RECHARGE HISTORY OF THE CAMBRO-ORDOVICIAN AQUIFER OF SE WISCONSIN

A geochemical tracer study of the Cambro-Ordovician aquifer within SE Wisconsin yields insights into the recharge history of this critical water supply aquifer and the glacier-aquifer interplay that occurred during the last glacial maximum. This aquifer contains a large volume of Pleistocene age water that is currently being unsustainably pumped by municipalities in SE Wisconsin. Well transects taken along the flowpath indicate distinct zones within the aquifer that contain water recharged before, during and after the last glacial maximum. Noble gas data in conjunction with stable isotopes and 14C ages indicate a cooling of at least 6.5–7 °C during the last glacial period compared with modern temperatures. Both stable isotope and excess air data indicate that none of the samples contain any significant portions of sub-glacial meltwater. Samples with 14C ages corresponding to the time when the study area was ice covered most likely infiltrated during short periods of ice retreat or represent recharge containing a significant proportion of precipitation rather than subglacial meltwater recharge. Further, all samples except for those recharged before the last glacial maximum, show a strong correlation between noble gas temperature and δ18O derived temperatures. By contrast, δ18O values of samples older than ~28 k.y. BP are too heavy with respect to their noble gas temperatures. This is ascribed to a stronger influence of an isotopically enriched moisture source from the Gulf of Mexico. The amount of excess air, which is closely linked to the magnitude of groundwater table fluctuations, increases shortly before and at the beginning of the last glacial period, suggesting that recharge dynamics changed considerably during that time period.