DAM FAILURE – HYDROGEOLOGIC CONSEQUENCES AND EFFECTS ON THE TITTABAWASSEE AND TOBACCO RIVERS AND THE GROUNDWATER SYSTEMS IN SOUTHERN GLADWIN COUNTY, MICHIGAN

In May 2020, a reservoir dam at the intersection of the Tittabawassee and the Tobacco Rivers collapsed after historic rainfall events. This resulted in catastrophic flooding in a cascade of hydrogeologic and hydrologic changes above and below the dam in southern Gladwin and northern Midland Counties Michigan. The formerly impounded Wixom Lake had artificially raised the local water table approximately 6 meters above the normal water table surface. Hundreds of the homes along the river supplied their water needs from shallow wells and sand points, and they experienced dry wells due to a lowering of the water table after this event. Along the two rivers, replacement water wells drilled since the event have shown that the water table has dropped up to 6 meters. Recent LiDAR imagery shows the water surface along the two rivers and the reservoir lake also exhibit a 6-meter drop compared to records prior to the event. A combination of water well records (Wellogic Database and more recent drillers reports) and a review of oil and gas well records indicated additional depth to bedrock data was needed to construct cross-sections and maps of the contact of the bedrock and glacial contact surface. Historic and recent water levels are presented to illustrate pre and post event levels. In order to better define the county bedrock contact surface, Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic data was collected in areas having limited data across the county study area to better develop a regional bedrock-glacial contact. Well records and HVSR data better defines the bedrock surface in this region and will allow a better understanding of hydrogeologic connections between the glacial sediment cover and the underlying Paleozoic and Jurassic bedrock. Gladwin County is a unique location in that there is an abundance of shallow drift wells and deep bedrock wells. Using calibrated passive seismic measurements, the bedrock surface is analyzed to determine the complexity of the bedrock surface and interaction with the glacial drift, which can determine if there are separate glacial and bedrock aquifer systems. The bedrock surface mapping and cross sections can provide a context for interpreting how the groundwater from the glacial material interacts with the bedrock material below.