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.

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 high­arsenic 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.

Origin and geochemistry of arsenic in surface and groundwaters of Los Pozuelos basin, Puna region, Central Andes, Argentina.

Los Pozuelos is a closed basin in the Puna region of NW Argentina, Central Andes. This is a semi-arid region where closed basins are the most important feature for the hydrologic systems. The center of the basin is occupied by a fluctuating playa lake called Los Pozuelos lagoon, which constitutes a UNESCO Biosphere Reserve. This is one of the most populated closed basins in the Argentinian Puna and residents use groundwater for drinking and cooking. Lowest concentrations of As and dissolved solids are in the headwaters of the rivers (1.46-27 µg/L) and the highest concentrations are in the lagoon (43.7-200.3 µg/L). In groundwater, arsenic concentrations increase from the outer ring aquifer (3.82-29.7 µg/L) composed of alluvial-alluvial fan sediments to the inner lacustrine aquifer (10-113 µg/L) that surround the playa lake. Moreover, high concentrations of As during the dry season (90.2 and 113 µg/L), Na/K mass ratios (0.2 and 0.3), and formation of Na-rich efflorescent salts suggest that high evaporation rates increases As concentration, while rainwater dilutes the concentration during the wet season. As(V) is the dominant species in all the water types, except for the lagoon, where As(III) occasionally dominates because of organic matter buildup. There are at least three potential sources for As in water i) oxidation of As sulfides in Pan de Azúcar mine wastes, and acid mine drainage discharging into the basin; ii) weathering and erosion of mineralized shales; iii) weathering of volcanic eruptive non-mineralized rocks. Because it is a closed basin, the arsenic released from the natural and anthropogenic sources is transported in solution and in fluvial sediments and finally accumulates in the center of the basin where the concentration in water increases by evaporation with occasional enhancement by organic matter interaction in the lagoon.