Groundwater Well Nitrate Trend Analysis in the Southern Willamette Valley
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In several areas around the state of Oregon, nitrate concentrations in groundwater are elevated above the drinking water maximum contaminant level of 10 mg nitrate-nitrogen L-1, posing a risk to human health. For this reason, the Oregon Department of Environmental Quality established several Groundwater Management Areas (GWMAs), including in the Southern Willamette Valley (SWV) in 2004. Nitrate concentrations in groundwater monitoring and drinking water wells within the SWV GWMA have been monitored since 2006. Stable isotopes of nitrate and water were added to the monitoring program in 2012, and wells were classified into six behavioral categories: leaching, stable, mixing, dilution, denitrification, and multi-process wells (Weitzman et al. 2021). Mann-Kendall trend analysis indicates that between 2008 and 2023. Nitrate concentrations significantly increased in 36% of wells, most of which were leaching and multi-process wells. Nitrate significantly declined in only 15% of wells, most of which were stable wells. with Well nitrate trends were associated with fertilizer inputs because agriculture accounts for nearly 65% of nitrogen inputs onto the landscape. Fertilizer inputs were estimated two ways, using extension recommended rates for crops around the wells, and using county fertilizer sales data. Even though estimated fertilizer application based on extension recommendations decreased around most wells from 2008-2023, county level fertilizer purchases actually remained constant from 1987-2017. Thus, increasing well nitrate trends may reflect continued fertilizer application in certain types of wells and not in other types of wells. Significant increases in nitrate concentrations for many wells across the SWV GWMA indicate that ongoing efforts to reduce nitrate contamination in groundwater and drinking water supplies have not been broadly successful, especially in wells where the nitrogen comes from leaching sources. Understanding well nitrate trends in response to nitrogen management is complicated by challenges in quantifying nitrogen inputs, use efficiency, legacies, and long-term groundwater dynamics.