Hydrogeochemistry and arsenic contamination of groundwater in the Jianghan Plain, central China

• 87% of groundwater samples had As concentrations exceeding the WHO guideline.
• Aquifers had strongly reductive conditions with abundant organic materials.
• Fe and Mn oxides/hydroxides may be the dominant sources of As in the groundwater.
• Potential mechanisms for As release include reductive dissolution of Fe and Mn oxides.

Although high arsenic groundwater has been reported in the northwest of China, no data describing groundwater arsenic in the river plains of central China have been published to date. In this study, 186 groundwater samples were collected from the Jianghan Plain between the Yangtze and Han rivers. Major constituents, trace elements, dissolved organic carbon (DOC), and stable hydrogen and oxygen isotopic compositions were analyzed to characterize groundwater chemistry and evaluate the factors controlling arsenic concentration and distribution in the plain. The results show that the groundwater is mainly HCO3–CaMg type with circum-neutral pH and moderate to high electrical conductivity. Negative Eh and high concentrations of DOC clearly indicate strongly reducing conditions with abundant organic matter in the groundwater aquifers. The characteristics of H/O stable isotopes demonstrate that the groundwater in the study area is recharged by local precipitation and there is a slow evaporation effect. Groundwater with high arsenic was found in wells at depths of 10 to 45 m along rivers. About 87% of the groundwater samples had As concentrations exceeding the WHO recommended value of 10 μg/L, up to 2330 μg/L. High concentrations of dissolved Fe (maximum value, 23 mg/L), Mn (maximum value, 5 mg/L) and P (maximum value, 4 mg/L) were also observed in groundwater. Among the wells, 89% and 98% exceeded the WHO guideline for Fe and Mn, respectively. The concentrations of total As measured in sediments from the Jianghan Plain were much higher than the global average, ranging from 11 to 108 mg/kg. All of the sediments had high Fe (Fe2O3 4–9%) and Mn (504–1064 mg/kg) levels, which is consistent with Fe and Mn oxides/hydroxides considered to be the dominant minerals containing As in sediments and the main sources for As in groundwater of the Jianghan Plain. The main potential mechanism for the release of As is the reductive dissolution of Fe and Mn oxides/hydroxides under reducing conditions, while microbial degradation of organic matter may also facilitate the release of arsenic into groundwater. The competitive effects of other anions may also be important factors since the concentrations of DOC, HCO3− and dissolved P were high in many of the groundwater samples.