Nitrate Contamination of Groundwater under Irrigated Coastal Plain Soils

To develop best management practices (BMP) for agricultural land to protect groundwater, data is needed on the leaching of N from irrigated coastal plain soils treated with poultry manure. This study was conducted to determine the vertical and seasonal patterns of NO3 leaching under such soils. Four commercially farmed corn (Zea mays L.) fields were studied, two receiving only fertilizer N (240 to 360 kg N ha-1 over a 2-yr period) and two with a continuing history of poultry manure applications (25-29 Mg ha-~ over 2 yr). In each field, a transect of four monitoring wells was installed 4 to 8 m deep (1 m below the seasonally low water table). Three additional wells were installed in forestiand adjacent to three of the fields. Groundwater and soils (to 1.5-m depth) were periodically sampled for analysis of NO3-N. Under the unmanured field, groundwater NO3-N concentrations averaged 15.1 mg L-~ during August through November 1986, while the corresponding figure for the manured fields was not significantly different at 18.3 mg L-~. Two months after spreading manure in November and December, as much as 104 mg NO3-N was measured in the groundwater under the manured fields. From December 1986 through September 1987 the groundwater under the manured fields had significantly higher NO3-N concentrations than did that under the unmanured fields (43.7 vs. 18.1 mg -~, respectively). Only for one well site with a buried A horizon did high CI to NO3-N ratios and low NO3-N concentrations indicate rapid denitrification. The forestland groundwater always contained < 1 mg NO3-N L-~, and high CI to NO3-N ratios, suggesting that NO3 in the cropland groundwater was lost after entering the forested areas, and that forests may therefore protect waterways from subsurface N contamination. N ITRATE CONTAMINATION of the Columbia Aquifer, a shallow unconfined aquifer of Maryland’s Coastal Plain, is of concern because of potential adverse effects on human and animal health, and because of its potential to contribute to eutrophication of the Chesapeake Bay. Several factors combine to make the NO3 contamination of this aquifer important to water quality in the Chesapeake Bay. First, the groundwater of the Columbia aquifer feeds into major tributaries of the Chesapeake Bay. Nitrogen is the nutrient most responsible for eutrophication of the brackish waters of the lower bay and its tributaries (Taft, 1982). Secondly, although Maryland normally has rainfall adequate for production of agronomic crops, the amount of irrigated cropland has been increasing dramatically in the past 15 yr. In 1975, the eastern shore of Maryland had 7500 ha of irrigated land, mostly in horticultural crops. This figure had increased to 13 950 by 1980 and 20 450 by 1985. In 1985, 47% of the irrigated land was devoted to corn and soybean [Glycine max (L.) Merr.]. Thirdly, NO3-N was found to be the most important All authors, Agronomy Dep., Univ. of Maryland, College Park, MD 20742. Contribution of the Agronomy Dep. and the Water Resources Res. Ctr., Univ. of Maryland. Financed, in part by the U.S. Dep. of the Interior, Office of Water Policy, as authorized by the Water Res. and Development Act of 1979 (P.L. 95-467). Received 12 May 1989. *Corresponding author. Published in J. Environ. Qual. 19:441-448 (1990). contaminant in the unconfined aquifers under the Delmarva Peninsula (Bachman, 1984). Irrigated corn, grown on light textured soils, may have a particularly great potential for NO3-N pollution. Farmers consider that the higher yield potentials of irrigated land justify higher rates of N fertilization. Typically, farmers apply 200 to 250 kg N ha-~ for irrigated corn and 120 to 150 kg N ha-1 for rainfed corn on these light-textured soils. An additional factor that may enhance the potential for groundwater contamination is the widespread application of poultry manure to irrigated land to supply N and dispose of the manure. The average rate of application by Delmarva farmers is >13 Mg ha-I of manure containing 24 kg N Mg -~. This equates to approximately 300 kg N ha-1. In some cases this amount of manure is applied twice a year to a given field. Also, commercial fertilizer N is often applied in addition to manure. These fertilization practices, along with irrigation of these droughty, light-textured soils, which have high permeabilities and leaching potentials, offer a very high potential for groundwater contamination with NO3. In order to develop BMP for Coastal Plain croplands, data is needed on the fate of NO3-N as a result of the irrigation, fertilization, and manure application practices common on these sandy soils. This study was initiated to: (i) characterize the depth and seasonal pattern of NO3-N leaching under center-pivot irrigated coastal plain soils, (ii) compare NO3-N leaching under fields treated with poultry manure and commercial fertilizer, and (iii) compare NO3-N in groundwater and in the soil profile under center-pivot irrigation to that under adjacent forestland. MATERIAL AND METHODS