A chemical and isotopic investigation of the groundwater in the Mahomet Bedrock Valley Aquifer: Age, recharge and geochemical evolution of the groundwater

The Mahomet Aquifer (MA) is a major source of freshwater for most of east-central Illinois. Increased use over the past decade has raised questions concerning how vulnerable the aquifer is to degradation and depletion. To help address these concerns the present study was conducted to determine the groundwater age, the geochemical processes controlling the water chemistry and regions of major recharge. Over 40 samples from the MA and overlying aquifers were analyzed for their chemical and isotopic composition, including δ 18O, δD, δ13C, δ34S, 14C, and 3H. Radiocarbon analyses were performed on dissolved inorganic carbon, dissolved organic carbon and methane. Results of this study indicate that the evolution of groundwater in the MA has been complex.; There are three distinct geochemical regions of the MA. The central and eastern regions are the most dilute and have the highest 14C activities, suggesting this is the area of most recent surficial recharge. The northeastern region has a reversed hydraulic head, the highest sulfate concentrations, intermediate chloride concentrations and low 14C activity, suggesting mixing with bedrock groundwater. The western region has the highest chloride, dissolved organic carbon and methane concentrations and a continuous westerly decrease in 14C activity, suggesting seepage from bedrock units, highly reducing conditions stimulating methanogenesis and isolation from surficial recharge.; Corrected 14C data for DIC from groundwater throughout the Mahomet Bedrock valley resulted in ages primarily ranging from modern to 12,000 radiocarbon years before present (RCYBP). The youngest ages in the MA, approximately 900 to 3000 RCYBP, were located in the central and eastern parts of the aquifer.; Corrected 14C data from the DOC (fulvic acid) did not give the same ages as calculated from the DIC results but did show some similar trends. A major complication for correcting the fulvic acid 14C data is determining how much dilution has occurred due to dissolution of buried peat and paleosols in the glacial till deposits.; The combined chemical and isotopic analyses on inorganic and organic constituents resulted in a conceptual model explaining the variable geochemical characteristics across the MA. The 14C data indicates that the MA is well protected from surficial contamination.