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An Isotopic View of Water and Nitrate Transport Through the Vadose Zone in Oregon’s Southern Willamette Valley’s Groundwater Management Area (S-GWMA)
Citation:
Brooks, J. Renee, Steve Hutchins, Bart Faulkner, S. Pearlstein, W. Rugh, K. Willard, R. Coulombe, AND J. Compton. An Isotopic View of Water and Nitrate Transport Through the Vadose Zone in Oregon’s Southern Willamette Valley’s Groundwater Management Area (S-GWMA). 27th PNW-SETAC Annual Conference, Corvallis, Oregon, March 08 - 10, 2018.
Impact/Purpose:
Groundwater nitrate contamination affects thousands of households in Oregon’s southern Willamette Valley and many more across the USA. The southern Willamette Valley Groundwater Management Area (GWMA) was established in 2004 due to nitrate levels in the groundwater exceeding the human health standard of 10 mg nitrate-N L-1. Much of the nitrogen (N) inputs to the GWMA comes from agricultural fertilizers, and thus efforts to reduce N inputs to groundwater are focused upon improving N management. However, the effectiveness of these improvements on groundwater quality is unclear because of the complexity of nutrient transport through the vadose zone and long groundwater residence times. Our objective was to focus on vadose zone transport and understand the dynamics and timing of N and water movement below the rooting zone in relation to N management and water inputs. Using stable isotopes of water, we are tracking water and nutrient movement through the soil, and exploring how these vadose zone complexities can be incorporated into practical understanding of the impacts of N management on groundwater inputs. This abstract contributes to SSWR 4.03C.
Description:
Groundwater nitrate contamination affects thousands of households in Oregon’s southern Willamette Valley and many more across the USA. The southern Willamette Valley Groundwater Management Area (GWMA) was established in 2004 due to nitrate levels in the groundwater exceeding the human health standard of 10 mg nitrate-N L-1. Much of the nitrogen (N) inputs to the GWMA comes from agricultural fertilizers, and thus efforts to reduce N inputs to groundwater are focused upon improving N management. However, the effectiveness of these improvements on groundwater quality is unclear because of the complexity of nutrient transport through the vadose zone and long groundwater residence times. Our objective was to focus on vadose zone transport and understand the dynamics and timing of N and water movement below the rooting zone in relation to N management and water inputs. Stable isotopes of water are a powerful tool for tracking water movement, and transit times. We established lysimeters at multiple depths and groundwater wells in a corn field in the GWMA, and have monitored nitrate and water isotopes biweekly for over a year. Our results indicate that vadose zone transport is highly complex, and the residence time of water collected in lysimeters was much longer than expected. During the fall wetup period, high nitrate concentrations were found in old irrigation water, and not precipitation at 0.75m depth. Water isotopes at 3 m depth also matched that of irrigation water but was low in nitrate concentrations. In March-April after 1m of precipitation fell, water isotopes at 0.75 depth finally match that of cumulative precipitation inputs, and had low nitrate concentrations. However, precipitation water in March-April carried high nitrate concentrations to 3m depth. We are exploring how these vadose zone complexities can be incorporated into practical understanding of the impacts of N management on groundwater inputs.