Graduate Thesis Or Dissertation
 

Chemical and isotopic patterns of nitrate variability in the southern Willamette Valley, Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s7526h63g

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  • A relatively stable, persistent and historical problem with elevated NO₃⁻ concentrations in rural drinking wells in the southern Willamette Valley, Oregon is evident. What is the origin of NO₃⁻ in rural drinking water wells in this area? The answer to the question is not simple. Many non-point sources contribute to the elevated levels of NO₃⁻ in ground water, including residential and agricultural. The objective of this study was to use isotopes of NO₃⁻ and other chemical indicators to determine the sources of NO₃⁻ in drinking water wells in the southern Willamette Valley, OR. Criteria for wells to be included in the study were (1) less than 75 feet in depth (2) installed after 1960 and (3) domestic use. Four hundred sixty-six wells met the criteria of the study and 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. Twenty 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 units play an important role in determining vulnerability of wells to NO₃⁻ contamination, with land use also being important, yet less significant. Attempts to determine a fingerprint of septic and agricultural sources of NO₃⁻ contamination were inconclusive, though various chemical indicators were found to suggest the origin of the NO₃⁻.
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