Weitzman Julie N, Brooks J Renée, Mayer Paul M, Rugh William D, Compton Jana E
ORISE Fellow at Pacific Ecological Systems Division, Center for Public Health and Environmental Assessment, Office of Research and Development, US Environmental Protection Agency, Newport, OR, United State of America.
Pacific Ecological Systems Division, Center for Public Health and Environmental Assessment, Office of Research and Development, US Environmental Protection Agency, Corvallis, OR, United States of America.
Environ Res Lett. 2021 Mar 24;16(4):1-45008. doi: 10.1088/1748-9326/abdcef.
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 (δHO = δH and δO) and nitrate (δNO = δN and δO) 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 δHO and δNO , 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 δHO and δNO values over time revealed the mechanisms controlling nitrate concentrations. Wells with stable δHO and δNO 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.
地下水的硝酸盐污染是一个全球性问题，尤其是在农业地区，数十年来氮肥的使用导致土壤和地下水中积累了大量氮素。当前管理措施与地下水硝酸盐动态之间的联系常常受到遗留效应以及与管理无关的其他过程的干扰。水（δH₂O = δH和δ¹⁸O）和硝酸盐（δ¹⁵NO₃⁻ = δ¹⁵N和δ¹⁸O）的双稳定同位素耦合分析是识别地下水污染来源和过程的有力方法。为了评估管理措施如何影响地下水硝酸盐含量，我们对从俄勒冈州南威拉米特谷（SWV）长期监测井采集的水样中的δH₂O和δ¹⁵NO₃⁻行为以及硝酸盐浓度进行了解释。各井的硝酸盐和水的来源各不相同，这表明该山谷浅层含水层中的硝酸盐浓度模式并不统一。分析一口井相应的δH₂O和δ¹⁵NO₃⁻值随时间的稳定性与变异性，揭示了控制硝酸盐浓度的机制。δH₂O和δ¹⁵NO₃⁻值以及硝酸盐浓度稳定的井受到一种具有较长停留时间的水源和一种硝酸盐来源的影响。其他井中硝酸盐浓度的变化归因于与替代水源的稀释、两种硝酸盐来源的混合或来自上覆土壤的遗留氮释放的差异。反硝化作用不是影响井中硝酸盐动态的重要过程。了解硝酸盐动态的驱动因素以及与遗留氮的相互作用对于管理水质改善至关重要。本案例研究说明了这种耦合稳定同位素方法何时何地可能为地下水硝酸盐污染问题的管理提供关键见解。
