Graduate Thesis Or Dissertation
 

A Trend Analysis of Nitrate in the Southern Willamette Valley Groundwater Management Area (GWMA)

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/cr56n703s

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  • The shallow aquifer in Southern Willamette Valley (SWV) has high levels of nitrate, and we are exploring the time trends in nitrate, and the hydrologic and land management factors that contribute to this problem. Nitrogen (N) inputs to farmland from fertilizer is thought to be the primary source of nitrogen to the Southern Willamette Valley (SWV) landscape. Elevated levels of nitrate in the shallow domestic wells have been measured at nitrate concentrations identified by the state of Oregon as a human health concern. In 2004, the Oregon Department of Environmental Quality (DEQ) designated a portion of the Southern Willamette Valley as a Groundwater Management Area (GWMA) where water quality data in 44 domestic and monitoring wells have been collected since 2005. Ranging from the northern boundary of Corvallis to the Springfield/Eugene region, the GWMA is characterized by substantial sources of nitrate which include agricultural fertilizers, home septic systems, concentrated animal feeding operations, and other non-point sources. This project was oriented around understanding how the groundwater nitrate levels changed since 2005. Using geospatial and statistical modeling techniques, as well as machine learning, representative prediction models were produced to give an indication of how the nitrate levels are changing over time, and to infer the likely sources. Using annual remote sensing landcover data dating back to 2004, we explored how land use and expected N fertilizer inputs over time based on locally-derived crop type have influenced monitoring well nitrate levels dynamics over the last 14 years. Approximately 57% of the wells in the study area increased between 2006 and 2019, and the total mean nitrate-N level in wells has increased from the 2006 through 2011 well mean of 5.41 mg nitrate-N/L to a mean of 6.28 mg nitrate-N/L from 2012 to 2019. Our findings indicate despite the greater awareness of the issue of groundwater nitrate contamination in the SWV GWMA, groundwater nitrate concentrations increased over the last 14 years, with the average monitoring well concentration now falling above the state of Oregon’s action level of 7 mg nitrate-N/L. Future efforts may need to find new approaches to improving drinking water quality in the area and continue to reach out to residents to ensure public safety.
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