The role of loess weathering in the groundwater chemistry of the Chaco-Pampean Plain (Argentina)

The Chaco-Pampean Plain (CPP), Argentina, is one of the most extensive loess areas in the world, higlhly productive in terms of agriculture and livestock and with a generalized groundwater signature of Na-HCO3 water type. High As concentrations have been often reported in aquifers throughout the entire CPP related to higher pH and alkalinity values. The processes leading to the groundwater from the humid Matanza-Riachuelo Aquifer System (MRAS) to acquire its alkalinity and pH have been investigated by means of detailed sampling of water and solid phases and geochemical modeling (Phreeqc code). The model has also been applied to two extra areas of the CPP with different climates: the semi-arid Rio Dulce Alluvial Aquifer (RDA), and the arid Northern area of the La Pampa province (NLP). The analyses of groundwater from the three different regions (from humid to arid) confirm the positive correlation between arsenic and the high pH and HCO3 concentrations. The model has offered insights into key soil-rock interaction parameters that determine the groundwater chemistry of the CPP. Thus, in addition to the already known calcite precipitation, Ca retention by the volcanic glass exchange complex has appeared as crucial for groundwater to acquire the characteristic Na-HCO3 signature. Moreover, the model has also highlighted the role of low CO2 pressures in attaining the high pH values measured in poorly vegetated soils. Of particular significance is the unexpected role played by the formation of secondary alumino-silicates in increasing (rather than decreasing) pH. Further investigations on the role of volcanic glass in the soil exchange properties and the chemical composition of the exchange complex together with the accurate de- termination of the clay mineralogy are needed to understand the high pH and alkalinity values of groundwater of this vast region.