DEVONIAN CARBONATE GROUNDWATER CHEMISTRY IN NORTHEASTERN MICHIGAN: A GEOCHEMICAL RECORD OF FOCUSED FRESHWATER RECHARGE TO THE MICHIGAN BASIN

The presence of a continental ice sheet over the northern margin of the Michigan Basin during the Pleistocene deeply affected the regional carbonate groundwater flow system. Significant amounts of glacial meltwater were recharged into the extensive Devonian carbonate subcrop exposed at the surface along the basin margin during the multiple advances and retreats of the Laurentide ice sheet. Continuing to the present, these Devonian carbonates provide a broad avenue for meteoric water recharge into the basin, both directly at the surface and through varying thicknesses and permeabilities of overlying glacial drift.

Devonian carbonate aquifers are exposed at the surface or beneath permeable glacial drift in much of northeastern Michigan. South of the northern basin margin, the same carbonate strata are confined by the Antrim and younger Mississippian shales. In turn, near the Lake Huron shoreline, meteoric recharge into the carbonates is restricted by impermeable Pleistocene-age lake deposits, which overlie the Devonian carbonates. To make the flow system even more complicated, much of the near-surface subcrop is karsted, and Devonian units beneath it contain localized lenses of evaporites. Each of these hydrogeologic factors contributes to changes in the groundwater chemistry laterally and with depth below the subcrop.

How has the carbonate groundwater system in the northeastern Michigan Basin evolved since the Pleistocene? What have been the primary sources of freshwater recharge to the carbonates in the basin? How does a karsted terrain at a northern latitude affect the geochemistry of the groundwaters? These are the central research questions that our group has focused on. In order to approach these problems from various angles, we have extensively sampled Devonian carbonate groundwaters and overlying glacial drift waters from household wells in northeastern section of the basin. We chose several transects that would allow us to constrain the changes in carbonate groundwater chemistry, both with depth and laterally. The major and minor ion and isotope chemistry of the waters serve as effective indicators of the source and subsequent evolution of groundwater in the Michigan Basin, and thus help to complete our historical record of freshwater recharge to the basin since the Pleistocene glaciation.