GIS-BASED THREE-DIMENSIONAL GEOLOGIC AND HYDROGEOLOGIC MODELING OF THE MILAN, OHIO 1:24,000 QUADRANGLE

The Central Great Lakes Geologic Mapping Coalition (CGLGMC) is a partnership among the state geological surveys of Ohio, Indiana, Illinois, and Michigan, and the U.S. Geological Survey. The mission of the CGLGMC is to produce detailed three-dimensional geologic maps and information, along with related digital databases, that support informed decision-making involving ground water, mineral-resource availability and distribution, geological hazards, and environmental management. The initial Ohio project for the CGLGMC was the geologic and ground-water modeling of the Milan Quadrangle in north-central Ohio. This area was modeled as ten lithologic units, including alluvium, beach ridges, lacustrine sand and clayey silt units, Wisconsinan till, and a significant pre-Wisconsinan buried valley aquifer. Tools in ESRI ArcGIS, including the Spatial Analyst extension, were used to analyze borehole and outcrop data, construct the bounding surfaces of each lithologic unit, and to produce raster data layers representing the three-dimensional framework of these units.

We used the detailed three-dimensional geologic model and merged it with an equally detailed groundwater-flow model to produce a more realistic understanding of the controls that glacial geology and geomorphology exert on shallow ground-water flow systems. The top of the geologic model was the surface topography (digital elevation model), which was also used to derive the drainage network that is an important boundary condition in the ground-water flow model. The bottom of the geologic model was the top surface of the Devonian Ohio Shale. Flow in the shallow saturated zone reflected strong control by surface topography and assumed hydraulic properties of the mapped sedimentary units. In contrast, the flow at depth was not strongly influenced by the topography of the Ohio Shale but did show some tendency for regional flow toward Lake Erie.

The resultant three-dimensional geologic model and companion ground-water modeling results can be used to produce a range of derivative products such as maps of recharge and discharge areas. Such products can be used to address the wide variety of water management, land use, environmental, and resource issues that are crucial to local, state, and federal agencies, private industry, and the general public.