Upward Trends and Lithological and Climatic Controls of Groundwater Arsenic, Fluoride, and Nitrate in Central Mexico

Abstract

Central Mexico is known for its high concentrations of geogenic arsenic (As) and fluoride (F−) in the groundwater; however, concentrations vary widely within the region. To identify specific hydrogeological processes that cause these variations, the study area was divided into four sections, each section with a particular lithology, climate, and land use. Nitrate was added to the analysis as a common anthropic contaminant in this area as one that is indicative of human and agricultural activities. Concentration maps, Na-normalized diagrams, Spearman correlation, and upward trend analyses were applied to 77 wells distributed across the four sections. Specific patterns of concentration emerged according to climate and the lithology of the exposed rocks. A sharp reduction of F− concentrations in the section where carbonate rocks outcrop suggested co-precipitation of F− with calcite. The Mann–Kendall method detected upward trends in 5 out of 54 wells for As and NO3–N and three for F− at a 95% probability level. Several wells with upward trends of As and NO3–N overlapped. Only one well showed a downward trend for NO3–N. The results show the degree to which lithology and climate affect groundwater quality, information that leads to a better understanding of the processes (and health hazards) that govern As, F−, and NO3–N concentrations, which could be construed to include the potential effect of human activities such as overfertilization and altering groundwater residence time via groundwater withdrawals.