Chemical and isotopic indicators of groundwater evolution in the basal sands of a buried bedrock valley in the midwestern United States: Implications for recharge, rock-water interactions, and mixing

Buried bedrock valley aquifers can be found throughout Canada and the northern United States where glacial deposits have filled in previously exposed bedrock valleys. The Mahomet bedrock valley is an east-west trending buried valley in central Illinois containing basal Pleistocene sands and gravels making up the Mahomet aquifer and the contemporaneous Sankoty Mahomet aquifer, which are the major sources of freshwater for east-central Illinois. The hydrochemical characteristics of the Mahomet and Sankoty Mahomet aquifers change significantly across the buried bedrock valley. To determine the geochemical processes controlling the chemistry of the water, possible groundwater mixing, and the regions of major recharge, over 80 samples from the Mahomet aquifer, the Sankoty Mahomet aquifer, and shallower aquifers were analyzed for their chemical and isotopic composition, including delta(18)O, delta D, delta(13)C, delta(34)S, (14)C, and (3)H. Four geochemical regions were observed across the aquifers. The central and eastern region of the Mahomet aquifer had dilute chemistry and medium (14)C activities, suggesting relatively recent recharge from the surface. The northeastern Mahomet aquifer region had variable sulfate and delta(34)S values, medium chloride concentrations, and low (14)C activity, suggesting mixing with bedrock groundwater along with sulfate reduction. The western Mahomet aquifer region had the highest chloride, dissolved organic carbon, and methane concentrations and showed a continuous decrease in (14)C activity, suggesting seepage from bedrock units, strong reducing conditions, and isolation from surficial recharge. Characteristics of the Sankoty Mahomet aquifer indicated rapid freshwater recharge and mixing with western Mahomet aquifer water. The delta D and delta(18)O values indicated little to no Pleistocene water in the Mahomet bedrock valley aquifer system, suggesting an age limit of ca. 11,000 yr B.P. for most of the groundwater. The tritium data indicated modern recharge in some shallower aquifers, but little to none in the Mahomet aquifer and Sankoty Mahomet aquifer, except near a river where stacked sands may have created a hydrologic window to the Mahomet aquifer. It appears that most of the Mahomet aquifer is well protected from surficial contamination. The approach used in this study enabled us to better understand and identify the processes that control the groundwater chemistry within the buried Pliestocene aquifer in central Illinois; processes that may be prevalent in other buried bedrock valley aquifers distributed throughout much of North America.