Ground-water conditions in the Green Bay area, Wisconsin, 1950-60

Abstract

The Green Bay area, which includes parts of Brown, Outagamie, and Shawano Counties, has an area of about 525 square miles in eastern Wisconsin at the south end of Green Bay. In 1960, it had a population estimated at 124,000; Green Bay, the largest city in the area, had a population of 62,888. The Green Bay area is underlain by a basement complex of crystalline rocks of Precambrian age. Sedimentary rocks of Cambrian, Ordovician, and Silurian ages overlie the crystalline rocks. These rocks are divided, in ascending order, as follows: The Dresbach Group, Franconia Sandstone, and Trempealeau Formation of Cambrian age; the Prairie du Chien Group, St. Peter Sandstone, Platteville Formation, and Maquoketa Shale of Ordovician age; and the Niagara Dolomite of Silurian age. The Maquoketa Shale and Niagara Dolomite are present only in the eastern part of the area. Unconsolidated deposits, largely of Pleistocene age and glacial origin, overlie the older rocks in most of the area. The rocks of the Dresbach Group, Franconia Sandstone, Trempealeau Formation, Prairie du Chien Group, and St. Peter Sandstone are connected hydraulically and can be considered to form one aquifer, called the sandstone aquifer. The sandstone aquifer is the principal source of ground-water supply in the Green Bay area and is one of the most productive water-bearing units in Wisconsin. All the public water supplies in the area, except the supply for the city of Green Bay, and many of the industrial water supplies are obtained from wells tapping the sandstone aquifer. Rates of discharge of individual wells range from about 200 to 1,000 gallous per minute. The city of Green Bay also obtained its water supply from wells tapping the sandstone aquifer until August 1957, when it began using Lake Michigan as a source of water supply. Several industries also use large quantities of surface water. The Niagara Dolomite, although largely undeveloped, is potentially an important aquifer, in the eastern part of the area. Small amounts of water are obtained from dolomite of the Platteville Formation and from sand and gravel deposits of Pleistocene age. Recharge to the sandstone aquifer in the Green Bay area is derived chiefly from precipitation that infiltrates at or near the outcrop area of the aquifer in northwestern Brown County, eastern Outagamie and Shawano Counties, and southern Oconto County. The amount of recharge is estimated to be at least 30 mgd (million gallons per day). Withdrawals of water from wells tapping the sandstone aquifer in the area began when the first well was drilled in 1886. The withdrawals gradually increased to an average of about 6 mgd in 1940, about 10 mgd in 1950, and about 13 mgd in January-July of 1957, after which time the city of Green Bay discontinued pumping from wells. From August 1957 through 1960, average annual withdrawals of water remained relatively constant at about 5 mgd. Water levels in wells tapping the sandstone aquifer persistently declined until August 1957 as a result of the gradually increasing withdrawals of water. In the area of concentrated ground-water withdrawals in downtown Green Bay, the piezometric surface, which had been about 100 feet above land surface in 1886, was about 340 feet below land surface in 1957. The cessation of pumping by the city of Green Bay in August 1957 resulted in a decrease in withdrawals of ground water from about 13.1 mgd in the first half of 1957 to about 5.3 mgd in the last half and a rapid recovery in water levels. ]n the area of concentrated withdrawals, the piezometric surface had recovered about 300 feet by September 1960. Rises in water levels were recorded throughout the Green Bay area, with the amount of the rise depending on the distance from the Green Bay city wells. In September 1960, water levels appeared to be affected more by local variations in the rates of pumping than by the recovery resulting from 1957 reduction in pumping. Much additional ground water could