Geological-geomorphological and geochemical control on low arsenic concentration in the Lerma valley groundwater between the two high arsenic geologic provinces of Chaco-Pampean plain and Puna

Abstract

Argentina is known for having one of the most extensive areas with high arsenic (As) concentration in groundwater in the world. These areas correspond to two geological provinces, the Altiplano-Puna plateau and the Chaco-Pampean plain. In this large territory, there are some specific environments where the As concentration in groundwater is lower, and in some cases within the recommended limits for drinking water. In our study, we analyze and interpret the low As concentrations reported for the Lerma valley, the easternmost intermontane basin of the Cordillera Oriental, located between the aforementioned high‑arsenic areas. The groundwater from this valley is used for the consumption of >600.000 inhabitants in the city of Salta and nearby towns. The incipient geological development of the valley since the late Miocene and the subsequent tectonic and climatic evolution favored low As concentrations with respect to the Altiplano-Puna plateau and the Chaco-Pampean plain. The high-energy sedimentary environments that characterized the area during Plio-Quaternary times and the composition of the sediments have controlled the characteristics of the multilayered aquifer. Moreover, the absence of geogenic As sources, climate, high rain infiltration rate, near neutral pH, redox conditions, and wells construction with screens settled in coarse productive layers favor groundwater of good quality. The geological and tectonic evolution of the Lerma valley could be extrapolated to other similar valleys in NW Argentina and can be a useful tool for exploration of good quality groundwater. This is of high importance in Latin American territories with high As concentration in groundwater such as Argentina.