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⁻¹ 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⁻¹ during August through November 1986, while the corresponding figure for the manured fields was not significantly different at 18.3 mg L⁻¹. 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 L⁻¹, respectively). Only for one well site with a buried A horizon did high Cl to NO₃-N ratios and low NO₃-N concentrations indicate rapid denitrification. The forestland groundwater always contained <1 mg NO₃-N L⁻¹, and high Cl 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. 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).