RAPID GROUNDWATER FLOW IN CARBONATE ROCKS BENEATH GLACIAL DRIFT, NORTH-CENTRAL, OHIO

In north-central Ohio the Bellevue-Castalia Karst Plain physiographic province (600 km sq.) is delineated by a lack of surface drainage. Glacial deposits 3 to 30 m thick cover limestones of Devonian & Silurian age. Sinkholes are generally shallow, but can be up to 1 km sq. in area. A few caves are known. Problems with groundwater contamination have occurred for >50 years. Paths for rapid groundwater movement occur along traditional karst pathways (solutionally enlarged joints and bedding planes), but also occur along purely mechanical features in the limestone, caused by natural dissolution of anhydrite at depth. As such, though this area is karst, it has an origin that is complex. Backwards modeling of spring waters shows that circa 20 cubic meters of rock are removed from depth each day by the major springs. Dye traces by others have shown rapid movement of water from a swallet to residential wells. A trace we conducted south of the “karst” area showed rapid acceptance of chase water, but resulted in non-recovery of the dye. This is attributed to movement of the water down in to the regional flow system. Geophysical (gravity) surveys on the margins of major sinkholes give some indication of their subsurface morphology, but are not definitive. Thick covering of drift can protect the underlying carbonate aquifer from contamination, but this is not guaranteed. Even in areas having few surficial karst features, rapid groundwater flow has been demonstrated.