Occurrence, distribution, hydrochemistry and origin of arsenic, fluoride and other trace elements dissolved in groundwater at basin scale in central Mexico

Abstract

The occurrence, distribution and origin of total inorganic arsenic (iAs), fluoride (F ^–) and other trace elements (Se, Sb, V, Cr, Mo, Re, Pb, Cu, Cd, Co y Zn) in the Independencia basin (IB) are investigated through hydrogeologic, hydrochemical and isotopic techniques. This basin, with an area of 7000 km², is tributary to the continental Lerma-Chapala basin of central Mexico. Analyses of groundwater samples from 246 wells, in both granular and fractured aquifers, indicate high iAs (0.025–0.120 mg/L) and F ^– (1.5–16 mg/L) concentrations in the northeastern part of the basin, in an area of about 500 km². Thirty three water samples from this area, where a regional drawdown cone has developed, indicate that the origin and hydrochemistry of iAs and F ^– are associated to high concentrations of HCO₃^– and Na⁺, mainly derived from the dissolution of sodium feldspar and other minerals present in the rhyolite-ignimbrite rocks that form the fractured aquifer, with long groundwater residence times up to 35,000 years. Dissolution of arsenic minerals is a secondary processes for iAs enrichment in groundwater, whereas F ^– enrichment is related to dissolution of fluorite, high concentrations of Li⁺ , Cl ^–, Cs, Br ^–, and thermal water. The abundance of iAs and F ^– does not correlate with that of heavy metalloids or oxide-forming elements, neither with other trace elements in solution. Stable isotopes, in 125 samples, indicated a meteoric origin of the groundwater, variable altitude of the recharge, and that water was unaffected by evaporation during recharge, including those with ages in the range of thousands of years of residence time in the aquifer. Besides, there are no evidence that the basin was closed throughout its geologic evolution. If criteria and policies for groundwater management are not established for the IB, the size of the areas impacted by high concentrations of iAs and F ^– may increase with time, which would affect the health of the inhabitants and the environmental and economic sustainability of the basin.