Sources of groundwater nitrate revealed using residence time and isotope methods |
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| Institution: | 1. School of Biological Sciences, University of Hong Kong, Pok Fu Lam, Hong Kong, China;2. Swire Institute of Marine Science, University of Hong Kong, Cape D''Aguilar, Hong Kong, China;3. State Key Laboratory of Marine Environmental Science, Xiamen University, PR China;4. Department of Earth Sciences, University of Hong Kong, Pok Fu Lam, Hong Kong, China;1. State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China;2. Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 51027, China;3. Institute of Applied Ecology, The Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China;4. Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall, West Lafayette, IN 47907, USA |
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| Abstract: | Nitrate concentrations approaching and greater than the maximum contaminant level are impairing the viability of many groundwater basins as drinking water sources. Nitrate isotope data are effective in determining contaminant sources, especially when combined with other isotopic tracers such as stable isotopes of water and 3H–He ages to give insight into the routes and timing of NO3 inputs to the flow system. This combination of techniques is demonstrated in Livermore, CA, where it is determined that low NO3 reclaimed wastewater predominates in the NW, while two flowpaths with distinct NO3 sources originate in the SE. Along the eastern flowpath, δ15N values greater than 10‰ indicate that animal waste is the primary source. Diminishing concentrations over time suggest that contamination results from historical land use practices. The other flowpath begins in an area where rapid recharge, primarily of low-NO3 imported water (identified by stable isotopes of water and a 3H–He residence time of <1 year), mobilizes a significant local NO3 source, bringing groundwater concentrations up to 53 mg NO3 L−1. In this area, artificial recharge of imported water via local arroyos increases the flux of NO3 to the regional aquifer. The low δ15N value (3.1‰) in this location implicates synthetic fertilizer. In addition to these anthropogenic sources, natural NO3 background levels between 15 and 20 mg NO3 L−1 are found in deep wells with residence times greater than 50 a. |
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