HIGH HEXAVALENT CHROMIUM GROUNDWATER FROM DEEP AQUIFERS IN CHINA (Invited Presentation)

Chromium (Cr) is mainly present as Cr(VI) and Cr(III) in natural water. In comparison with Cr(III), Cr(VI) is more toxic and can cause severe negative health effects in humans as a mutagen, teratogen, and carcinogen. High Cr groundwater has been mainly observed in shallow aquifers, which were sourced from serpentinized and ultramafic rocks and sediments. Oxidation of Cr(III) to Cr(VI) has been believed to be the major process of Cr mobilization from aquifer solids, normally in the presence of Mn oxides. After oxidation, desorption of Cr(VI) contributed to Cr enrichment in groundwater in alkaline pH conditions, due to the presence of Cr(VI) as oxyanions. In addition to ultramafic rock aquifers, high Cr groundwater has been found in sandstone aquifers in Brazil and metamorphosed rock aquifers in eastern U.S. However, no data are available for occurrence of high Cr groundwater in deep aquifers.

This study found the occurrence of high Cr groundwater in deep aquifers from the North China Plain (NCP) and the northwest of Shanxi province. Shallow groundwater generally had low Cr concentrations (<10 μg/L). High Cr groundwater normally had high NO₃^- concentrations and high ORP, but low concentrations of dissolved Fe and Mn. Dissolved Cr was predominantly present as Cr(VI) (>90%) in deep groundwater. Both D and ¹⁸O isotopes and hydrogeochemical characteristics indicate that groundwater Cr was sourced from aquifer solids, instead of anthropogenic activities. Cr contents of aquifer solids were not abnormal high, which fell in the range of natural geological rocks and sediments. High Cr groundwater corresponded to the aquifer sediments with high relative contents of Mn oxide-bound Cr and low relative contents of unweathered silicate-bound Cr. Those data indicate that groundwater Cr was sourced from unweathered silicate-bound Cr in the presence of Mn oxide. In deep aquifers with oxic-suboxic conditions, the presence of Mn oxides induced the oxidation of Cr(III) to Cr(VI), which is more mobile in aqueous systems. The produced Cr(VI) was desorbed from solids into groundwater under alkaline pH conditions. This study suggest the geogenic occurrence of high Cr(VI) concentrations in deep groundwater from both sediment and rock aquifers containing Mn oxides.