Geochemistry of thermal waters and arsenic enrichment at Antsirabe, Central Highlands of Madagascar

Abstract

Thermal waters in the Central Highlands of Madagascar around Antsirabe were investigated using a combination of hydrogeochemical and isotopic methods, geochemical speciation and inverse geochemical modeling. Thermal waters at Antsirabe have temperatures from 35.2 to 47.7 °C, are highly mineralized (EC up to 5.87 mS/cm) are of Na-HCO3-Cl type and have elevated concentrations of arsenic (up to 0.597 mg/l), Sr (up to 8.05 mg/l) and Li (up to 2.83 mg/l). About 25 km west of Antsirabe, a thermal spring at Betafo has a temperature of 53.6 °C, but its mineralization is much lower (EC 0.72 mS/cm) and its water is of Na-HCO3-SO4 type. Concentrations of As, Sr, and Li at Betafo are much lower, only 0.006 mg/l, 0.72 mg/l, and 0.063 mg/l, respectively. Calculated reservoir temperatures using quartz and chalcedony geothermometers are up to 153 °C at Antsirabe and 108 °C at Betafo. Values of δ2H and δ18O are above the WMWL, indicating exchange with silicates like micas. Values of δ13C(DIC) are very enriched (up to −0.45‰) in Antsirabe samples, but depleted with a value of −21.52‰ in a Betafo sample. Values of δ34S(SO4) are close to −4.0‰ in all samples, suggesting an origin of sulfate from a Na-sulfate mineral. Values of 87Sr/86Sr ratios in Antsirabe samples suggest interactions with reservoir rocks. The principal difference between both sites seems to be in the significant input of magmatic-origin CO2 at the Antsirabe site (up to 61.4 mmol/l as determined by inverse geochemical modeling) with resulting higher dissolution rates of As-containing silicate minerals such as micas. There is probably no such input at the Betafo site. In spite of relatively lower As concentrations compared to geothermal waters at global tectonic plate margins, concentrations of As at Antsirabe can represent a serious environmental problem.