STUDY OF NITRATE IN SOILS AND AGRICULTURAL DRAINS OF THE LOWER YAKIMA VALLEY

Nitrate in the groundwater of the lower Yakima Valley, Washington has long been known to frequently exceed the EPA maximum contaminant level standard for potable water (10 mg/L). In this research, nitrogen and oxygen isotopic signatures were determined for nitrate in leachates of soils collected in the lower Yakima Valley, Washington. Isotope signatures for nitrate from soil leachate largely resembled those from nitrate in groundwater in a local EPA study which was largely attributed to a mixture of manure and fertilizer. However, they also resemble isotopic values attributed to naturally occurring nitrate at the nearby Hanford site, Washington. A mass balance calculation based on Δ17O suggests there is a 9% atmospheric contribution to nitrate in soil accumulations below the top of caliche layers at several locations. This contribution concurred with other research in areas with similar Mean Annual Precipitation (MAP) values. We argue this implies the nitrate in these caliche soil samples at depth may have a largely non-anthropogenic origin. We suggest the flushing of naturally occurring soil nitrate to groundwater during land use conversion to irrigated agriculture may represent a significant nitrate input to groundwater in this region.