| Summary: | Study region: The Arsenic (As) concentrations found in Ong Phra sub-district, Dan Chang district, and Suphan Buri province are approx. 20 times higher than the average concentration of As in agricultural soils of Thailand. Study focus: The objectives of this current study were to investigate the As contamination in soil and groundwater and further develop the risk assessment map of As from contaminated soils releasing to shallow groundwater by combining DRASTIC method with contamination factor (CF) and mobility factor (MF). New hydrological insights for the region: According to the sequential extraction result, the concentrations of total As in 39 soil samples ranges from 4.8 to 1070.4 mg/kg. The highest As concentration in shallow groundwater was found at well number 3, located within <0.5 km. downstream from the old tin mine. The Index of DRASTIC groundwater vulnerability, is in the range of 59–147, which is categorized into three vulnerability zones as follows: low (<100), moderate (100–130) and high (>130). The high vulnerability area, found along the waterway, is approximately 35 km2 (18.65% of the study area). Interestingly, the DRASTIC index of the old tin mine is indicated as the zone of moderate and high vulnerability. The findings in this study reveal that the integration of the DRASTIC risk map with CF can well elucidate As contaminated in shallow groundwater. Keywords: Arsenic, Contaminated soil, Groundwater, Old tin mine, Sequential extraction
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