The ground-water system in the Twin Cities Metropolitan Area includes five aquifers; two of these aquifers the Prairie du Chien-Jordan and the Mount Simon-Hinckley supply about 80 percent and 10 percent, respectively, of the ground water pumped for public supply. Water levels and changes in water levels in these two aquifers differ greatly in the Twin Cities Metropolitan Area. The Mississippi, Minnesota, and St. Croix Rivers are in hydraulic connection with and influence the pattern of flow in the upper aquifer, the Prairie du Chien-Jordan. Water generally flows toward these streams from water-level highs northeast, northwest, and south of Minneapolis and St. Paul. Consequently, heavy pumping has caused only localized cones of depression in the potentiometric surface of this aquifer. In contrast, the Mount Simon-Hinckley, which has only a slight hydraulic connection to the streams, is greatly influenced by pumping. Pumping in the urban centers of Minneapolis and St. Paul has caused a large cone of depression in the Mount Simon-Hinckley potentiometric surface. During 1971, the measurable cone was centered in eastcentral Hennepin County, was about 25 miles in diameter, and was as much as 150 feet deep at its center.

Between 1971 and 1980, average water levels in the Prairie du Chien-Jordan aquifer changed less than 5 feet in most of the study area, but rose or declined as much as 25 feet locally in response to pumpage and recharge. During this period, seasonal declines of water levels from winter to summer lessened, and the area where these declines exceeded 10 feet decreased.

In contrast, between 1971 and 1980, average water levels in the Mount Simon-Hinckley aquifer rose as much as 60 feet in the center of the cone of depression in response to decreased pumping. Also, the measurable cone of depression contracted from about 25 miles to about 15 miles in diameter. However, because of increased summer pumping due to below-average precipitation during 1980, seasonal water levels declined much more and over a wider area during 1980 than in 1971.

Water-level data suggest that (1) little variation in annual pumpage between 1971 and 1980 from the Prairie du Chien-Jordan aquifer produced generally stable water levels in that aquifer, (2) decreased annual pumpage from the Mount Simon-Hinckley aquifer from 1971 to 1980 caused water levels in that aquifer to rise, and (3) a greater seasonal component of pumpage from the Mount Simon-Hinckley aquifer than from the Prairie du Chien-Jordan produced larger and more widespread seasonal water-level declines in the Mount Simon-Hinckley than in the Prairie du Chien-Jordan, particularly during dry years.