Coupling the dual isotopes of water (δ2H and δ18O) and nitrate (δ15N and δ18O): a new framework for classifying current and legacy groundwater pollution

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 (delta H2O = delta H-2 and delta O-18) and nitrate (delta NO3-= delta N-15 and delta O-18) 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 delta H2O and delta NO3-, 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 delta H2O and delta NO3- values over time revealed the mechanisms controlling nitrate concentrations. Wells with stable delta H2O and delta NO3- 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.

Automated summary

Nitrate contamination of groundwater is a global issue, impacted by decades of fertilizer use in agricultural regions. Stable isotope analysis can help identify nitrate sources, with management practices influencing nitrate concentrations.