Citation:

Weitzman, J., J. Renee Brooks, P. Mayer, W. Rugh, AND J. Compton. Coupling the dual isotopes of water (δ2H and δ18O) and nitrate (δ15N and δ18O): A new framework for classifying current and legacy groundwater pollution. Environmental Research Letters. IOP Publishing LIMITED, Bristol, Uk, 16(4):045008, (2021). https://doi.org/10.1088/1748-9326/abdcef

Impact/Purpose:

Shallow groundwater of the Southern Willamette Valley (SWV) of Oregon contains nitrate (NO3-) concentrations exceeding the human health standard of 10 mg NO3--N L-1, prompting Oregon Department of Environmental Quality (ODEQ) to designate the region as a Groundwater Management Area (GWMA) in 2004. However, 15 years after the SWV-GWMA designation, ODEQ found that NO3- still exceeded drinking water standards. Researchers at the Pacific Ecological Systems Division, CPHEA, ORD, EPA have been collaborating with ODEQ to help understand the mechanisms of spatial and temporal variability in NO3- concentrations across the SWV-GWMA, establish when and where H2O and NO3- isotopes are appropriate indicators of N pollution, and inform management of N in groundwater by identifying sources of N.

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. 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 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 were influenced 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. This case study illustrates when and where such coupled stable isotope approaches might provide key insights to management on groundwater nitrate contamination issues.

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