The purpose of this study was to; (1) determine the extent of well water contamination with nitrate and pesticides, and to understand the relationships among nitrate, pesticide, dissolved chloride, dissolved sulfate, well age, and well depth in the southern Willamette Valley, Oregon; and, (2) to investigate local residents' perceptions and opinions regarding water quality issues. Three data sets were used. First, the Oregon Department of Environmental Quality (ODEQ) sampled untreated well water for nitrate contamination from 500 households in the winter of 2000-2001. Second, a 100 household subset of the original 500 households was re-sampled in the summer of 2002 to be tested again for nitrate and further analyzed for pesticides, dissolved sulfate, and dissolved chloride. The third data set included responses to a mail survey from a stratified subset of residents of the original 500 households whose well water was sampled by the ODEQ.
Concentrations of nitrate during the 2000-200 1 sampling period had a mean of 4.0 mg/L, and ranged from .05 to 22.6 mg/L in 476 sample wells. Concentrations exceeded the national standard of 10 mg/L in 7.4% of these samples, which increased to 9.3% in wells less than 45 ft. deep. The mean nitrate concentration for wells sampled in 2002 was 10.8 mg/L, with 48% exceeding the national standard. Eighty-one of the 100 wells tested were found to contain at least 1 pesticide. The most common pesticides found were atrazine (73% of the wells) and its breakdown product desethylatrazine (80% of the wells). No significant association was found between nitrates 2000-2001 levels and either well depth or well age. Well depth and well age were found to have a statistically significant inverse relationship. Dissolved sulfate and nitrate (2002 sampling period) were significantly associated. Nitrate concentrations were not significantly correlated with any of the other independent variables.
The response rate for the surveys was 51%. Seventy-two percent of the respondents do not use a water treatment device and 28% use treatment devices. There was no difference in the use of water treatment devices based on nitrate concentrations in the well water. Respondents who used activated charcoal treatment devices had significantly lower nitrate concentrations than respondents who used reverse osmosis treatment systems. There was no significant relationship between respondents' description of well water quality and nitrate concentrations. However, respondents who did not own agricultural land were more likely than those who owned agricultural property to indicate concern over agricultural fertilizer and pesticide use harming well water quality. No significant relationship was found between nitrate contamination levels and the frequency of septic tank pumping, the distance of the septic tank from the well head, or the distance from the septic tanks drain field. Nitrate contamination levels were found to be significantly different between geologic units. Nitrate levels in geologic unit Qg₁ were significantly higher than all other geologic units, with the exception of Qalc. Typically, households located in areas where the Holocene alluvium of the Willamette River and the Pleistocene sand and gravel post-Missoula flood deposits dominated the geologic structure were at greater risk of high nitrate contamination levels.
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