Shallow Groundwater Quality Beneath an Intensive Multiple‐Cropping System using Center Pivot Irrigation

Abstract

Abstract A study was conducted to compare shallow groundwater NO 3 ‐N concentrations under intensive multiple cropping systems with those from under nearby nonagricultural sites. The intensive multiple cropping systems involved center pivot irrigation and sprinkler‐applied fertilizer. The study was conducted on a sandy Coastal Plain soil by weekly measurement and sampling of observation wells located under a center pivot area and at two forest sites. Nitrate‐N concentrations under the center pivot area ranged from < 1 mg/L to about 133 mg/L, with a mean of 20 mg/L ( n = 360, SD = 19.5). In contrast, samples from the forest sites had NO 3 ‐N concentrations ranging from < 0.1 mg/L to just over 1 mg/L. Mean NO 3 ‐N concentrations under the center pivot area were found to vary seasonally according to cropping and hydrologic patterns such that the mean values for March–May, June–August, September–November, and December–February were 7, 21, 27, and 21 mg/L, respectively. The lower value for March–May indicated that winter rains leached most of the root zone NO 3 ‐N beneath the wells by March, and that there was a 2 to 3 month lag between spring‐applied N and its appearance in shallow groundwater. The study raises concern about shallow groundwater quality under intense multiple cropping systems on sandy soils. Although studies in this physiographic area of the Coastal Plain have shown that relatively impermeable subsoil horizons cause shallow groundwater to flow laterally to the larger drainage networks, and that alluvial forests then remove most of the N, the results of this study imply that groundwater NO 3 ‐N contamination could be a problem in other physiographic areas having sandy soils lacking such subsoil horizons.