RISK ASSESSMENT OF ARSENIC FROM CONTAMINATED SOILS TO SHALLOW GROUNDWATER IN ONG PHRA SUBDISTRICT CHANGWAT SUPHAN BURI, THAILAND

Abstract

Arsenic (As) is a global pollutant and people are exposed to As through food and drinking water. In the previous research, total As concentration in agricultural soils in Thailand has an average concentration of 30 mg/kg. However, some areas were found many times of As concentration in soils higher than the mean As concentration, especially soil samples collected in Tambon Ong Phra , Amphoe Dan Chang, Changwat Suphan Buri. However, the total contents of As in soils do not provide enough information to indicate the potential risk into the groundwater environment. Therefore, in this study, the DRASTIC vulnerability integrated with the mobility factor (MF) and the contamination factor (CF), derived from the sequential extraction procedure, has been developed to construct the risk assessment map of As contaminated soils into the shallow groundwater system. The 39 soil samples were analyzed As in each fraction using the sequential extraction technique. The concentrations of total As, sum of As in each fraction, in soil ranged from 4.8 – 1,070.4 mg/kg, with an average value of 171.8 mg/kg. The results of sequential extraction revealed that most As were in the oxidable and residual fractions, leading to low potential of releasing into the surrounding environments. The Index of DRASTIC vulnerability, overlaid of 7 thematic layers, was in the range of 59 – 147 and was categorized into three zones according to the groundwater vulnerability classification as follows: low (index <100), moderate (index 100–130) and high (index > 130 ). Around 18.65 % of the area showed high vulnerability, found along the waterway, and around 34.83 % and 46.47 % of the area were in the zones of moderate and low vulnerability, found in the plain and mountainous areas, respectively. Interestingly, the DRASTIC index of the old tin mine was indicated as the zone of moderate and high vulnerability. Moreover, the investigation of As concentrations in groundwater was carried out in dry and wet seasons and found that As in well no. 3 was higher than the drinking water standard only in the dry season. The distribution maps of As in shallow groundwater were generated and further used to compare with the risk maps. Finally, two risk maps were created by integrating the DRASTIC map with MF and CF and found that the risk map generated from CF was able to explain As contaminated in shallow groundwater better than the other risk map derived from MF.