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Environ Sci Technol. 2016 Apr 5;50(7):3521-9. doi: 10.1021/acs.est.5b04986. Epub 2016 Feb 9.

Aquifer Arsenic Cycling Induced by Seasonal Hydrologic Changes within the Yangtze River Basin.

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Earth System Science Department, Stanford University , Stanford, California 94305, United States.
Environmental Sciences Department, University of California - Riverside , Riverside, California 92521, United States.
Department of Geosciences, Boise State University , Boise, Idaho 83725, United States.
State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences , Wuhan, Hubei 430074, People's Republic of China.


Consumption of groundwater containing >10 μg L(-1) arsenic (As) adversely impacts more than 100 million people worldwide. Multiyear trends in aquifer As concentrations have been documented, but strong seasonal variations are not commonly observed. Here we report dramatic seasonal changes in As concentrations and aquifer chemistry within the Jianghan Plain of the Yangtze River, China. At some wells, concentrations fluctuate by more than an order of magnitude within a single year (100-1200 μg L(-1)). Groundwater extraction and sustained water levels of surface channels during the dry season induces a strong downward hydraulic gradient, seasonally supplying oxidizing (oxygen, nitrate) water to the otherwise anoxic aquifer. Oxygen and/or nitrate addition promotes a transient drop in As concentrations for 1-3 months. When recharge ceases, reducing, low-arsenic conditions are reestablished by reactive, endogenous organic carbon. Temporal variability in As concentrations is especially problematic because it increases the probability of false-negative well testing during low-arsenic seasons. However, periods of low As may also provide a source of less toxic water for irrigation or other uses. Our results highlight the vulnerability and variability of groundwater resources in the Jianghan Plain and other inland basins within Asia to changing geochemical conditions, both natural and anthropogenic, and reinforce that continued monitoring of wells in high-risk regions is essential.

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