ASSESSMENT OF GROUND WATER SUPPLIES ADJOINING THE GREAT LAKES WATERSHED

In southeastern Wisconsin, 500,000 people outside the Great Lakes watershed rely on ground water as their sole source. Historical use has not considered sustainability or the impacts of continued exploitation. The result is rapidly declining water levels in the confined Cambro-Ordovician aquifer, indicating ground water overdraft and escalating pumping costs. Alternative sources of water are being explored.

The regional planning commission is examining development options seeking those that are cost-effective, and which minimize impacts to surface water baseflows and reduce drawdowns. Among those under consideration are:

1. Shifting extraction from the confined aquifer to an unconfined system,

2. Diverting water from Lake Michigan outside the Great Lakes watershed, and

3. Managing the ground water system by reducing demand and augmenting recharge.

While all will reduce drawdown in the confined aquifer, each alternative will have consequences extending beyond simple recovery of water levels.

In order to compare the viability of different options, quantitative measures of impacts and sustainability, in addition to drawdown, cost and legality, are needed. Three indices which compare ground-water budget components derived from a regional flow model will be presented:

1. The ratio of demand to natural supply provides a measure of sustainability,

2. A ratio which shows the relative impact of human activity on the ground water budget,

3. A measure of the reduction of ground water discharge to surface water bodies.

The first two are modifications of ratios developed by Weiskel, et al (Water Res Res, 43(4)).

These indices are defined and their historical distribution in southeast Wisconsin is presented and interpreted. In the region's confined aquifer the first two produce results comparable to, but different from, drawdown. However, all three of these measures fail miserably to assess the effects of pumping in unconfined aquifers. Here surface water bodies are the ultimate source of most of the water extracted from wells and baseflow reduction is the primary impact of ground water extraction.

The hydrologic effects of development options which have been completely simulated will be presented and compared, although no decisions on the best approach will have been made at the time of this GSA meeting.