LINKS BETWEEN CA/MG MOLAR RATIOS AND CR(VI) DISTRIBUTION IN THE CENTRAL OKLAHOMA AQUIFER

The need to obtain fresh drinking water in the Central Oklahoma Aquifer (COA) grows as regional water demand outpaces increases in surface water resources. However, groundwater drawn from the COA frequently contains elevated levels of naturally occurring hexavalent chromium (Cr(VI)), leading to some of the highest regional water system Cr(VI) contents as measured in national surveys. Our research team investigated chromium cycling processes in the Central Oklahoma Aquifer through field collection of colloids from groundwater, experiments with outcrop materials, and mineralogical/ geochemical investigations of sediment core samples. We further analyzed both depth-specific and mixed-depth groundwater chemical data to explain Cr distribution in the context of regional groundwater chemistry. We created thermodynamic models to explain Cr(VI) occurrence in terms of water-rock interactions. Our conceptual model for Cr(VI) occurrence involves a sequence of mineral-water interactions that occur along hydrologic flowpaths during aquifer recharge. Dissolution of Mn(II)-bearing dolomite raises pH and leads to production of Mn(III/IV) oxide minerals, leading to both Cr(III) oxidation and Cr(VI) anion desorption. We noted Cr concentrations above 50 ppb were found in waters with Ca/Mg ratios between 1.1 and 1.5 regardless of pH due to carbonate mineral dissolution/ solubility equilibria, while low Cr waters had Ca/Mg ratios greater than 1.5. However, these trends emerged only when using data associated with a single depth of collection within the aquifer. Most wells in the COA include multiple perforations or screened intervals that allow mixing of groundwater from multiple sandy layers. On Piper plots, groundwaters with the >60 ppb Cr project to distinctly lower Ca/Mg molar ratios than groundwaters containing elevated As rather than Cr. This work emphasizes the importance of carbonate dissolution and equilibria on trace metal cycling and introduces Ca/Mg molar ratios in depth-specific groundwater geochemistry as a predictor of elevated Cr levels in the COA.