Two computer models were developed to investigate possible hydrologic effects within the Indiana Dunes National Lakeshore caused by planned dewatering at the adjacent Bailly Nuclear Generator construction site. The dewatering, which is scheduled to last for about 18 months, would cause ground-water levels to be drawn down 20 to 30 feet (6 to 9 metres) to an elevation of 4 ft (1.2 m) below Lake Michigan datum under the approximately 7-acre (2.8-square hectometre) construction site, which is about 800 ft (245 m) west of the Lakeshore property. The results of this study have been used by the National Park Service to help evaluate the environmental impact of the dewatering, particularly the effects on the ecosystem of the lakeshore.

The aquifer underlying the area is about 50 ft (15 m) thick and is composed of fine sand with layers of medium sand and gravel. All natural ponds in the area are separated from the aquifer by a low-permeability layer of silty organic muck and clay that ranges from about 1 to 4 ft (0.3 to 1.2 m) in thickness. This pond-bottom material acts only to retard the flow of water either to or from the aquifer beneath the ponds, depending upon the relative head in the pond and the aquifer. Elsewhere, the aquifer is under water-table conditions.

The model analysis indicates that the planned dewatering would cause a drawdown of about 4 ft (1.2 m) under the westernmost pond of the Lakeshore and that this drawdown would cause the pond to go almost dry--less than 0.5 ft (0.15 m) of water remaining in about 1 percent of the pond--under average conditions during the 18-month dewatering period. When water levels are below average, as during late July and early August 1974, the pond would go dry in about 5 1/2 months. However, the pond may not have to go completely dry to damage the ecosystem. If the National Park Service's independent study determines the minimum pond level at which ecosystem damage would be minimized, the models developed in this study could be used to predict the hydrologic conditions necessary to maintain that level.