Changes in global climate, such as warmer temperatures, threaten the sustainable use of surface water bodies as drinking water sources (Barbarossa et al., 2021). For example, warmer temperatures within the Laurentian Great Lakes (e.g., the western basin of Lake Erie) have exacerbated harmful algae blooms, decreasing the water quality (Gibbons and Bridgeman, 2020) and threatening drinking water supplies for coastal communities. Harmful algae blooms are now annual occurrences in the western Lake Erie basin, and in 2014, it led to a “do not drink” advisory for the Toledo area in Ohio, USA (Stauffer et al., 2019), highlighting the need for exploring alternative and sustainable sources of potable water such as groundwater.
The Toledo area in Lucas County, northwestern Ohio (Fig. 1), is home to approximately five hundred thousand people. The groundwater withdrawal rate in the Toledo area is estimated at about 27 million gallons per day (Grannemann et al., 2000), and only about 15% of the wells drilled are used for domestic water supply and irrigation (ODNR, 2022). The regional groundwater system is poorly understood and likely underutilized. To evaluate whether groundwater could serve as a backup source of drinking water and continue to sustain the rare ecosystem of oak savanna and grassland prairie in the region, there is a need for an improved understanding and management of groundwater resources in the region.
The aquifer system in northwest Ohio has been variously described over the past few decades as surficial sand of the Oak Openings Region (OOR, Fig. 1A) overlying a confining till unit that rests unconformably above carbonate bedrock (Shade and Valkenberg, 1975; Breen and Dumouchelle, 1991) or a simpler stratigraphy of till overlying carbonate bedrock (Venteris, 2007). Regional studies considered each of the sand and carbonate bedrock hydrostratigraphic units as an isolated aquifer system (Shade and Valkenberg, 1975; Breen and Dumouchelle, 1991) or focused on the upper few meters of sand (Wijayarathne and Gomezdelcampo, 2019). Recharge avenues and solute pathways to the carbonate aquifers were studied by estimating evapotranspiration and measuring piezometric heads within the deeper aquifer system showing the possibility of direct recharge through the confining unit, with implications for inter-aquifer water exchange (Martin-Hayden et al., 1999). Also, the possible occurrence of fractures and joints within the till (Fig. 1D) challenges the assumption that the confining till unit may be impermeable (Brockman and Szabo, 2000). Surficial geology mapping (Larsen et al., 2005) and investigations of the post-glacial history of the region (Anderson, 2011; Fisher et al., 2015) described rhythmically laminated silt and clay lacustrine couplets (rhythmites) overlying till that previously was unrecognized or combined with till as a confining unit for the carbonate bedrock (Fig. 1C). The rhythmites are not considered in hydrogeological models in the region (Shade and Valkenberg, 1975; Wijayarathne and Gomezdelcampo, 2019). Given that rhythmites and till are deposited in different environments and by different processes, their hydraulic properties are expected to be different, and they should be treated as separate hydrostratigraphic units. By recognizing the region’s more varied and complex surficial sediments overlying the carbonate bedrock, this study evaluates the subsurface hydrostratigraphy in greater detail than previous studies and offers an improved conceptual hydrogeological model of the OOR.
For this study, a representative 3-D geological model (Hassen et al., 2016) is developed from multiple datasets, including well-logs, outcrops, and geophysical data. In groundwater studies, 3-D geological modeling is used to represent aquifer-aquitard geometry, heterogeneity, and boundary conditions which serve as the basis for any hydrogeological conceptual model and, therefore, a necessary step toward building a numerical groundwater flow model (Robins et al., 2005; Enemark et al., 2019). However, a common problem with 3-D geological models is obtaining hydrostratigraphic data that meets the required quality, quantity, and spatial distribution, especially in regional studies where high costs of acquiring detailed, high-density subsurface data generally result in dependence on less reliable data that still needs to be standardized and validated (Ross et al., 2005; Ventris, 2007). The shortage of input data to the models is often compensated by combining geological data with meteorological (Karlović et al., 2021), hydrogeochemical (Kpegli et al., 2018), and geophysical data (Francés et al., 2015; Somaratne et al., 2018).
High-quality geophysical data provide representative structural constraints when interpolating borehole data and estimating heterogeneities within a geologic model across a large spatial scale (Pasquet et al., 2015; Lekula et al., 2018). The fast and non- to minimally-invasive nature of collecting geophysical data is advantageous when collecting subsurface data and characterizing its stratigraphic heterogeneity. Several studies have demonstrated the advantage of using electrical resistivity and electromagnetics (Doro et al., 2013; Francés et al., 2015; Dias and Bacellar, 2021; Márquez Molina et al., 2021), seismic (Gerber et al., 2018), and airborne electromagnetics (Vittecoq et al., 2019) to develop hydrogeological conceptual models over well-log data alone. Barfod et al. (2018) highlighted the importance of using geophysical data to constrain multiple-point statistical (MPS) tools in developing a hydrostratigraphic model. Geophysical methods can characterize the subsurface lithology of glacial and deglacial sediments where outcrops are limited.
For this study, much of the geophysical data was collected at a newly established hydrogeophysics field research laboratory at the Stranahan Arboretum (STAR) research site of the University of Toledo, Ohio (Fig. 2). Additional geophysical data from select sites elsewhere in the OOR compliments the work at STAR. Our results challenge the existing two- to three-layer conceptual models for the regional groundwater system over much of the area, and we update the hydrostratigraphic model using geophysical methods to better assess their layer distribution and continuity. We emphasize separating the glaciolacustrine rhythmites from the underlying till because the rhythmites are more permeable than the till and could result in enhanced vertical flow to and through the till to the carbonate aquifer.