Nitrogen pollution indicators in groundwater: The role for the dual-isotope analysis of water and nitrate
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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. Agricultural land use practices were the largest contributor of N inputs (90%) across the SWV-GWMA, including production of grass and legume seeds and field crops/vegetables. ODEQ focused on improving N management strategies by increasing the agricultural community’s awareness of best management practices that reduce N inputs. However, 15 years after the SWV-GWMA designation, ODEQ found that NO3- still exceeded drinking water standards. We quantified the dual stable isotopes of H2O (δ2H and δ18O) and NO3- (δ15N and δ18O) in aqueous samples collected from SWV-GWMA monitoring wells from 2016-2019 to track water sources and groundwater residence times, as well as to elucidate sources and transformations of N in the groundwater. We also assessed how management impacted groundwater N by evaluating whether variations in NO3- levels and N sources were linked with changes in crop types and fertilizer/manure N inputs.
We found that the source of NO3- in SWV-GWMA monitoring wells tended to be either synthetic fertilizer or septic/manure. For example, a fertilizer N source was suspected when groundwater NO3- concentrations were high, but δ15N-NO3- values were low. When concentrations of NO3- and values of δ15N-NO3- were both high, the suspected source of N was septic and/or manure waste. And, in wells where lower NO3- concentrations were associated with higher δ15N-NO3- values, the occurrence of N transformation processes was inferred. Further analysis of the dual-isotopes of H2O and NO3- revealed that some variation in NO3- concentrations over time were likely due to denitrification, while other wells varied in NO3- concentration due to alternating water sources throughout the year. This effort is intended to characterize 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.