Project Packet - Laserfiche WebLink

Home Browse Search

natural points of discharge for ground water in the Prairie du Chien -Jordan aquifer. Water -level contours on maps ;„ B, and C on plate 1 that locally are irregularly shaped or closed indicate that major pumping centers disrupt the natural flow pattern in the Prairie du Chien -Jordan aquifer by diverting ground water enroute to the major streams. In some areas, such as near the depression in the water -level surface in southwestern Ramsey County (pl. lA and 1B), pumping may have reversed the natural direction of flow and caused water from the Mississippi River to enter the aquifer. However, despite heavy pumpage that averaged about 154 Mgal/d from 1976 through 1979 (Horn, 1983, table 7 , the water -level surface of the Prairie du Chien -Jordan aquifer has no large cones of depression. This indicates a highly transmissive aquifer (Woodward, 1984) that is in good hydraulic connection with the overlying drift and the major streams. Water -level changes in the Prairie du Chien -Jordan aquifer between 1971 and 1980 (pl. 1E) were detertr`ned by comparing water levels measured during January -March 1971 (pl. UU with chose measured during January -February 1980 (pl. 1B). Water levels measured during winter were chosen for this comparison because they seem to represent "average" annual water levels and because water levels are fairly stable during the winter months. Plate lE shows that water levels in different parts of the Prairie du Chien -Jordan aquifer rose as much as 20 ft and declined as much as 30 ft between winter 1971 and winter 1980. Water levels rose more than 5 ft in eastern Dakota County south of the junction of the St. Croix and Mississippi Rivers, throuyl"ut most of Washington County, in central Ramsey County, and in eastern Hennepin County. Water levels declined more than 5 ft in central Dakota County and in central Hennepin County. The greatest water -level changes are centered around single wells, and reflect reductions or increases in ground -water pumpage by .industries and municipal] ities. For example, the rise in water levels in northeastern Dakota County near the Mississippi River is due to reduced pumpage by the meat -packaging industry in that area (Horn, 1983, fig. 1). Also, the water -level declines in asatral Fmimpin County are probably cue to increased pumping from the Prairie du Chien -Jordan aquifer by suburban municipalities west of Minneapol i a The general rise in water levels in Washingtron County is probably related to long- term climatic trends, rather than changes in pumping. This phenomenon is the object of studies by Federal, State, and local agencies because of its possible relationship to high lake levels in the area (D. C. Gillies, U.S. Geological Survey, St. Paul, Minn., oral common., 1983). Althouglh rater lr4 els cvenged si gnif i cant ly in certain parts of the 'IM iM Cities area between 1971 and 1990 (pl. 1E), hbere was no region -wide trend for the Prairie du Chierw-Jordan aquifer. This is not surprising considering that average daily pumpage from the aquifer remained fairly constant for 197D-79, averaging .152.7 Mgal. Table 1 below shows average daily pumpage for each year and indicates that pumpage peaked in 1974 arxi has declined singe then. 9