Sedimentogenesis and hydrobiogeochemistry of high arsenic Late Pleistocene-Holocene aquifer systems

Geogenic enrichment of arsenic (As) in groundwater has been a topic of worldwide concern over the past several decades due to the severe health threat to an estimated over one hundred million people. Despite the genetic diversity and the spatial variability of As concentrations, the most common geogenic As-enriched groundwater occurs in flat, low-lying river floodplains and fluvial-lacustrine plains. Within river systems draining the Himalaya, tectonic movement, sedimentological processes and palaeoclimatic optima after the Last Glacial Maximum have created favorable conditions for the formation of high-As aquifer systems mainly within the Late Pleistocene-Holocene deposits. The co-deposition of As-bearing Fe oxides and reactive organic matter within sediments of deltas/floodplains or fluvial-lacustrine plains forms the premise for As release under reducing geochemical environments. However, depending on the reactivity of organic matter and As-bearing Fe oxides, as well as the availability of aqueous sulfate, geochemical mechanisms controlling As enrichment in the groundwater differ between the coastal floodplains of South and Southeast Asia and the fluvial-lacustrine plains of North and Central China. The indigenous geomicrobes thriving to adapt to specific aquifer environments may mediate Fe-S-As biogeochemical cycling responsible for As speciation transformation, dissolution/desorption, and hence mobilization. Hydrologic processes regulated by the local and regional groundwater flow regimes control As transport as well as its spatial distribution. Special attention must be paid to anthropogenic perturbation from intensive groundwater pumping and land use changes that affect the aquifer hydrological conditions and the biogeochemical processes, thus altering groundwater As levels. To decipher the patterns of As variation within the aquifers, an integrated approach characterizing the major sedimentological, hydrobiogeochemical and anthropogenic processes influencing As release and transport needs to be applied to ensure sustainable water supply for domestic and agricultural needs.