NUMERICAL MODELING OF THE EFFECTS OF A QUARRY DEWATERING OPERATION IN NORTHWEST OHIO

The effects of a large pumping stress on a steady state aquifer system are reasonably well understood, but if the same stress is imposed on system with naturally varying water levels the response will be different. Since natural systems rarely maintain a steady state it is the later scenario that is most relevant to studying the impact of a large pumping stress on aquifers. Dewatering for quarry operations is one of the main sources of large pumping stresses and these operations often conflict with domestic and agricultural water uses. Greater understanding of the effects of quarry dewatering on aquifers could help minimize these water use conflicts. This study focuses on an aggregate quarry in northwestern Ohio. The dewatering for this quarry has produced a cone of depression in two bedrock aquifers, covering an area >10 sq mi, and resulted in over 100 complaints from owners of nearby domestic water wells. The carbonate and shale bedrock in the area dips towards the Michigan Basin to the northwest. The upper affected aquifer is the Tenmile Creek Dolomite, the subcrop unit at the quarry but confined by the Antrim Shale to the northwest. Beneath the Tenmile lies the Silica Shale, which confines the lower aquifer in the Dundee Limestone and Detroit River Group. The Dundee becomes the subcrop unit one mile southeast of the quarry. The area is overlain by thick glacial deposits and the quarry sits on the Oak Openings Sand, a northeast trending beach deposit. The quarry began mining this sand in ‘68, and the Tenmile in ’93. In ‘96 the Silica confining bed was breached, but mining of the Dundee did not begin until ‘99. Dundee mining ended in ‘01 and the quarry ceased operations at the end of ‘02. 3 sets of over 50 synoptic water level measurements have been taken since ‘01, in addition to measurements taken in complainants’ wells by the Ohio DNR. Water levels have also been collected from drillers’ reports. Water levels in the aquifers declined 10-15 feet from the 50’s to the 80’s, and declines averaged 20-30 feet during the 90’s but generally are smaller away from the quarry. Water levels in the Dundee have shown recovery since 2001, but those in the Tenmile have been constant or continued to decline. A 3D transient flow model is used to simulate the water levels in the area and differentiate the effects of quarry dewatering from the natural decline in water level.