Changing Ground Water Supply Options in Southeastern Wisconsin

摘要

Southeastern Wisconsin consists of a seven county area that includes Milwaukee and the surrounding metropolitan area. The area comprises only about 5% of the land mass of Wisconsin, but contains over 37% of the state's population and produces a significant amount of the state's economic output. Most municipal water in the region comes from several surface water plants along the Lake Michigan shore. These plants supply 208 million gallons per day (mgd) to the major urban centers adjacent to the lake. Approximately 62% of the land area of southeastern Wisconsin lies west of the subcontinental divide that separates the Lake Michigan Basin and the Mississippi River Basin. A compact between the Great Lake States and Canada prevents any new diversion of surface water of the Great Lakes Basin. This prevents most of the fast growing suburban communities from receiving Lake Michigan water. A total of 93 mgd of ground water is pumped for water supply by fifty municipal systems, most of which tie on the western side of the subcontinental divide.rnThe major ground water source has been the deep sandstone aquifer. The deep sandstone aquifer consists of a series of Cambrian and Ordovician sandstone, shale and dolomite units. The sandstone aquifer is between 500 and 2500 feet thick in the region. It is recharged by surface precipitation in the extreme western portion of the region, but is highly confined throughout the rest of southeastern Wisconsin. The sandstone aquifer has produced the vast majority of municipal supply for ground water systems for over 70 years.rnDecades of over pumpage have created a cone of depression in the sandstone aquifer over 500 feet below original predevelopment heads. Over the last 20 years, the potentiometric head has been declining by over 10 feet a year in the center of the cone and by over 5 feet a year over much of southeastern Wisconsin. Water quality of sandstone aquifer wells has begun to change in the area of heaviest pumpage. Total dissolved solids and gross alpha levels have risen sharply in several wells are beginning to experience problems with nuisance bacteria as a result. These problems all appear to be related with prolonged overdraft of the aquifer.rnIn response to problems with the sandstone aquifer, municipalities have begun to develop two shallow unconfined aquifers that are present in most of the region. These aquifers receive more recharge than the sandstone aquifer and are believed to have a higher sustainable yield. However, these wells are harder to site due to higher vulnerability to contamination and wide variations in well yield. In addition, over development could harm surface water features. In spite of these difficulties, these aquifers will probably be used to reduce the demand on the sandstone aquifer and provide most of the new capacity.rnWhile the current water supply is sufficient for immediate and near term needs, significant changes will have to occur to prevent areas of shortage in the future. Recently, there have been several positive steps toward rational regional water resource planning. A regional ground water model is being completed by a consortium of state and federal agencies. The model is directed