Nitrogen Pollution Indicators in Groundwater: The Role for Dual-Isotope Analysis of Water and Nitrate

Nitrate contamination of groundwater is a concern across the US, particularly in agricultural regions where 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 is often confounded by the legacy effect, and other processes unrelated to management. Analyzing the coupled water (δH2O = δ2H and δ18O) and nitrate (δNO3- = δ15N and δ18O) stable isotopes can be a powerful indicator 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 2012-2019 from 39 long-term monitoring wells in the Southern Willamette Valley (SWV), Oregon, where agricultural land uses cover ~93% of the area. The source(s) of nitrate to each well were unique, suggesting that the nitrate concentration patterns seen across the uniform shallow aquifer of the valley can be quite complex. Analyzing the stability versus variability of a well’s corresponding δH2O and δNO3- values revealed the mechanism(s) controlling nitrate concentrations. Wells with stable nitrate concentrations and isotopes appeared to be influenced by one water source with a long residence time and one nitrate source, as evidenced by their stable δH2O and δNO3- values. 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. Understanding the drivers of nitrate dynamics and interaction with legacy N, which serves as a long-term source for mineralization and nitrate leaching, is crucial for properly designing intervention efforts to address particular sources for 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.

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.

Citation

Weitzman, J., J. Renee Brooks, P. Mayer, W. Rugh, AND J. Compton. Nitrogen Pollution Indicators in Groundwater: The Role for Dual-Isotope Analysis of Water and Nitrate. American Geophysical Union Annual Meeting, virtual, Virtual, December 07 - 11, 2020.