Nutrient Management for Water Quality Protection : A Case Study of Delaware

Transcription

1 Scientific registration n o : 1947 Symposium n o : 25 Presentation: Poster Nutrient Management for Water Quality Protection : A Case Study of Delaware's Inland Bays Watershed Gestion des nutriments pour une protection de la qualité des eaux. Etude de cas sur un bassin versant de l île Delaware (Etats Unis) SIMS J. Thomas (1), PRICE Kent S. (2) (1) Dep. Plant and Soil Sciences, Univ. of Delaware, Newark, DE , USA (2) College of Marine Studies, Univ. of Delaware, Lewes, DE, 19958, USA Introduction: Delaware, located on the Delmarva Peninsula (Delaware-Maryland-Virginia) in the Atlantic Coastal Plain of the United States, is bordered by two large and extremely important estuaries, the Chesapeake and Delaware Bays (Figure 1a). It is also the location of a national estuary, the Inland Bays (Figure 1b: Rehoboth Bay, Indian River Bay, and Little Assawoman Bay). Water quality in all these estuaries has been impaired by years of point and nonpoint source pollution, particularly by inputs of nitrogen (N) and phosphorus (P). While point source discharges of nutrients have been reduced, due to improvements in municipal wastewater treatment facilities, agricultural contributions of N and P via nonpoint source pollution (runoff and ground water discharge) remain substantial. For example, from 44-55% of the N and from 29-53% of the P entering Delaware s Inland Bays was attributed to agriculture by Ritter (1992). Point sources contributed less than 12% of the N and from 0 to 25% of the P. The importance of the Inland Bays for fishing, recreation, tourism, and as a vital estuarine ecosystem has stimulated numerous research and educational programs over the past 25 years. Many have focused on the development and implementation of «best management practices» (BMPs) that could be used to minimize the impact of agricultural nutrients on water quality. Despite these efforts little improvement has been observed in water quality in the Inland Bays, prompting increased public concern and demands for greater restrictions on agriculture. Most recently, questions have arisen about the impact of the intensive animal agriculture practiced on this peninsula on ground and surface water quality. More than 700 million broiler chickens are produced on Delmarva each year and land application of the wastes from this industry has been identified as a potentially major source of N and P to the Chesapeake Bay and to Delaware s Inland Bays. Our objectives are to describe the water quality problems in the Inland Bays watershed, review the challenges faced by agriculture in reducing nonpoint source pollution, and outline some immediate and long-term solutions that should be considered if we are to restore the health of the Inland Bays. 1

2 Figure 1. Maps of Delaware and the Delmarva Peninsula, illustrating (a) the proximity of the state and peninsula to the Chesapeake and Delaware Bays, and (b) the location of the Inland Bays National Estuary. 2

3 Delaware s Inland Bays Estuary and Watershed: Delaware s Inland Bays consist of three small, shallow, interconnected water bodies with a total surface area of ~80 km 2. Rehoboth and Indian River bays are similar in size (~38 km 2 ) and depth (< 2 m), while Little Assawoman Bay has a total surface area of only 9 km 2 and a mean depth of 1.5 m. The bays are surrounded by tidal marshes, forests and agricultural cropland and receive fresh waters from numerous small tributaries and from direct ground water discharge. Salt water enters from the Atlantic Ocean through several natural inlets and engineered canals. Tidal flushing of the bays occurs about every days for Rehoboth Bay and every 80 days for Indian River Bay; flushing times are not available for Little Assawoman Bay. More than 75 km 2 of the bays are classified as shellfish waters but about 30% of these waters are not approved for shellfish harvesting due to high coliform bacteria levels. The Inland Bays drain a watershed with a total land area of ~800 km 2, divided into 16 drainage basins within three main sub-watersheds (Figure 1b). Land in the watershed is primarily used for agriculture (43%), followed by forests (39%), urban areas (11%), and wetlands and water bodies (6%). The population density is ~50 people per km 2, projected to grow by 30% in the next decade as urbanization of the watershed continues. The importance of tourism to the Inland Bays watershed is illustrated by the fact that over five million people per year visit the beaches adjacent to the bays, contributing more than $250 million annually to the local economy. Coordinated efforts to identify and control water quality problems in the Inland Bays began nearly 30 years ago, in In a task force appointed by Delaware s Governor identified 15 key environmental issues that needed to be addressed if the health of this ecosystem was to be restored and suggested that the five most important were eutrophication, nitrate contamination of ground waters, bacterial contamination of shellfish, adverse effects of dredging, and saltwater intrusion. From a coordinated effort between local, state, and federal agencies was conducted to address many of these problems. Examples include identification of the Inland Bays as an ERES (Exceptional Recreational and Ecologically Significant) water body and later as a national estuary, a five year HUA (Hydrologic Unit Area) project conducted by the Cooperative Extension program to educate the agricultural and rural communities about nonpoint source pollution, and the WE C.A.R.E. (Comprehensive Agricultural Resources Effort) water quality and nutrient management planning program. Most of these efforts culminated in a six year effort by the Inland Bays Management Conference, a collaborative body of concerned citizens, to prepare the Comprehensive Conservation and Management Plan (CCMP) for the Inland Bays, which was approved in The priority areas and proposed solutions identified in the CCMP are summarized in Table 1. Closely related to the CCMP was the establishment in 1994, by the Delaware state legislature, of the Center for the Inland Bays. The Center is a nonprofit organization with a dual role: (i) to oversee and facilitate the implementation of the CCMP and, (ii) to develop a long-term approach for the use and enhancement of the watershed. The activities of the Center are led by a Board of Directors acting upon the advice of a Citizens Advisory Committee (CAC) and a Scientific and Technical Advisory Committee. 3

4 Table 1. Priority environmental problems and suggested environmental action plans from the Inland Bays Comprehensive Conservation and Management Plan (1995). Environmental Problems Nutrient overenrichment (Eutrophication): Caused by excessive N and P entering the bays from point and nonpoint sources. Results in excessive growth of microscopic plants and algae and in nuisance seaweed blooms. Depletions in oxygen concentrations in waters as algae and seaweed decompose and turbid waters that block sunlight penetration to the bottom of the bays negatively affect fish and benthic organisms. Agriculture has been identified as the major nonpoint source contributor of N and P to Indian River Bay and Little Assawoman Bay and of N to Rehoboth Bay; a municipal sewage treatment plant is the main source of P to Rehoboth Bay (Ritter, 1992). Habitat Loss: Eutrophication, sedimentation, bulkheading, dredging, and urbanization have drastically altered the habitats in and around the Inland Bays (waters, shorelines, uplands). Submerged aquatic vegetation (SAV) has disappeared, more than 20% of the tidal wetlands and more than half the freshwater wetlands have been destroyed. These changes have resulted in a loss of biological diversity and declines in the populations of desirable fish and shellfish. Proposed Solutions in the CCMP I. Education and Outreach Action Plan: Major efforts are underway to educate all users of the Inland Bays (boaters, homeowners, developers) on the need to protect the bays to preserve them for future generations. II. Agricultural Action Plan: General goal is to reduce the nonpoint source loading of nutrients and sediments from agriculture by better nutrient management practices and soil conservation planning. Examples include improved soil testing, better and more efficient use of animal wastes, managing and planting buffer strips. Support research on nutrient management for water quality protection. III. Industrial, Municipal, and Septic System Action Plan: Develop a pollution control strategy for point sources and septic systems. Tie future development to construction of a sewage treatment infrastructure. IV. Land-Use Action Plan: Review existing land use planning strategies and require environmentally sensitive development in key areas. V. Habitat Protection Plan: Create resource protection zones, develop ordinances for habitat protection, set limits on building near shorelines, develop comprehensive water-use plan, investigate alternatives to current dredging and bulkheading practices. Agriculture in the Inland Bays and Nonpoint Source Pollution: Arable agriculture predominates in the Inland Bays watershed where the major crops are soybeans, corn, wheat, barley, and vegetables for processing and fresh market. Agriculture is dominated by a large and extremely intensive poultry industry that produces approximately 70,000,000 broiler chickens per year in a watershed that has 28,000 ha of cropland. Put differently, the Inland Bays watershed has ~12% of the total cropland in Delaware and ~30% of the total 4

5 broiler production. Poultry production, however, is not uniformly distributed over the watershed, however, but is concentrated much more intensively in the Indian River and Little Assawoman bay sub-watersheds than in the Rehoboth Bay sub-watershed (610, 2360, and 7575 birds/ha/yr, respectively). Soil types and hydrologic conditions vary between watersheds as well. Well-drained and excessively well-drained soils predominate in the Rehoboth sub-watershed (Ultisols and Entisols) while poorly drained and very poorly drained soils (Umbraquults). characterize the Little Assawoman sub-watershed. Soils in the Indian River sub-watershed are intermediate between the two, but are primarily well-drained. Large areas of the Indian River and Little Assawoman sub-watersheds are extensively ditched to lower the water table and accelerate natural drainage; the ditches are directly connected to natural tributaries that flow to the Inland Bays. Recent studies have shown that surface water in agricultural drainage ditches can have concentrations of N and P high enough to be of environmental concern. The ground water aquifers in this watershed are shallow (< 10 m) and have been shown in numerous studies to be contaminated by nitrate-nitrogen. Ritter (1992) estimated that, in a normal rainfall year, 75% of the N and 77% of the P that entered the Inland Bays from agricultural lands came from via ground water discharge. The magnitude of the agricultural contribution of N and P to the Inland Bays has prompted a variety of studies on the specific nutrient sources and agricultural management practices that are of greatest concern. Land application of poultry litter (a mixture of poultry manure and sawdust or wood shavings) is generally regarded as a major area of importance because of the intensity of animal production in the watershed and the fact that alternatives to land application of animal wastes are not available at present. Overuse or poorly timed applications of commercial fertilizers can also be a factor, particularly in vegetable crop production where high rates of fertilizer N and P are commonly used. The situation with the poultry industry is highly problematic. Nutrient budgets constructed for individual farms or for larger areas, such as the three sub-watersheds in the Inland Bays, clearly show that large surpluses of N and P are often present on farms in this watershed (Table 2). The major cause of the surpluses is the fact that much larger quantities of N and P are imported to farms as feed than are exported in crops and animal products. Any commercial fertilizers that are purchased add to the nutrient surplus. The combination of nutrient surpluses and the absence of other options for poultry litter use besides application to agricultural crops has resulted in the build-up of P to extremely high levels in soils on poultry farms and more frequent ground water contamination by nitrate-n in areas with high animal densities. 5

6 Table 2. Estimated nutrient budgets for a poultry farm in the Inland Bays ( Sims, 1997). Nutrient Inputs or Outputs Nitrogen Phosphorus Inputs (kg/farm/yr) Animals Animal Feed Biological N Fixation Fertilizers Outputs (kg/farm/yr) Harvested crops Animals sold ,700 3,300 4,500 10,800 20, , ,500 2,500 Nutrient Balance (Inputs-Outputs, kg/ha/yr) Assumes the farm produces 330,000 chickens per year and has 75 ha of cropland. Sims (1997a) reported that 91% of the agricultural soils tested for P in Sussex County, DE (site of the Inland Bays watershed) between 1992 and 1996 were rated as «optimum» (27%) or «excessive» (64%) in P relative to crop requirements. The average soil test P value was 89 mg/kg, relative to an optimum agronomic value of 25 mg/kg (Mehlich 1 soil test). The well-known role of P in eutrophication and recent concerns that toxic aquatic dinoflagellates (e.g. Pfiesteria spp.) may threaten not only fish but human health when surface waters become nutrient enriched have emphasized the need for accelerated efforts to prevent nonpoint source pollution of the Inland Bays and their tributaries. Improving Water Quality in the Inland Bays: The Future Efforts to improve water quality in the Inland Bays and thus to restore the health of these ecosystems must have both terrestrial and aquatic components. One of the first steps is to identify the characteristics associated with a healthy estuarine ecosystem so that we will have a set of criteria by which to judge the success, or failure, of our actions. Price (1997) compared the Inland Bays with several coastal bays in Maryland as part of an Environmental Monitoring and Assessment Program (EMAP). This study evaluated five parameters which have been used to characterize ecosystem health in the Chesapeake Bay restoration efforts (critical values in parentheses, healthy ecosystems have values lower than the critical value): Light attenuation coefficient (1.5 k d /m), total suspended solids (15 mg/l), chlorophyll a (15 µg/l), dissolved inorganic N (10 µm), and dissolved inorganic P (0.67 µm). All three of Delaware s Inland Bays consistently exceeded these critical values and hence would be considered impaired ecosystems, unable to support submerged aquatic vegetation (SAV) or diverse biological communities. Only 10 and 5% of the areas sampled in the Rehoboth and Indian River Bays met all five criteria; none of the areas in the Little Assawoman Bay met all five criteria. In contrast, 45% of southeastern Chincoteague bay in Maryland met all five criteria, had higher benthic diversity, lower seaweed abundance, higher percentages of SAV and greater populations of more desirable fish, many of which are sensitive to the low oxygen concentrations commonly observed in the Inland Bays. Price (1997) suggested that, based on comparison of the Chincoteague bay and the Inland Bays, a ten-fold reduction in N and P loadings to the Inland Bays would be needed to restore the ecology of the bays. More importantly it appears that a framework is being developed by which we can relate water quality to ecosystem health and, from this, model the changes in terrestrial inputs 6

7 needed to restore the Inland Bays. Sims (1997b) reviewed the environmental challenges faced by intensive animal agriculture and suggested that it will take much more than traditional approaches to nutrient management and soil conservation to resolve environmental problems such as those in the Inland Bays watershed. Of prime importance is the need to systematically address the nutrient mass balance issue and the fate of the surplus N and P. Alternatives to land application must be developed and/or animal wastes must be moved to areas where soils are deficient in N and P. Examples include using poultry litters as a renewable fuel source for bio-energy facilities, composting or pelletizing litters to produce value-added products that can be more easily re-distributed to other regions, and using poultry composts to produce synthetic topsoils for non-agricultural uses (road construction, urban and industrial development, land reclamation).where agricultural land application continues it is important to use new soil testing methods to identify soils with the greatest potential for P loss to waters and to avoid litter applications to these soils. As with all nutrients proper application timings and methods are needed to prevent losses. It may be necessary to use chemical stabilization of P in litters (e.g. reducing soluble P in litters by adding alum) or to install enhanced buffer strips amended with by-products (e.g. iron oxides) to decrease soluble P and increase P sorption capacity in agricultural fields close to waterways. Restoration of the health of the Inland Bays will require significant improvements in nutrient management by agriculture. Many of these improvements are not economically justifiable from a crop production perspective, hence there is a need for a collaborative effort among all concerned parties (agriculture, state and federal agencies, environmental action groups, scientists, etc.) to identify the path forward and to monitor the cost and the success of more intensive nutrient management efforts for water quality protection. References Price, Kent S A framework for a Delaware Inland Bays Environmental classification. Proc. 3 rd Environ. Monitoring Assess. Prog. Albany, NY April 8-11, Ritter, W. F Delaware s Inland Bays: A case study. Environ. Sci. Health 27: Sims, J. T. 1997a. Phosphorus soil testing: Innovations for water quality protection. Proc. 5 th Intl. Symp. Soil and Plant Analysis. Minneapolis, MN, August 2-7, Sims, J. T. 1997b. Agricultural and environmental issues in the management of poultry wastes: Recent innovations and long-term challenges. p In. J. Rechcigl and H. MacKinnon (eds.) Agricultural Uses of By-Products and Wastes. ACS, Washington DC Keywords : nutrient management, water quality, eutrophication, agriculture. Mots clés : gestion des nutriments, qualité des eaux, eutrophisation, agriculture, gestion de bassin versant, contrôle des sources de pollution 7