Natural occurrence and controls of arsenic in groundwater in a semiarid basin in the Mexican Altiplano

The sources, mobility, and controlling processes of arsenic (As) in an aquifer system in the Cerritos-Guadalcazar area of Central Mexico were investigated. High As concentrations (up to 128 mu g/L) in groundwater were found to be associated with both primary sources (rocks and tailings) and secondary sources (soil and sediments). The high As concentrations in the otherwise pristine Cerritos shallow granular aquifer are the result of a distant source of As-rich sediments eroded from a mineralized area in the recharge zone of a karstic aquifer. An intermediate groundwater flow system delivers the As load from the karstic to the granular aquifer. In addition, identification of a Na-Cl water type in the shallow granular aquifer indicates a regional flow; however, additional information is needed to corroborate this assumption. Local flows in the karstic aquifer are Ca-HCO3 water type with low (<1 mu g/L) dissolved As concentrations. Principle axis factor analysis shows distinct groupings associated with different aquifer characteristics. The sulfate factor dominates in the southern area of the granular aquifer, which explains 30% of the total variance. The arsenic-uranium factor is expressed in samples from deep and dug wells tapping the basin sediments. A conceptual model was developed where the path of the As is shown. Three groups of samples showed negative correlation among deuterium excess and As concentrations. Evaporation modeling to simulate the As enrichment showed that evaporation processes play an important role in As behavior. This research can provide a tool for further studies in the Sierra Madre Oriental.