Environmental isotopes as tracers of mining activities and natural processes: A case study of San Antonio de los Cobres River Basin, Puna Argentina

Abstract

High concentrations of arsenic (As) in water have been detected in the San Antonio de los Cobres (SAC) River. Historically, the upper basin has been exposed to mining districts spread in the high altitudes of the Puna Argentina, intense evaporation and discharge of hot springs from a near geothermal area (Tuzgle-Tocomar Complex). This work evaluate possible sources of As in surface waters, applying major and minor ions and trace elements, as well as multi-isotope analysis (3H, 2H/H, 13C/12C,18O/16O and 34S/32S). Four main water types were defined: waters with no signs of contribution from mineralized areas or hot springs (P); waters with thermal origin (T); waters with natural sulphide oxidation (M); and acid mine drainage waters (AMD) waters from abandoned mine wastes. δ34S-SO42− used in combination with sulphate concentration, discriminate sulphate derived from sulphides oxidation and those from thermal waters inputs. All sampled surface water result from mixing between two or three end-members (P-M; P-AMD; P-M-T). The proposed mixing models estimate their relative contribution. δ18O-SO42− and δ18O-H2O corroborate the influence on the SAC River of water coming from AMD rather than from natural sulphides oxidation. δ13C-DIC identifies samples with thermal influence. δ18O-H2O and δ2H-H2O deviate from meteoric water, indicating evaporation of 7% to 17%. The distribution of arsenic along the San Antonio River together with the δ18O-SO42− values show that both, natural sulphides oxidation and thermal springs contribute to highly rise concentrations. The current drinking water for the town of SAC is not being affected by the AMD.