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

The shallow aquifer in Southern Willamette Valley has high levels of nitrate-N, and we are exploring the role of geomorphology and historic land use in contributing to this problem. Nitrogen inputs to farmland from fertilizer is thought to be the primary source of nitrogen to the Southern Willamette Valley (SWV) landscape. With the valley’s surface land use averaging 85% agricultural between 2004 through 2017, elevated levels of nitrate-N 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 monitoring wells have been collected since 2005. Ranging from the northern boundary of Corvallis to the Springfield/Eugene region, substantial sources of nitrate include agricultural fertilizers, home septic systems, and potentially several concentrated animal feeding operations, among other non-point sources. This project is oriented around understanding how the groundwater nitrate levels have changed since 2005, including why 57% of the wells in the study area are increasing, as well as why the total nitrate-N level has increased from the 2005 through 2011 well mean of 5.46 mg/l to a mean of 7.10 mg/l from 2012 to 2019. Using geospatial and statistical modeling techniques, as well as machine learning, representative prediction models were produced to give an indication of how the nitrate-N levels are changing over time, and to infer the likely sources. Using annual remote sensing landcover data going 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-N levels dynamics over the last 14 years.

Impact/Purpose

Nitrate contamination of shallow groundwater in Oregon's southern Willamette Valley has been a concern for the state agencies for over two decades. Several thousand residents were estimated to use drinking water wells with nitrate concentrations above the state's action level of 7 mg/L, the Oregon Department of Environmental Quality (DEQ) designated a portion of the Southern Willamette Valley as a Groundwater Management Area (GWMA) in 2004. The GWMA formation led to a great deal of effort and communication to improve nitrogen management in the southern Willamette Valley. In 2012, state agency staff came to EPA to ask for assistance in understanding the factors affecting nitrate leaching and whether the improved agricultural practices have led to improvements in drinking water quality. Recent work examining the well data from 2005-2019 indicates that 57% of the wells in the study area still are increasing, and mean well nitrate-N levels have increased by approximately 2 mg/L, with the average now rising to be just above the state action level. Together with staff from EPA and ORISE examining the water and nitrate isotopes, OSU student Cody Piscitelli is examining the role that different crops and other N sources are affecting specific wells, to help better understand what factors could improve water quality in the area.

Citation

Piscitelli, C., P. Adamus, J. Compton, Jacqueline Brooks, AND J. Weitzman. A Trend Analysis of Nitrate in the Southern Willamette Valley Groundwater Management Area (GWMA). American Geophysical Union, San Francisco, CA, December 09 - 13, 2019.