to be completed sometime in 2000 (Box 9-1). When finished, this study will provide a definitive statement about the effectiveness of urban BMPs. OTHER MITIGATION OPTIONS Regional Stormwater Control Facilities Location as well as type of BMPs play an important role in control of nonpoint source runoff. For example, stormwater and combined sewer
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution BOX 9-1 National Stormwater Best Management Practices Database In response to the need for a centralized, easy-to-use, scientifically-sound tool for assessing the appropriateness of stormwater runoff BMPs under various conditions, the Urban Water Resources Research Council of the American Society of Civil Engineers has entered into a cooperative agreement with the EPA to develop a National Stormwater BMP Database (http://www.bmpdatabase.org). The long-term goal of the project is to promote technical design improvements for BMPs and to better match their selection and design to the local stormwater problems being addressed. The database, which was released in late 1999, contains data from BMP evaluations conducted over the past 15 years (ASCE and EPA 1999). Database updates will be made available as additional BMP evaluation data are gathered. overflow controls can be distributed at critical points throughout the watershed, such as at discharge locations. Another option is to collect nutrient-laden stormwater and control/treat it at a more centralized or downstream location. Such a regional facility may have advantages such as lower capital costs, reduced maintenance, and greater reliability (Stutler et al. 1995). Alternatively, a regional collection system may be better from the standpoint of the location of the receiving water discharge. For instance, the City of San Francisco collects combined sewage in large storage tanks placed along the Bay-side of the city. The combined sewage is then pumped across the dividing hills for treatment and discharge into the Pacific Ocean in the southwest corner of the city. In this way, combined sewer overflow loadings to San Francisco Bay are avoided, except for very high storm events. Hydrologic/Hydraulic Alterations in the Watershed The history of human development is one of encroachment upon wetlands and waterways, and loss of wetlands in coastal areas is substantial. An outstanding example is the Kissimmee River system of central Florida, which drains to Lake Okeechobee and ultimately, through the Everglades system to Florida Bay. Although not a coastal system, Lake Okeechobee has reacted to loss of upstream wetlands and attendant nutrient filtering with massive eutrophication problems since flood control facilities that straighten, narrow, and reduce the length of the river were built in the 1960s (Koebel 1995; SFWMD 1998; Koebel et al. 1999). The South Florida Water Management District is now working with the Corps
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution of Engineers to restore wetlands for purposes of phosphorus removal from the predominant inflow, the Kissimmee River. Restoration of the wetlands themselves, as well as enhancement of hydraulic connections, can provide a nutrient removal function that helps mitigate the overall trend of increased loadings from tributary watersheds and growing urban areas. Restoration of non-tidal wetlands as a management strategy has received much attention for the Mississippi Basin (NOAA 1999b), and is also a part of the Chesapeake Bay Program strategy. Circulation Enhancement Coastal developments, bridges, jetties, causeways, breakwaters, and flood control structures often lead to altered circulation patterns in an estuary. Reduced ocean exchange leads to lack of flushing, longer residence times, and more time for eutrophication processes to proceed. An extreme example is finger canals (Figure 9-4) that support extensive residential development, a pattern that has now been prohibited in most states. Finger canals lead to dead-ends and stagnant water, ripe for eutrophication and other pollution hazards. In many cases, circulation can be enhanced by tidal pumping promoted through new connectivity via culverts, pipes, and bridges. Small differences in tidal amplitude at multiple outlets, coupled with frictional resistance that depends on the flow direction, can create a net circulation through a looped system. Hence, where reduced flushing and increased residence time can contribute significantly to eutrophication problems, engineering alternatives may exist for mitigation through hydraulic controls. General guidance on flushing characteristics of estuaries and implications for mixing, residence times, and eutrophication potential can be found in references such as Officer (1976), McDowell and O’Connor (1977), Fischer et al. (1979), and Kjerfve (1988). In Situ Biological Treatment Options Eutrophication and other effects of nutrient over-enrichment in a waterbody are affected in part by the grazing activity of animals—both zooplankton and benthic filter feeders—on phytoplankton. The abundance of zooplankton that feed on phytoplankton is in part regulated by the abundance of zooplanktivorous fish, and these in turn are regulated in part by the abundance of higher predators (Carpenter et al. 1985). To some extent, nutrient problems in lakes can be managed by managing the populations of predatory fish, with the effect cascading down to zooplanktivorous fish, zooplankton, and then phytoplankton. The same principles apply to estuaries and coastal waters (Ingrid et al. 1996). However,
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution FIGURE 9-4 Finger canals in the Port Charlotte, Florida area, in 1974. Systems of canals with dead-ends have reduced circulation and flushing and can lead to accumulation of pollutants and nutrients, with resultant eutrophication problems (photo by W. Huber). whereas lakes can be considered relatively closed systems with regard to fish populations, fish readily migrate between estuaries and coastal waters. While fishery practices in coastal areas may have impacts on nutrient enrichment in coastal areas, it would be exceedingly difficult to manage this through manipulations of fishery populations. Benthic filter feeders such as oysters, mussels, and many species of clams can have a major influence on phytoplankton populations in coastal waters (Lucas et al. 1997; Meeuwig et al. 1998). In fact, it has been suggested that eutrophication of Chesapeake Bay is due in part to loss of oyster populations there: when oyster populations were high in the bay, they may have filtered the water as frequently as once per day on average, which would have been a significant control on phytoplankton abundance. Currently, oysters are believed to filter the water of Chesapeake Bay on average only once per year (Newell 1988). The data behind this
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution speculation are sparse, but the logic is sound, and such grazing activity currently is a major factor regulating primary production in San Francisco Bay (Lucas et al. 1998) and in many small estuaries in Prince Edward Island, Canada (Meeuwig et al. 1998). Manipulation of benthic filter feeding organisms deserves further study as a possible mechanism for partially managing nutrient loads in estuaries. Control of Bloom Incidence through Nutrient Reductions Options for control of harmful algal blooms (HABs) include prevention, control, and mitigation (Boesch et al. 1997) (Box 9-2). HAB algae, just like all plants, require certain major and minor nutrients for their nutrition. These can be supplied either naturally or through human activities, such as pollution. As described in Chapter 4, a strong case has been made in several areas of the world that increases in pollution are linked to increases in the frequency and abundance of red tides (e.g., Smayda 1990; Okaichi 1997). It follows that a reduction in pollution can sometimes lead to a decrease in HAB frequency or magnitude. It should be emphasized, however, that it is exceedingly difficult to predict with any certainty what the effect of pollution control strategies will be on HAB incidence, except in situations where the pollution loading is massive (e.g., in Tolo Harbour or the Inland Sea of Japan [Box 4-2]) where it is now clear that increasing pollution was associated with increasing algal biomass, and therefore with more red tides/HABs. Given the high pollution loads to many estuaries and coastal waters, there is little doubt that these inputs contribute to some of the harmful blooms that occur. What is not clear, however, is the nature of that linkage—how much, and in what specific ways, the pollution must change before the number of HABs will decrease to “acceptable” levels. What is needed from a management perspective is the development of quantitative relationships between nutrient loading parameters and HAB incidence, such as the relationship between nitrogen:phosphorus ratios and dinoflagellate abundance described by Hodgkiss and Ho (1997) for Tolo Harbour, Hong Kong. However, the validity of such a relationship needs to be evaluated more thoroughly and if found to be robust, expanded to include other watersheds and hydrographic systems before it can be used to justify major policy decisions on water quality options in any particular region. In general, the argument can be made that to reduce HAB incidence in an area, strict pollution control regulations should be instituted. However, a reduction in pollution loading will not lead to a complete absence of red tides/HABs—it is likely to reduce red tides in general, but some toxic species that thrive in relatively clean waters may find the new conditions suitable for growth. Given these uncertainties, it is difficult to jus-
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution BOX 9-2 Can Harmful Algal Blooms be Controlled with a Natural Parasite? Could naturally occurring parasites be tapped as a biological tool to help control harmful algal blooms? New evidence is emerging that this might be possible (Delgado 1999). During one red tide covered by a paralytic shellfish poison-producing dinoflagellate called Alexandreum catenella in Catalonia, Spain, scientists noticed that a decline in the bloom corresponded with the presence of unknown round cysts. Further work showed that these cysts infected the A. catenella cells. They named the new organisms “diablillo parasites” (a Spanish word related to devil) and conducted further research to understand how they attacked the algae. The new diablillo parasite develops rapidly, so hundreds of new parasites per infected host can grow in about the 48 hours it takes for the host to reach maturity. Additional work continues to see if the parasite could be used to control harmful algal blooms in the natural environment. One feature of the parasite is that it only infects motile cells, and not the quiet cell stages of the hosts, and this could limit its ability to totally destroy problem algae populations in natural settings. Scientists continue to work to culture the parasite on non-toxic and widespread dinoflagellate hosts, which might facilitate its use in different geographic areas. tify major pollution reduction programs solely on the basis of an expected reduction in HABs. Instead, reductions of nutrient inputs into coastal waters should be rigorously pursued as a key element of general estuarine and coastal management. The potential benefits of reductions of nutrient loadings in terms of decreased frequency and severity of HABs should be one of several considerations driving pollution policy decisions in estuarine and coastal management programs, but it should not be the sole justification. Marketable Permits Prior to 1990, marketable permits played only a minor role in environmental policy design, but the 1990 Clean Air Act Amendments promoted the use of marketable permits for sulfur dioxide emissions and provided an impetus for increased attention to this policy instrument. There are now a number of trading programs in place. As noted earlier, EPA advocated the use of a tradable permit system for reductions in NOx emissions. In addition, several watersheds are experimenting with trading programs as a means of meeting water quality goals at lower cost. For example, the State of Connecticut estimates that trading will reduce the
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution state’s cost of meeting its target level of nitrogen removal by more than $200 million (CDEP 1998). Although tradable permit systems have the potential to reduce the costs of achieving water quality goals, they have drawbacks that could limit their use. These include the administrative burden of operating a trading market (if it is publicly run) and the difficulty of establishing appropriate trading ratios when the environmental impact of a given discharge level varies by site. NEXT STEPS FOR SOURCE REDUCTION Much research has been conducted seeking agricultural, forest, and urban management practices that reduce the potential for nonpoint nutrient export. Yet we have not been successful at implementing cost-effective remedial measures in certain critical areas such as animal waste management. In most cases we know how to minimize nutrient export and input to coastal waters (the science is there), so that the major barriers to implementation now involve overcoming economic constraints, societal pressures, and political forces. New mechanisms to encourage implementation of BMPs, and remedial strategies in general, are necessary. A critical component for facilitating widespread BMP implementation is by funding of cost-share programs and development of alliances among stakeholders. Stakeholder alliances encourage collaborative rather than adversarial relationships among affected groups. There is a considerable scientific basis for reduction of nutrient releases from agriculture sources (e.g., enzyme adjustments for poultry and hogs to promote efficiency of phosphorus uptake; genetically-engineered corn to reduce unavailable phosphorus content). But there is a tendency on the part of farmers to over feed and urban dwellers to over fertilize—that is, to provide more nutrient supplements than are scientifically justified. Agricultural practices that reduce nutrient export must continually be communicated to end users in an effort to overcome the intuitive but false premise that “more is better.” Many approaches can help people manage fertilizer application. The most fundamental principle is to supply only the nitrogen and phosphorus needed to meet the needs of the current crop, and to apply them in synchrony with crop use. Manure generated from confined animal feeding operations has a significant potential to discharge nutrients to receiving waters. Many options are available to mitigate this source. Entrepreneurial activities should be encouraged to take advantage of management practices that require more than just field-scale activities (e.g., transport of manure from one location to another). Methods for managing nitrogen and phosphorus transport may differ
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution because nitrogen losses can occur from any location in a watershed via subsurface pathways, while phosphorus loss occurs most often in areas prone to surface runoff. Hence, remedial strategies for nitrogen may be applied to the whole watershed, whereas the most effective phosphorus strategy would be a combination of stringent measures at the most vulnerable sites to minimize loss of phosphorus in surface runoff with simple measures over the whole watershed to avoid excessive nutrient buildup, and thereby limit losses in subsurface flow. Extreme events must also be considered when designing phosphorus management strategies because one or two extremely high flows may be responsible for the bulk of annual phosphorus export. There is a serious lack of post-implementation monitoring to assess the effectiveness and long-term viability of BMPs. Monitoring programs should be established to determine the long-term effectiveness of BMPs on nonpoint nitrogen and phosphorus reduction. A database of effective measures and design parameters should be maintained by appropriate federal agencies (e.g., EPA for urban areas and U.S. Department of Agriculture [USDA] for agricultural areas). The cost of implementing control measures is an important planning consideration, and databases of BMP effectiveness should also include relevant cost data. Economic data is also needed to help determine the economic benefits of management strategies and to see implementation costs in relation to the relative costs of different problems caused by nutrient over-enrichment. The Clean Air Act may be as important as the Clean Water Act in protecting the nation’s coastal waters from nitrogen pollution. Air pollution policy for nitrogen control is driven mainly by concerns other than nutrient over-enrichment (e.g., smog and human health, ozone, acid rain, global warming), but addressing these concerns can yield some nutrient-related benefits. Thus the effects of nitrogen on coastal waters should be considered in the formulation of air pollution policy. While eutrophication reduction is an additional benefit of air pollution control, policymakers will need to recognize that year-round emission control is necessary to affect eutrophication, not just summer controls as are used to combat smog. Also, the full range of nitrogen emissions need attention, not just NOx, and thus ammonia-based scrubbers are inappropriate. Although biologically-based secondary treatment of municipal wastewater is practiced at many U.S. cities and has some indirect nutrient benefits, point source discharges from publicly owned treatment works can still constitute a significant source of nutrients to coastal waters. In general, nutrient reduction strategies should address the least cost solutions first. Advanced waste treatment options of point sources for additional nutrient removal are often cheaper (on the basis of dollars per kilogram of nitrogen or phosphorus removed) than is control of nonpoint sources and
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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution should be examined carefully when planning strategies for nutrient reduction. The larger the tributary area to the coastal waters, the more important is source control in the control of coastal nutrient over-enrichment. However, managers have options beyond source control. For coastal waters with smaller tributary areas, mitigation strategies such as enhancement of coastal wetlands might be a possibility. Most nutrient management schemes rely on a combination of measures. In all cases, maintenance of natural systems, including water column biota and shellfish, is important. Economic incentives, such as tradable permits, have potential to be used to facilitate the design of comprehensive cost-effective management strategies.
Representative terms from entire chapter: