EPA Study on Nitrate Concentration Trends in the GWMA Monitoring Wells
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Have groundwater nitrate concentrations changed since the 2004 designation of the southern Willamette Valley Groundwater Management Area (GWMA)? This is a key question for the GWMA board, the community, and the state. The GWMA was established to address concerns about high nitrate levels in groundwater, and DEQ has been measuring nitrate concentrations in wells throughout the GWMA since 2006. Recently, Oak Ridge Institute for Science and Education Research Fellow Lena Wang and EPA Office of Research and Development (EPA-ORD) scientists Renée Brooks and Jana Compton teamed up with Oregon DEQ to analyze the long-term well monitoring data in the GWMA to address this question. The team examined the data from the monitoring wells sampled by DEQ from 2006-2024. These wells include a mixture of existing drinking water wells and groundwater monitoring wells installed by DEQ within the GWMA. Wells were sampled during May each year to capture the highest yearly nitrate concentrations. Median nitrate concentrations in the SWV-GWMA well network did not change during the monitoring period between 2006 and 2024 (Figure 1a). In the individual wells, statistically significant increases were seen in 39% of wells, while only 17% decreased, with the majority (44%) having no significant change over time (Figure 1b). Going back to that original question: Are groundwater nitrate concentrations changing since the formation of the GWMA in 2004? In general, median nitrate concentrations in the SWV-GWMA well network did not change during the monitoring period since 2006. However, more wells increased in nitrate concentrations than decreased, and the number of wells above both the trigger level and the MCL thresholds have increased. These findings indicate that the nitrate concentrations are not improving based on the long-term data collected, and in many wells, especially where leaching is the source of water and nitrate, concentrations are increasing.