Hydrochemistry of hot springs in geothermal fields of central, northern, and northeastern Taiwan: implication on occurrence and enrichment of arsenic

Abstract

Elevated arsenic (As) levels in geothermal systems have been identified in many places of the world. In this study, hydrochemical features and occurrence of As in hot springs from three geothermal fields located in northern, northeastern (NE), and central Taiwan were characterized. The results showed remarkable differences in pH and dissolved ions among these springs, due to their strong association with different geological origins. The acidic springs in the north field were associated with igneous rocks and were of Na–Ca–SO4 type, whereas neutral-alkaline springs in the NE and central fields were distributed in sedimentary and metamorphic rocks and were of Na-HCO3 and Na–Ca–HCO3 types. Geochemical modeling suggests that ionic variation is redox-sensitive, interrelated, and consistent with mineral saturation. Dissolution of carbonate minerals in sedimentary and metamorphic rocks would result in enrichment of HCO3 −, whereas oxidation of sulfur minerals associated with igneous rocks might be responsible for the low pH and elevated sulfate concentrations. Stable isotope signatures showed depletion of δD values in hot springs associated sedimentary and metamorphic rocks compared to those from igneous rocks. This suggested their mixing with local meteoric water or salt water, and their chemical compositions were influenced by water–rock interaction. Aqueous As concentrations were less than 100 µg/L in most springs except the one in the north field, where it peaked at 3867 µg/L. The elevated As level was presumably source-dependent and the presence of As-bearing pyrite in acidic-sulfate spring might be responsible for the geochemical anomaly of high As.