Investigation of sources of groundwater nitrate contamination in the Burbank-Wallula area of Washington, U.S.A.

Abstract

During the fall and early winter of 1980, groundwater samples were collected from 38 public supply and domestic wells in the Burbank-Wallula area of Washington. Groundwater nitrate-nitrogen (NO 3-N) concentrations ranged from 3.9 to 32 mg l −1 and averaged 10.5 mg l −1. Nitrogen isotope composition ( δ 15N) of the NO 3-N ranged from +1.3 to +16.0‰ and averaged +4.1‰. The preponderance of low δ 15N-values suggests that agricultural leachates (from oxidation of soil humus or use of nitrogen fertilizers) are the primary source of contamination for the area. Fourteen of the 38 wells were sampled at least twice in a four-month period. The relative invariability in NO 3-N levels and δ 15N in these wells suggests little or no temporal change in either the source or the magnitude of the source. Two additional samples collected in May 1981 from monitoring wells on the perimeter of a cropped field spray-irrigated with cattle waste from an anaerobic lagoon had δ 15N-values for the NO 3-N of +12.0 and +18.7‰, respectively. The high δ 15N-values in these wells indicate that the NO 3-N is predominantly derived from animal waste.