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Abstract: Arsenic-enriched groundwater has been a pressing human health issue for more than a decade, with tens of millions of people worldwide being at risk of chronic As poisoning through the consumption of As-burdened groundwater. To elucidate the importance of dissolved S on the scale of As concentrations, the composition of groundwater samples from 926 locations spanning over the floodplains of three severely arsenic affected regions in Asia (Bengal-, Mekong-, Red River deltas), were assessed. A binary mixing model based on Cl− or B as conservative tracers implies that two types of water may be regarded as end-members with respect to groundwater composition in these deltas, namely surface derived water (approximated by river water) and saline water identical to residual sea water. Six redox zones were distinguished by comparing the model-calculated concentrations with the measured values. Only one zone (denoted methanogenic) had very high average As concentrations and they were significantly higher than in the other zones – for all three regions, regardless of applying Cl− or B as a tracer in the model. Average As concentrations±standard error in the methanogenic zone were 182±23μgL−1 (n =50%), 41±6μgL−1 (n =43%), and 61±20μgL−1 (n =24%) in the Mekong, Red River and Bengal delta, respectively. Arsenic levels were significantly lower in the SO4-reducing and the Fe-reducing zones, where averages were 23±7μgL−1 (n =27%, zone I), 14±3μgL−1 (n =48%, zone S) and 26±9μgL−1 (n =64%, zone S). These results suggest that a sufficient supply of inhibits the release of As to groundwater and that reduction may be as important as Fe reduction in controlling the enrichment of As in groundwater. [Copyright &y& Elsevier]