INFLUENCE OF GROUNDWATER PUMPING ON INORGANIC AS MOBILITY IN SHALLOW AQUIFER

Inorganic arsenic (As) is one of the main contaminants in the continental water bodies. The solubility and mobility of inorganic As is controlled by the alteration of redox condition in water. In Yun-Lin county of southwestern Taiwan, the As concentrations in ~34% shallow groundwater (<70m) monitoring wells exceeded the WHO guideline of 0.01 mg/L. At present, the groundwater is still widely provided to meet irrigation needs. The purpose of this study is to evaluate the groundwater pumping affecting As mobility in shallow aquifer. The highest groundwater As was found in a shallow (13.7m) monitoring well (CK) of Yun-Lin county with an average concentration of 0.42±0.17 mg/L. Hence, the core samples are collected nearby CK station with a total length of 24m for the analyses of As, Fe, and Mn contents. The sequential extraction and X-ray absorption near-edge structure spectra (XANES) are adopted to determine the As contents in different mineral phases and As speciation, respectively. The results of chemical analyses show that the highest As content of core samples was 62.5 mg/kg with the depth of 14.2m. The peaks of Fe and Mn content were also found in the range of 14-17m. The results indicate that the groundwater As mainly originate from the As-riched sediment. The results of sequential extraction show that 5.7% and 68.9% of As concentration of the sample with a depth of 14m is associated with the crystalline metal oxides dissolution phase and sulfide oxidation phase, respectively, which is different from the sample with a depth of 5m (70.2% and 26.2%, respectively). Following the reductive dissolution of As-rich Fe oxyhydroxide, released As may co-precipitate on the surface of sulfide under reductive condition. Groundwater in tens of the private dug wells (depths <20m) in this area are used as irrigation demands. Long-term (>20 year) pumping allows atmospheric oxygen to enter into shallow aquifers. According to the analytical results of XANES, the arsenate is the dominant As species of the sample with highest As. The transformation of inorganic As speciation may cause the desorption of As. Hence, extensive pumping promotes the re-oxidation of As-containing sulfide minerals and the transformation of As speciation, which are responsible for further As release to groundwater.