INCREASING ARSENIC AND FLUORIDE CONCENTRATIONS IN AN INTENSIVELY PUMPED AQUIFER SYSTEM IN CENTRAL MEXICO

Arsenic (As) and fluoride (F) are geogenic contaminants which occur in aquifers in a wide range of geologic and climate settings. International agencies recommend screening all ground water used for drinking for As and F (WHO/UNICEF Sustainable Development Goals, 2015-2030). Sampling, however, is typically only performed once because of a prevailing view that As and F concentrations are stable. Recent studies, however, question this view. The objective of this study is to explore the extent and causes of observed increases in As and F concentrations by comparing changes in the spatial distributions of As and F across two decades within an intensively pumped aquifer system, called the Independence Aquifer (AI) in Mexico. The findings may be a harbinger for analogous aquifer systems around the world. We hypothesize that increases in As and F will be greatest in places with the most intense groundwater pumping. Arsenic increases may be caused by near-surface processes such as the oxidative weathering of sulphide-bearing minerals in the vadose zone or reductive dissolution of iron oxides from wastewater infiltration. Fluoride increases may be caused by fertilizers on farmland. Finally, upwelling of deeper, more mineralized and possibly even geothermal groundwater may cause both to increase. Major ions, trace metals and water isotopes were compared between 246 groundwater samples taken in 1999-2000 and 36 samples taken in 2015-2016. In 2000 and 2016, 8 (93/246) and 28 (7/26) percent of wells exceeded the Mexican As drinking water limit of 25 µg/L, respectively. During the same years, 8 (8/99) and 39 (14/36) percent of wells exceeded the Mexican F drinking water limit of 1.5 mg/L, respectively. Changes in the regional patterns of water isotopes in the AI between 2000 and 2016 suggest enhanced downward flow of shallow groundwater into the deeper aquifer in the center of the basin. On the other hand, a positive correlation between lithium and F implicate upwelling of geothermal waters. Multivariate and geochemical modeling were used to identify which minerals may be causing the increases in As and F concentrations over time. Future work includes sampling 60 of the same wells sampled in 2000 and developing a logistic regression model to predict which areas of the AI are likely to experience increasing As and F over coming decades.