Identifying and quantifying geochemical and mixing processes in the Matanza-Riachuelo Aquifer System, Argentina.

The Matanza-Riachuelo River Basin, in the Northeast of the Buenos Aires Province, is one of the most industrialized and populated region in Argentina and it is worldwide known for its alarming environmental degradation. In order to prevent further damages, the aquifer system, which consists of two overlaid aquifers, is being monitored from 2008 by the river basin authority, Autoridad de la Cuenca Matanza-Riachuelo. The groundwater chemical baseline has been established in a previous paper (Zabala et al., 2016), and this one is devoted to the identification of the main physical and hydrogeochemical processes that control groundwater chemistry and its areal distribution. Thirty five representative groundwater samples from the Upper Aquifer and thirty four from the deep Puelche Aquifer have been studied with a multi-tool approach to understand the origin of their chemical and isotopic values. The resulting conceptual model has been validated though hydrogeochemical modeling. Most of the aquifer system has fresh groundwater, but some areas have brackish and salt groundwater. Water recharging the Upper Aquifer is of the Ca-HCO3 type as a result of soil CO2 and carbonate dissolution. Evapotranspiration plays a great role concentrating recharge water. After recharge, groundwater becomes Na-HCO3 , mostly due to cation exchange with Na release and Ca uptake, which induces calcite dissolution. Saline groundwaters exist in the lower and upper sectors of the basin as a result of Na-HCO3 water mixing with marine water of different origins. In the upper reaches, besides mixing with connate sea water other sources of SO4 exist, most probably gypsum and/or sulfides. This work highlights the relevance of performing detailed studies to understand the processes controlling groundwater chemistry at regional scale. Moreover, it is a step forward in the knowledge of the aquifer system, and provides a sound scientific basis to design effective management programs and recovery plans.