Identifying groundwater arsenic contamination mechanisms in relation to arsenic concentrations in water and host rocks

Leaching and oxidation of high arsenic (As) host rocks tend to be induced by circulation of deep geothermal waters, which increase As concentration in shallow groundwater. The purpose of this study is to identify the mechanism of groundwater As contamination in relation to leaching and oxidation along the border between the South Minahasa and Bolaang Mongondow districts, North Sulawesi, Indonesia. This region contains Miocene sedimentary rock-hosted disseminated gold deposits associated with hydrothermal alteration in a fault zone. Abnormally high As concentrations were observed in hot and cold springs and in surrounding shallow groundwater for a total mineralization area of 8 × 10 km². Two methods were adopted in this study: (1) microscopic and spectroscopic analyses of rock samples for mineral identification and (2) geostatistics for spatial modeling of As concentrations in groundwater. Jarosite was identified as the chief fill mineral in rock defects (cracks and pores). The presence of this mineral may indicate release of As into the environment, as can occur as an alteration product derived from oxidation and leaching of pyrite, As-rich pyrite or sulfide minerals by geothermal waters. Moreover, As concentrations in groundwater were estimated using geostatistics for spatial modeling. The co-kriging map identified local anomalies in groundwater As concentrations over the permissible limit (10 ppb). Such anomalies did not appear through ordinary kriging. Integration of the results indicates that As contamination in shallow groundwater probably is controlled by heterogeneous distributions of jarosite and variations in intensity and extent of hydrothermal activities.