Sources of nitrate in northwestern Nevada groundwater, 1985-2014 of the requirements for the degree of Master of Science in Hydrogeology
Groundwater data from across northwestern Nevada was compiled to determine variables that affect nitrate concentrations in the region. Variables used in this study that may influence increased nitrate concentrations include land-use type (undeveloped/natural, agricultural/rural, low-residential, and high-residential land), septic tank density, soil properties (well vs poorly drained), depth to water, and geology. A Kendall's Tau rank correlation test was used to determine trends in groundwater nitrate concentrations over the 30 years between 1985 and 2014. On the basis of the correlation test, nitrate concentrations in groundwater available across northwestern Nevada exhibit increasing trends. A principal component analysis was used to reduce the dimensions of the variables to form three significant groups out of possible correlated variables. The three groups that best explain the data are 1) Soil permeability and septic use (well drained soils, septic density, and low-residential), 2) Urban areas (anthropogenic influences), and 3) Geology (textural variation of geologic deposits). Samples of nitrogen (δ15N-NO3) and oxygen (δ18O- NO3) isotopes from Churchill, Washoe, and Douglas counties were used to fingerprint nitrate sources. The nitrate isotope values all plot within the septic and manure range and because most samples were collected in residential areas, the likely source of nitrate is septic tanks or leaky sewer lines. Isotope samples from agricultural wells are within the soil-N range and have low concentrations of nitrate (< 5 mg/L). These factors indicate that natural nitrification in the soil, rather than the flushing of excess fertilizer application may be influencing the isotopic values of the water sampled from wells under agricultural fields. Statistical analyses also suggest that septic tanks are most likely the major contributor of nitrate to groundwater in the region as opposed to agricultural practices or natural sources.
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