Abstract
Abstract. An integrated hydrogeochemical study was carried out to realize the occurrence of arsenic (As) in a saline aquifer. Saline groundwater was mostly concentrated in the uppermost aquifer, and non-saline water was in the lower aquifer in the study area. High As concentrations were found in both the uppermost and lower aquifers. No correlation among salinity, well depth and As concentration was observed. Various forms of Fe oxyhydroxides were identified in the magnetic fractions, which were concentrated by high gradient magnetic separation (HGMS) technique, revealing that the redox cycling of Fe occurred in the subsurface. High levels of Fe, HCO3-, DOC and NH4+ concentrations accompanying alkaline pH in the As-rich groundwater were consistent with the mechanism triggered by the microbially mediated reductive dissolution of Fe oxyhydroxides. A proposed threshold value of 50μg L-1 As concentration was used as an indicator for identification of active proceeding reductive dissolution of As-bearing Fe oxyhydroxides in the saline aquifer. Desorption behaviors of As were relevant to its valence in the sediments and the co-existence of anions. Experimental and numerical results showed that additions of Cl- and SO42-, which represent the main anions of saline water, had minor effect on leaching sedimentary As. Although bicarbonate addition resulted in less As desorption than that of phosphate on a molar basis, the contribution of bicarbonate to the total release of As was greater than phosphate due to the much higher concentration of bicarbonate in shallow groundwater and the associated microbial mediation. Collectively, the chemical effect of saline water on the As release to groundwater is mild in the coastal aquifer.
Highlights
Arsenic (As) in groundwater exerts a serious environmental issue, notably in South Asia where local residents rely heavily on groundwater for drinking and agricultural purposes (Mandal et al, 1996; Smedley and Kinniburgh, 2002; Rodriguez-Lado et al, 2008; Shamsudduha et al, 2009)
Plots of major ions on the Piper diagram showed that most samples, 12 out of 28 wells, were categorized into Group 4 of saline type, and 5, 7 and 4 samples were in Groups 1, 2, and 3, respectively (Fig. 2)
Seawater intrusion plays a minor role on the local groundwater salinization, due to aquifers that are enclosed by the aquitard, and does not directly contact the seawater (Liu et al, 2003b)
Summary
Arsenic (As) in groundwater exerts a serious environmental issue, notably in South Asia where local residents rely heavily on groundwater for drinking and agricultural purposes (Mandal et al, 1996; Smedley and Kinniburgh, 2002; Rodriguez-Lado et al, 2008; Shamsudduha et al, 2009). For the Choushui River alluvial fan, a neighboring region of Chianan Plain, As-contaminated aquifers are found and distributed in the coastal area, with around 70 % of monitoring wells exceeding the drinking water quality standard in Taiwan (0.01 mg L−1) (Liu et al, 2006; Taiwan Sugar Company, 2006). For these two catchments, high As concentrations were observed at different depths, major characteristics of groundwater as well as the dominant release mechanism of As were similar, at the BFD-endemic area (Wang et al, 2007; Lu et al, 2011).
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