Midwest Paleozoic Carbonate aquifers

Waterfall in Mammoth Cave Nat. Park. Recent rainfall has activated a large waterfall in the so-called Mammoth Dome, cascading water down on the path used by visistors through the cave. (Photo by Dan Doctor)

Disappearing stream, in Kentucky. USGS Hydrologist Chuck Taylor stands next to a stream that enters the subsurface through a cave entrance. (Photo by Chuck Taylor)

A "blue-hole" spring, Orangeville Rise, Indiana. Sixty feet in diameter. (Photo by Chuck Taylor)

Underground stream in Mammoth Cave Nat. Park. USGS scientists observe a stream located at the base of the explorable cave system. (Photo by Dan Doctor)

The porosity of carbonate and dolomitic units in Midwest Paleozoic rocks has been enhanced by dissolution, and in many areas these rocks have undergone extensive karst development. The greatest karst development as occurred in Devonian-Silurian and Cambrian-Ordovician rocks. The vertical sequence of the aquifers and confining units varies across the region.

In Iowa and Michigan, solution-enhanced dissolution of Mississippian dolomitic rocks has resulted in karst flow systems.

Limestones in the Devonian-Silurian aquifer system in Ohio, Indiana, Illinois, Iowa, and Wisconsin have undergone extensive karst development. The limestones outcrop in some areas, and in others are overlain by up to several hundred meters of unconsolidated Quaternary deposit. Throughout this area, large well yields are possible from these units.

Some limestone and dolomites Cambrian-Ordovian rocks underlying the Silurian-Devonian aquifer also have undergone extensive karst development. These are water-producing units in parts of Illinois, Wisconsin, Minnesota and Iowa. In southeastern Minnesota and northern Iowa these are referred to as the upper carbonate aquifer, and is extremely productive.

Because of the prevalence of agriculture and livestock in this part of the United States, contamination of karst aquifers with nutrients, pesticides, and bacteria is of much concern. In Iowa, for example, regulations have been developed for some confined animal feeding operations in karst terrain.