Improved management of agricultural drainage ditches for water quality protection : An overview

Groundwater nitrogen (N) and phosphorus (P) transport from ditch-drained, cultivated fields has not been adequately investigated in the Chesapeake Bay watershed.We monitored hydrology and groundwater Nand P concentrations in 26 shallow (~3 m [10 ft]) wells for 27 months on a heavily ditched, poultry-grain farm on Maryland's Lower Eastern Shore. Water tables fluctuated above and below shallow ditches, but were always higher than deep ditches. Thus, groundwater flow to shallow ditches was intermittent, but flow to deep ditches was continuous.Water tables rose rapidly with rain, but drained back from 15 to 60 cm (6 to 24 in) the first day after rain. The rate of water table fall decreased rapidly thereafter. Water tables frequently perched on top of subsoil clay horizons. Although perching persisted only 24 to 48 hours, nutrient transport could be accelerated if rapid, lateral movement of water to ditches occurs. Frequent and widespread concentrations ofgroundwater N0 3 -N greater than 10 mg Lshow subsurface N loss from the farm is probable. High concentrations ofdissolved P existed in groundwater, but P movement in groundwater was restricted. Rain infiltrating through topsoils mobilized soil P into groundwater and moved considerably high concentrations of P as deep as 1.5 m (4 ft), where elevated P concentrations persisted for days or weeks. Groundwater P concentrations were greatest where high water table hydrology combined with the greatest soil P concentrations. Delivery of groundwater P to shallow ditches was apparently controlled by near-ditch soil P conditions, while P delivery to deep ditches was controlled by how deep groundwater flowed. Therefore, limiting soil P accumulation in near-ditch zones may help reduce P delivery to shallow ditches, and increasing the length of groundwater flow paths through low-P subsoils may help reduce P delivery to deep ditches. Peter A. Vadas is a soil scientist at the Dairy Forage Research Center, USDA Agricultural Research Service (ARS), Madison, Wisconsin. M.S. Srinivasan is a hydrologist at the Invermay Agricultural Centre, AgResearch limited, Mosgiel, New Zealand. Peter J.A. Kleinman and John P. Schmidt are soil scientists at the Pasture Systems and Watershed Management Research Unit, USDA ARS, University Park, Pennsylvania. Arthur L. Allen is an associate professor at the University of Maryland Eastern Shore, Princess Anne, Maryland. nearly 600 million birds at a wholesale value of nearly $2 billion. Nearly all of the 750,000 tons of poultry manure annually produced Nitrogen (N) and phosphorus (P) are the primary nutrients that accelerate eutrophication in fresh surface waters (Carpenter et at. 1998). Eutrophication is an aging process where influxes of nutrients promote excessive plant and algae growth and can limit water use for recreation, industry, and drinking. On the Delmarva Peninsula in the Mid-Atlantic region of the United States (figure 1), the Chesapeake Bay and Delaware and Maryland's Coastal Bays are subject to eutrophication-related algal blooms (Boesch et al. 2001; Boynton 2000). The Delmarva Peninsula is home to one of the most concentrated poultry industries in the United States, and annually produces Water Quality Protection: Symposium. College Park, MD: University of Maryland. www.sawgal.umd. edu/DrainageDitches/2006_tourandsymposium/. Skaggs, R.W, M.A. Breve, and ].W Gilliam. 1994. Hydrologic and waterquality impacts ofagriculcural drainage. Critical Reviews in Environmental Science and Technology 24:1-32. Skaggs, R.W, and].V Schilfgaarde, eds. 1999. Agricultural drainage. Agronomy Monograph 38. Madison, WI: American Society of Agronomy-Crop Science Society ofAmerica-Soil Science Society ofAmerica. Skaggs, R.W, G.M. Chesheir, and B.D. Phillips. 2005a. 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