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Arsenic (As) contamination of groundwater in Bangladesh is a serious natural calamity and public health hazard. High As concentrations in groundwater ( > 10 mu g I-1) in generally occur in Holocene alluvial aquifers. In this study, hydrochemical characteristics and factors influencing As distribution and release were investigated in Prayagpur, Southwestern Bangladesh. Fifty groundwater samples from the tubewells to depths from 21 m to 81 m were collected for hydrochemical and isotopic analysis. The results showed that As concentration in groundwater varied from 6.05 to 590.7 mu gI(-1) (average 58.31 mu g I-1), and about 82% samples contained As exceeding 10 pg I-1. Elevated As concentration mostly occurred in shallow aquifers, which was associated with reducing conditions. High As groundwater was distinguished with weak alkaline pH value, moderate HCO3- and total dissolve solid (TDS), and low NO3- and SO42 concentrations. The groundwater was dominated by Ca center dot Mg-HCO3- type water that was different from other high As aquifers distributed in the arid area. Moderate alkalinity and the reductive condition, resulted from biochemical reactions were the key controlling factors of As mobilization in aquifers. Environmental isotopes (delta O-18 and delta H-2) confirmed that evaporation did not play a vital role in the process of As enrich