Citation:

Weitzman, J., J. Compton, AND Jacqueline Brooks. Science to inform groundwater nitrate management in the Southern Willamette Valley GWMA. Yakima Groundwater Management Area¿s Implementation Committee Meeting - over teams, Yakima, WA, July 13, 2023.

Impact/Purpose:

):  Julie Weitzman (former ORISE fellow based at CPHEA-PESD) was invited by Washington State Department of Public Health to present on EPA work using stable isotope approaches to understand sources and processing of nitrate in the southern Willamette Valley’s Groundwater Management Area (GWMA).  Jana Compton will provide background information on nitrate sources and management in the GWMA, Julie will discuss how isotope analysis is informing their work and Renée Brooks will describe how the data has been used within the southern Willamette Valley GWMA.  The Yakima GWMA committee is developing their approaches for improving groundwater nitrate levels and exploring the role of stable isotopes within their programs, and our presentation will inform that work. 

Description:

Nitrate contamination of groundwater is a concern globally, particularly in agricultural regions where decades of fertilizer nitrogen (N) use has led to a legacy of N accumulation in soils and groundwater. Linkages between current management practices and groundwater nitrate dynamics are often confounded by the legacy effect, and other processes unrelated to management. Here we present how a coupled analysis of dual stable isotopes of water (δH2O = δ2H and δ18O) and nitrate (δNO3− = δ15N and δ18O) can be a powerful approach to identify sources and processes responsible for groundwater pollution to a Yakima working group dealing with nitrate groundwater issues. To assess how management practices impact groundwater nitrate, we interpreted behavior of δH2O and δNO3−, together with nitrate concentrations, in water samples collected from long-term monitoring wells in the Southern Willamette Valley (SWV), Oregon. The source(s) of nitrate and water varied among wells, suggesting that the nitrate concentration patterns were not uniform across the shallow aquifer of the valley. Analyzing the stability versus variability of a well’s corresponding δH2O and δNO3− values over time revealed the mechanisms controlling nitrate concentrations. Wells with stable δH2O and δNO3− values and nitrate concentrations wereinfluenced by one water source with a long residence time and one nitrate source. Variable nitrate concentrations of other wells were attributed to dilution with an alternate water source, mixing of two nitrate sources, or variances in the release of legacy N from overlying soils. Denitrification was not an important process influencing well nitrate dynamics. Understanding the drivers of nitrate dynamics and interaction with legacy N is crucial for managing water quality improvement. We emphasize how this case study illustrates when and where such coupled stable isotope approaches might provide key insights to management on groundwater nitrate contamination issues, and discuss how such an approach might help their work within the Yakima River Basin.

Record Details: