MANGANESE-BEARING DOLOMITE DISSOLUTION DRIVES HEXAVALENT CHROMIUM OCCURRENCE IN THE CENTRAL OKLAHOMA AQUIFER

Municipalities in central Oklahoma rely increasingly on water drawn from the Central Oklahoma Aquifer (COA) as surface water resources have not scaled to current demands. The primary aquifer unit in the COA is the Permian Garber-Wellington formation, which consists of sequences of sandstone, mudstone, and dolostone conglomerate. While water quality in the COA is generally good, groundwater drawn from certain regions frequently contains elevated levels of naturally occurring hexavalent chromium. We analyzed groundwater parameters obtained from the National Water Information System (NWIS) to evaluate the geochemical conditions associated with high Cr levels in the COA. In addition, bulk mineralogy and chemistry, selective chemical extractions, and microscale elemental analyses were performed on samples from the USGS/EPA Norman Arsenic Test Hole Core (NATHC) to further investigate the mineralogic hosts of Cr in the COA. Cr concentrations above 50 ppb were found in waters with Ca/Mg between 1.1 and 1.5 regardless of pH. Geochemical analyses on core samples indicate that most COA Cr is contained in Fe oxides and clays as isomorphic substitutions for Fe(III). Dolomite in the COA contained up to one weight % Mn. Furthermore, bulk chromium in core samples was inversely correlated with Mn-oxide content (r = -0.883, p<0.01). Here, we propose a release mechanism involving a sequence of coupled mineral-water interactions along hydrologic flow paths during aquifer recharge. Dissolution of Mn-bearing dolomite raises pH and releases Mn, leading to precipitation of Mn(III/IV) oxide minerals. These processes result in conditions that favor both anion desorption and Cr oxidation. The conversion of Cr(III) to Cr(VI) on the surfaces of resulting Mn oxides is hydrologically limited, occurring to the greatest extent in the most permeable rocks. Overall, this work emphasizes the importance of carbonate dissolution/precipitation on trace metal cycling and introduces Ca/Mg as a useful predictor of elevated Cr levels in the COA.