Natural arsenic contamination of Holocene alluvial aquifers by linked tectonic, weathering, and microbial processes

[1] Linked tectonic, geochemical, and biologic processes lead to natural arsenic contamination of groundwater in Holocene alluvial aquifers, which are the main threat to human health around the world. These groundwaters are commonly found a long distance from their ultimate source of arsenic, where chemical weathering of As-bearing minerals occurs. We propose a ‘‘GBH-As’’ model that ties together all of the important tectonic, biologic, and hydrologic processes that cause natural As release in groundwater in Holocene terrestrial deposits. Processes highlighted by the ‘‘GBH-As’’ model can explain the movement of arsenic from lithosphere to hydrosphere. However, we propose a factor that has increased dissolved As concentrations in Holocene aquifers to levels where human health is threatened: mechanical weathering associated with Pleistocene glaciation. Our model invokes erosion of mountain belts aided by glaciers, transport of arsenic by surface waters, adsorption of As by stream sediments, and deposition of stream sediments and organic matter in alluvial deposits. Subsequently, Fe(III)-reducing bacteria present in alluvial aquifers cause the release of sorbed As to groundwater under moderately reducing conditions.