Understanding arsenic and uranium sources and mobility in groundwater flow systems from two areas in San Luis Potosi, Mexico. Cerritos and Villa de Reyes Basins

Arsenic (As) and Uranium (U) are trace elements present in (ground) waters. Worldwide As and U have been identified in groundwaters with elevated concentrations above the reference levels set by the World Health Organization (WHO) threatening the human health especially in arid to semi-arid areas where the groundwater is the main source not only for drinking water for the population but also for the industry. In addition, water in the vadose zone and shallow groundwater are influenced by evaporation, in particular in these regions. This study deals with geogenic, anthropogenic, and the evaporation effect on As and U concentrations in two regions in the Mexican Altiplano, Cerritos Basin (CB) and the Graben of Villa de Reyes (GVR) in which elevated concentrations of As and U in groundwater have been detected. Even though they are located in different physiographic entities with different, geological, calcareous rocks affected by a mineralized granitic intrusive body characterize the CB and igneous rocks characterize the GVR, and hydrogeological characteristics they have in common elevated As and U concentrations in groundwater. Therefore, the main objective of this study is to understand the processes and dynamics controlling As and U mobility in groundwater flows systems as well as the identification of the primary and secondary sources and the interaction between the geological media and the groundwater. The chemical characterization of the water samples gave that in both areas the pH is in circum-neutral conditions. At the CB 73% of the samples are within the As Mexican threshold for water for human consumption; however, for the (WHO) guideline just 55 % are within. In the case of U, currently the Mexican drinking standard does not include the U in the mandatory elements; however, 96% of the samples in the area are within the WHO guideline. At the GVR all the samples are within the As Mexican threshold and 36% are within the WHO guidelines. For U 97% of the samples are within the WHO critical value. The evaluation of the water types in the CB showed the influence of the in the calcareous units being the Ca-HCO${}_3$ the main water type and at the south the wells tapping the gypsum units the main water type was the Ca-SO${}_4$. On the other hand, at the GVR the main water type was the Na-HCO${}_3$ with temperatures up to 42.4 °C and is distributed along the GVR tapping the basin fill sediments in which the highest As concentrations were found. One of the processes identified by Nicolli et al. (2010) for As and U enrichment in shallow groundwater is the evaporation process.