SORPTION CONTROLLED ARSENIC MOBILITY IN GROUNDWATER OF THE ZIMAPÁN VALLEY, MEXICO

High arsenic concentrations (ranging > 1 mg/L) are present in groundwater of the Zimapán Valley. Located in central Mexico, this valley hosts ore deposits that have been mined historically for lead, silver, and zinc. Groundwater contamination by arsenic has been previously attributed to the dissolution of arsenic-bearing minerals naturally present in mineralized zones of the highly fractured cretaceous limestone aquifer. Secondary sources include the dissolution and leaching of mine tailings and deposition of arsenic-rich smelter emissions. Although arsenopyrite oxidation and scodorite dissolution/precipitation reactions may be important sources for arsenic in the valley, several geochemical lines of evidence point toward adsorption/desorption as an important process controlling subsequent arsenic mobility. Wells with the highest arsenic concentrations also contained the most reduced water in the valley. Arsenic adsorption is redox sensitive, and arsenic is well known to desorb and mobilize at more reducing conditions. In addition, neither increases in SO₄^- nor decreases in pH were correlated with increasing arsenic concentrations, as would be expected if arsenopyrite oxidation were the sole mechanism responsible for the release and transport of arsenic. Finally, a number of adsorbent minerals, such as Fe(OH)₃, PbCO₃, and ZnCO₃, are stable in groundwater of the Zimapán Valley. The objective of this study is to better understand the role of sorption processes controlling arsenic mobility within groundwater of the Zimapán Valley. Using PHREEQC, inverse geochemical modeling with surface complexation was performed. Changes in arsenic mass were attributed to adsorption/desorption of iron oxyhydroxides and carbonate minerals, and as the result of alteration of arsenic-sulfide minerals. Preliminary results indicate both processes likely occur concurrently and sorption processes play a significant role in arsenic mobility of the groundwater system.