ARSENIC ENRICHMENT IN SHALLOW FLUVIAL AQUIFERS IN CERRITOS BASIN, MEXICO

Cerritos Basin (CB) in central Mexico has elevated (up to 128 µg/L) arsenic (As) concentrations in groundwater. CB is located in the Sierra Madre Oriental physiographical entity with a semi-arid climate. Its hydrogeology is composed of shallow aquifers of heterogeneous fluvial sediments with general flow direction from NW to SE and groundwater levels ranging from 2.5 to 34 m below surface. In total 95 water samples were collected for major and minor ions and trace elements, a sub-set of 47 were analyzed for δ¹⁸O and δ²H. The aim was to assess the As enrichment in the shallow aquifer, its evaluation according to the Mexican and the World Health Organization (WHO) guidelines for drinking water, and its spatial distribution. In total 73% of the sampled sites are within the As Mexican threshold; however, for the WHO guideline, only 55% are within. The As spatial distribution was assessed using Principal Axes Factor. The outcome was 4 factors explaining 61% of the total variance based on the orthogonally rotated solution, setting aside a huge group of samples from limestones surrounding the basin. One factor assigns samples with high As concentrations tapping the basin fill. The highest As concentrations are located in the shallow aquifer at the Cerritos valley. Evaluating the isotopic data and the local meteoric water line, strong secondary evaporation was identified, therefore, deuterium excess (DE) was calculated. The DE showed shallow wells that were classified in 3 groups tapping the basin fill with elevated As concentrations, each one depending on local conditions. Geochemical modeling was done in order to evaluate the evaporation process, considering two assumptions i) the chloride concentration was used as aquifer evaporation indicator; ii) the lowest As concentration for each group was the initial concentration. The modeling showed that in each group adsorption/desorption processes are presented indicating that in addition to the evaporation process they play an important role in the As behavior. The study showed that the shallow aquifer in the CB has high As concentrations above the WHO and Mexican guidelines for drinking water in which the evaporation and adsorption/desorption processes play an important role.