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... Full description

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Bibliographic Details
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doi: 10.1130/B26574.1
Authors:Hackley, K.C.; Panno, S.V.; Anderson, T.F.
Volume Title:Geological Society of America Bulletin
Source:Geological Society of America Bulletin, 122(7-8), p.1047-1066. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7606
Publication Date:2010
Note:In English. With GSA Data Repository Item 2009286. 92 refs.; illus., incl. sects., geol. sketch map
Subjects:Aquifers; Bedrock; Buried valleys; C-14; Carbon; Cenozoic; Clastic sediments; D/H; Deuterium; Experimental studies; Geochemistry; Ground water; Hydrochemistry; Hydrogen; Isotope ratios; Isotopes; Mixing; O-18/O-16; Oxygen; Paleohydrology; Pleistocene; Quaternary; Radioactive isotopes; Recharge; S-34/S-32; Sand; Sediments; Shallow aquifers; Stable isotopes; Sulfur; Tritium; Water-rock interaction; Illinois; Midwest; United States; Central Illinois; Mahomet Aquifer; Sankoty Mahomet Aquifer
Coordinates:N400000 N410000 W0873000 W0900000
Record ID:2010050185
Copyright Information:GeoRef, Copyright 2020 American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States, Reference includes data supplied by the Geological Society of America
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Description
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 δ18O, δD, δ13C, δ34S, 14C, and 3H. Four geochemical regions were observed across the aquifers. The central and eastern region of the Mahomet aquifer had dilute chemistry and medium 14C activities, suggesting relatively recent recharge from the surface. The northeastern Mahomet aquifer region had variable sulfate and δ34S values, medium chloride concentrations, and low 14C 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 14C 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 δD and δ18O 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 Pleistocene aquifer in central Illinois; processes that may be prevalent in other buried bedrock valley aquifers distributed throughout much of North America.