Mapping Pennsylvanian stratigraphy and elevation of the fresh-saline groundwater interface using geophysical well logs in Carroll County, Ohio

Abstract

Carroll County in eastern Ohio overlies the western portion of the Appalachian Basin and is dominated by surface sediments consisting of Pennsylvanian fluvial deposits. These deposits are approximately 30.48 meters (1,000 feet) thick and contain complex patterns of sandstone, siltstone, shale, and coal beds and are the main source of underground drinking water for the county. The distribution, geometry, and connectivity of porous sandstone beds plays a large role in the flow and recharge of meteoric water in this aquifer, and thus the elevation of the interface of fresh and deeper saline waters. Previously, the elevation this interface was not mapped in the study area due to stratigraphic complexity. This study’s purpose is to map this interface, which involves first delineating the Pennsylvanian fluvial stratigraphy in order to map the structure and thickness of connected porous sandstone beds using well logs. Specific conductance data collected from fluid samples gathered while drilling through the aquifer are then used in conjunction with well logs to allow two methods from which to calculate total dissolved solids (TDS) of formation water at each well log location. Total dissolved solids (TDS) interface elevations are determined at each well location for transitions of 1,000, 5,000, and 10,000 mg/L. These elevations are combined with the results of the stratigraphic analysis, which drive the interpretation of TDS interface contour geometries based on thickness and distribution of connected sandstone units. The integration of fluid and stratigraphic data results in a set of contour maps for each TDS transition elevation that allow for accurate prediction of each interface elevation at any location within the study area, which can help with protection and planning regarding freshwater resources.