What happens when the confined Cambrian-Ordovician aquifer in SE Wisconsin is “dewatered”?

Background/Need: The Cambrian-Ordovician aquifer has long been an important source of municipal water supply in Wisconsin, and recent trends are of some concern for future supply. Pumping has drawn down the potentiometric surface (hydrostatic pressure) of this deep aquifer system by over 400 ft during the 20^th century. Wisconsin state observation wells show that head in the deep aquifer system continues to decline at a rate of 7 ft/yr and will eventually dip into the top of the St. Peter sandstone, causing dewatering as air enters the pore space. Computer simulations cannot account for reduction in hydraulic conductivity caused by progressively developing unsaturated conditions near the well bore, which forms the principal avenue for air to reach the aquifer. It is not clear exactly what happens under
these conditions in the real world because few observations have been made of this phenomenon in deep aquifers where field data are rarely available.

Objectives: The goal of the research reported here was to investigate possible dewatering phenomena in the Cambrian-Ordovician aquifer system in southeastern Wisconsin. The specific objectives were 1) to investigate how unsaturated conditions might develop in a physical sand-tank model, 2) to attempt to verify the development of such hydrogeologic conditions in the field, and 3) to predict the long-term impact on water supply and quality by observing the evolution of head in the vicinity of model pumping wells.