The Paleozoic rock of Brown County includes formations of Cambrian, Ordovician, and Silurian age. These formations are eastward-dipping sedimentary rock that rest on Precambrian crystalline rock and are overlain by Pleistocene deposits. The units that are the principal sources of ground water were grouped into three aquifers (upper, St. Peter, and Elk Mound), and the less permeable units are grouped into three confining units (Maquoketa-Sinnipee, St. Lawrence, and Precambrian). The geologic and hydraulic characteristics of the aquifers and confining units are estimated from logs of more than 1,000 Brown County wells, from results of a packer test, and from published values.

Recharge to the water table, which was estimated at five monitored sites, ranges from 1 to 6 inches a year; most recharge is contributed by spring snowmelt and rainfall. A cone of depression caused by pumping the deeper aquifers in the Green Bay metropolitan area induces flow from the upper aquifer to the underlying st. Peter aquifer throughout most of the county. Several reaches of Duck Creek and the Suamico River also contribute water to the aquifers. A

bout 13 million gallons per day of ground water was pumped in Brown County during 1979, 63 percent of which was from wells open to both the St. Peter and Elk Mound aquifers. Municipal and industrial water users pumped 9.44 million gallons per day or 72 percent of the ground water withdrawn in 1979.

Most ground :water in the county is a calcium magnesium bicarbonate type. However, water from wells sampled in an area between the Fox River and Silurian escarpment have elevated levels of sodium (44 milligrams per liter) and sulfate (226 milligrams per liter). Water from wells that tap rocks older than Silurian dolomite contains high concentrations of strontium (more than 2.4 milligrams per liter) and fluoride (more than 0.85 milligrams per liter).

A three-dimensional digital model was used to simulate flow in the ground-water system. Model results indicate that sources of ground water pumped from wells tapping the St. Peter and Elk Mound aquifers in Brown County, 1979, include 4.8 million gallons per day of underflow, most of which enters the county across the west border; 1.9 million gallons per day of flow from vertical leakage within the county; and 1.5 million gallons per day from storage. The model is most sensitive to the horizontal hydraulic conductivity of the upper aquifer. Vertical hydraulic conductivity of the confining units and recharge rates to the water-table aquifer are the least well-defined model parameters.