Chemical and Isotopical Patterns of Nitrate Contamination in the Southern Willamette Valley, Oregon

A persistent problem with elevated NO₃ concentrations in rural drinking wells in the southern Willamette Valley, Oregon has been documented since the 1930's. We explore the origin of this contaminant. The objective of this study was to use isotopes of NO₃ and other ionic chemical indicators to determine the sources of NO₃ in drinking water wells in the southern Willamette Valley, OR. Many non-point sources were found to contribute to the elevated levels of NO₃ in ground water, including high-density residential and high-intensity agricultural. 466 wells met the criteria to be included in the study: (1) less than 75 feet in depth (2) installed after 1960 (3) domestic use and (4) be located in the southern Willamette Valley. 120 wells were sampled during the summer of 2003. Geologic units, dominant land use and soil types were determined for each well in an attempt to determine vulnerability of wells for NO₃ contamination. 20 drinking water wells were selected to undergo isotopic and further chemical analyses. In order to determine the chemical and isotopic fingerprints of the dominant sources of NO₃ contamination soil samples were augered from 10 septic drain fields and water samples were collected below 10 agricultural fields. NO₃-N concentrations in the study area ranged from below detection (<0.20 mg/L) to 13.70 mg/L, with a mean concentration of 4.81 mg/L. There was a statistically significant trend (i.e. P < 0.05) in NO₃-N with well depth, well age, pH and SO₄. Findings suggest that geologic setting is an important factor in predicting vulnerability, with land use also being important but less so. Determination of septic and agricultural sources of NO₃ contamination were inconclusive, though various chemical indicators were found to suggest the origin of the NO₃.