Natural arsenic contamination in alluvial aquifers of Chianan Plain, Taiwan

Abstract

Geological and geochemical analyses were performed on groundwater and sediment samples collected from Budai and Yichu deep drilling sites in the Chianan Plain of Taiwan to examine the distribution and mobilization of arsenic in alluvial aquifers. The Chianan Plain is an area historically known for the unique occurrence of As-related endemic cases of Blackfoot disease (BFD). Groundwater at these drilling sites is mainly HCO3-Na-Cl or Na-Cl type and contains high levels of Sr and Br, indicating the influence of saltwater intrusion. However, the concentrations of As, Mn, Fe, Ba, and Si are significantly higher than those in seawater, implying that possible sources of As are either the reductive dissolution of Fe- and Mn-oxyhydroxides or chemical breakdown of silicate minerals such as biotite. Lower concentrations of PO43-, NO22-, and NO3- (<1 mg/kg) in groundwater suggest that ionic competition of nitrate and phosphate for sorbing sites likely is not the major mechanism for As mobilization. Sediment arsenic concentrations exhibit large depth dependence and range from 1.9 to 32.3 mg/kg and 1.7 to 23 mg/kg at the Budai and Yichu sites respectively. Arsenic levels in these sediments are notably higher than those in Bangladesh and West Bengal (generally < 10 mg/kg). Surface hydrologic transport modeling suggests that arsenic derived from dewatering fluids from mud volcanoes or weathering of potential source rocks in the headwaters may be routed through stream-channel networks and deposited in the Chianan Plain. Results of grain size and total organic carbon analyses indicate that high levels of arsenic are likely associated with fine-grained (silt and clay fraction) sediments and organic matter. Sequential extraction analysis reveals that Mn- and Fe-hydroxides and organic matter are the major leachable solid phases that host arsenic. More than 60% of arsenic is incorporated in silicates and other recalcitrant solid phases. The poor correlation between As and Fe in Na3P2O7-extracted fractions suggest that As may be associated in part with organic matter or humic substances in addition to Fe-bearing phases in sediments. Results of sediment bulk geochemistry and sequential leaching analysis indicate that dissolved As loads in groundwater represent a relatively small fraction of total As sources in sediments. The results of aquifer flushing models indicate that most initial mobile arsenic in a considerable part of alluvial aquifers, without new inputs from sediments, may be flushed out in a few thousand years. The modeling results are consistent with the young ages (<10,000 years) of most As-rich groundwater estimated for Holocene floodplain aquifers in Bangladesh and Taiwan.