Project Packet - Laserfiche WebLink

Home Browse Search

have had measuring -point altitudes surveyed to withi- 0.1 ft. Measuring -point altitudes for the remaining wells were estimated fL,xn topographic maps to an accuracy within 5 to 10 ft. The water -level altitudes used for the maps in this report, therefore, are accurate to within 5 to 10 ft, and adequately reflect regional ground -water flow. If detailed water -level data for a small local area are needed, the records of wells in that area need to be examined The accuracy of water -level data from individual wells in the network also may differ because of well --construction methods. Most wells in the network are open to the full thickness of the individual aquifer; these water levels repre- sent the average water level for the aquifer. However, some wells are open only to part of the aquifer; water levels in these wells represent only the average water level for the section to which the well is open and may differ from the average water level of the aquifer at the same point. However, water Levels from wells near each other but completed to different depths are simi- lar. This indicates nearly vertical equipotentials within the aquifers, sug- gesting that the water levels used in this report represent the altitude to which static water would rise in each aquifer. A measurement strategy was form .fated to obtain a regional picture of water levels and to investigate long-term and seasonal water -level changes. The strategy has four basic features. First, recognizi.ig that all wells should be measured simultaneously to represent the water -level surface at an instant of time, measurements were made over as short a time as possible (about 3 weeks). Secondly, measurements were made so tnat all wells within a pumping center were measured within a day or two, rather than over the 3-week period Thirdly, municipal and other high -capacity wells were turned off the night before water -level measuuensents were made so that localized, short-term effects of pumping were minimized. Finally, water 7wels were measured twice during the year, once during winter (January -February, when pumping was at a minimum and once during summer (August) when pumping was near maximum. In the Twin C_ties area, ground -water withdrawals in summer are commonly more than twice the withdrawals in winter because of use of ground water for air conditioning, lawn sprinklinq, and crop irrigation (Horn, 1983). IFMW LEVE S AND 11=t--•iZVISL. (WAM IN IM PMURN W ®I AQIIM The maps on plat- 1 (A, B, and C) show that water levels in the Prairie du Chien -Jordan aquifer are highest (more than 900 ft above sea level) in nortt�Prn waah,r.gton County, central Hennepin County, and southern Scott and Dakota Counties. The lowest water levels (less than 700 ft above sea level) occur along the Mississippi River where the river leaves the southeastern corner of the study area. The water -level contours indicate that ground water generally flows from the water -level highs toward the major streams, the Mississippi, Minnesota, and St. Croix Rivers. The water -level contours also indicate that water flows toward some smaller streams, such as the Vermillion and Cannon Rivers in the southeastern part of the study area. This general flow pattern indicates that the major streams in the Twin Cities area are the principal. 7