Intrusion of As-rich groundwater into previously As-free aquifers due to extensive groundwater extraction is an increasing cause of concern, especially in Southeast Asia. We studied in-situ As adsorption processes by exposing synthetic Fe-mineral coated sand, as well as original sediment material, to high As groundwater under reducing conditions. Samples were placed in three monitoring wells located in a well-investigated study area (Van Phuc, Vietnam). The amount of adsorbed As reflected an As adsorption potential in the following order: hematite sand > ferrihydrite sand > original Pleistocene sand > goethite sand, and increased proportionally to the concentration of dissolved As in the respective groundwater. Results further indicate transformation of the exposed Fe-minerals, which is presumably controlled by biogeochemical processes. This alteration is expected to cause pronounced changes in the As adsorption potential of the Fe-minerals over time, which is subject to ongoing research.
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