INCREASING FLUORIDE CONCENTRATIONS AND CHANGING RECHARGE PATHWAYS WITHIN AN INTENSIVELY PUMPED AQUIFER IN CENTRAL MEXICO

Fluoride (F), naturally found in aquifers around the world at toxic concentrations, causes disease in millions of people. The long-term stability, however, of those concentrations within intensively pumped aquifers is poorly characterized. We assessed long-term changes in the spatial distribution of F concentrations in an intensively pumped aquifer within the semi-arid, inter-montane Independence Basin in central Mexico between 1999 and 2016. Although generally stable across the basin, F concentrations increased in some localities by as much as 4 mg/L. Changes in recharge pathways to the deep aquifer were identified by analyzing changes in δ²H, δ¹⁸O and Cl/Br mass ratios. In 1999, δ²H and δ¹⁸O values suggested the aquifer was recharged in the mountains. In 2016, however, substantial increases in δ¹⁸O values in the center of the basin suggest recharge water is derived from rainfall that had experienced increased rates of evaporation. In 1999, the mass ratio Cl/Br in groundwater was consistently ~90, matching local rainfall. In 2016, however, this ratio varied greatly suggesting input from irrigation return flow and urban sewage. Step-wise multiple regression was used to quantify the effect of physical and chemical parameters on F concentrations. In 1999, Li (6.8±1.7) and Na (0.01±0.004) drove F concentrations (R²=0.54). In 2016, Na (0.013±0.0018), HCO₃ (0.004±0.001), Ca (-0.0018±0.00045), and Mg (-0.055±0.023) drove F concentrations (0.78). Irrigation pumping and urban expansion within inter-montane basins drive mixing of disparate groundwater chemistries. Further, pumping introduces new sources of recharge to aquifers inducing changes in aquifer chemistry including increasing concentrations of geogenic toxic elements.