Spatial variation of groundwater arsenic distribution in the Chianan Plain, SW Taiwan: Role of local hydrogeological factors and geothermal sources

Abstract

We present here major ion, trace element, stable and radioisotope data based on forty-six groundwater samples collected from various locations along few selected profiles across the Chianan Plain, southwestern Taiwan including the area affected by well known Blackfoot disease manifested by peripheral vascular gangrene. The objective of the study was to understand the role of local hydrogeology in terms of spatial variation of arsenic concentration in groundwater wells of the entire Chianan Plain and the foothill belt of the Central Mountain Range. An attempt has also been made to assess the contribution of nearby geothermal sources to the arsenic budget in groundwater of the Chianan Plain. Our study shows a gradual increase in all major and trace ion concentrations including total arsenic from foothill belt (arsenic: median = 4 μg/L, range = 0–667.6 μg/L, sample number n = 16) to coastal zones (arsenic: median = 42.74 μg/L, range = 0.14–348.6 μg/L, n = 15) of the plain. Inverse geochemical modeling shows that Ca may be exchanged on clays, and that the degree of the exchange increases from the foothill to the coastal zones. Inverse geochemical modeling further suggests that the oxidation of organic matter (CH2O) required in various east-west profiles across the plain to balance the total bicarbonate concentration and CO2 input from organic matters significantly increases from the foothill to the coastal zones with transfer coefficients ranging from 1.55 × 10−2 to 1.69 × 10−5 mol/L. High concentrations of tritium (mean = 1.33 ± 0.11 TU; n = 4) in foothill groundwater and low concentration of tritium in groundwater of central zone suggest gradually increasing water–rock interaction from the foothill to the coastal part. Few elevated arsenic (median = 171.8 μg/L, maximum = 667.60 μg/L, minimum = 24 μg/L; n = 6) hotspots are identified in the foothill belt. Available lithologs and aquifer test data suggest that the presence of impermeable clay around those pockets possibly inhibits vertical and lateral flushing of the aquifer and aids strong water–rock interactions subsequently leading to release of arsenic into groundwater. Using oxygen isotope and chloride mass balance method, we estimated that geothermal sources can recharge a maximum of 4% of groundwater in proximal aquifers and contribute <2% of average As concentration in the groundwater of Chianan Plain. Our preliminary observations thus show some arsenic enrichment in foothill aquifers, providing a necessity of detailed study of the aquifer systems in these understudied regions. Moreover, our research indicates that the contribution of arsenic from geothermal sources is insignificant, which stands in contrast to earlier studies suggesting that mud volcanoes and thermal springs in the Western Foothill Belt of the Central Mountain Range were potential sources of groundwater arsenic in the Chianan Plain aquifers.