T328 Using Natural and Anthropogenic Tracers to Determine Recharge and Residence Times within Arid to Semi-Arid Inter-Montane Aquifer Systems

Wednesday, 25 October 2017
1:30 PM - 5:30 PM
Room 618/619/620

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

Paper number: T328-14
Program: Technical Sessions
Time: 4:55 PM – 5:10 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-300529

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