The land-lake interface is a unique zone where terrestrial and aquatic ecosystems meet, forming part of the Earth’s most geochemically and biologically active zones. The unique characteristics of this interface are yet to be properly understood due to the inherently high spatiotemporal variability of subsurface properties, which are difficult to capture with the traditional soil sampling methods. Geophysical methods offer non-invasive techniques to capture variabilities in soil properties at a high resolution across various spatiotemporal scales. We combined electromagnetic induction (EMI), electrical resistivity tomography (ERT), and ground penetrating radar (GPR) with data from soil cores and in-situ sensors to investigate hydrostratigraphic heterogeneities across land-lake interfaces along the western basin of Lake Erie. Our Apparent electrical conductivity (ECa) maps matched soil maps from a public database with the hydric soil units delineated as high conductivity zones (ECa > 40 mS/m) and also detected additional soil units that were missed in the traditional soil maps. This implies that electromagnetic induction (EMI) could be relied upon for non-invasive characterization of soils in sampling-restricted sites where only non-invasive measurements are feasible. Results from ERT and GPR are consistent with the surficial geology of the study area and revealed variation in the vertical silty-clay and till sequence down to 3.5 m depth. These results indicate that multiple geophysical methods can be used to extrapolate soil properties and map stratigraphic structures at land-lake interfaces, thereby providing the missing information required to improve the earth system model (ESM) of coastal interfaces.

The National Science Foundation provided support to the American Geophysical Union (AGU) to engage its relevant community and help clarify the need for a Near-Surface Geophysics (NSG) Center and identify how it could advance key science questions, provide benefits for society, and develop the geophysical workforce of the future. This report synthesizes the broad input from the community. The listed authors represent the Steering Committee, led by Sarah Kruse and Xavier Comas, and AGU staff leads. They were responsible for most of the editing and connective writing. The major conclusions are: ● The capability and importance of NSG is expanding rapidly, and NSG is providing key science and knowledge to many specific scientific challenges in diverse disciplines–from ecology and anthropology to hydrology, oceanography, cryosphere science, soil and critical zone science, and more. ● This has been thanks to diverse new instruments and approaches, expanded monitoring, improved resolution, interoperable data sets, and new computing power and approaches, among other developments. ● As a result, advancing NSG is critical to addressing many societal challenges at local to global scales. Human society depends on and interacts with the NSG environment in deep and diverse ways at all scales. ● Despite these developments, integration of NSG approaches and awareness of these across related disciplines are not nearly robust enough for these needs. ● Major challenges include providing equipment and training around its use, developing and deploying new equipment and sensors, developing interoperable data, and developing computation techniques. ● In particular, educating both current researchers and developing an NSG-enabled workforce is a major challenge. ● Integrating education with societal and scientific challenges provides a great opportunity and means to expand inclusivity and diversity in the Earth sciences and to address climate justice and equity challenges. ● Thus there was a strong consensus for support of an NSG Center designed to address these challenges and needs and to foster convergent science, provide broad and hands-on educational training, and engage communities and the public meaningfully. ● We were not charged with envisioning the specific model for a Center—and indeed emphasized that the term “Center” was generic and did not necessarily imply that these efforts were envisioned to be in one location–but note that NSF is supporting important complementary facilities include the new EarthScope Consortium combining IRIS and UNAVCO, NCALM, and CTEMPS. ● In sum, we strongly encourage the NSF to take the next step in considering the best implementation model for a NSG Center that addresses these needs, enables these opportunities, and leverages and complements existing efforts.

The Oak Openings Region of Northwest Ohio has a unique shallow sandy aquifer that is responsible for the wet prairie ecosystem above it. However, groundwater flux and contaminant transport within the 1 – 3 m thick sandy aquifer and a potential flow exchange with the deeper carbonate aquifer in the post glacial regional aquifer system are not well understood. In this study, integrated geophysical methods involving electrical resistivity tomography (ERT) and ground penetrating radar (GPR) are co-located to delineate the sandy aquifer unit at the Stranahan Arboretum and the Sandhill Crane Wetland sites in Toledo and Swanton, Ohio. Parallel ERT profiles were acquired using a SuperSting R8 resistivity meter with a dipole-dipole configuration and unit electrode spacing of 1 m while the GPR profiles were acquired using a PulseEKKO Pro 250 MHz radar system. Additionally, we obtained soil samples extending to a depth of 2.5 m at six locations on three of the profiles at each site. The sand samples were analyzed for their grainsize and to estimate the hydraulic conductivity (K) of the aquifer. Multiple slug tests were also used to estimate the variation in K. We found that the sandy aquifer is somewhat disconnected at the Stranahan Arboretum, with the thickest lenses around 10 - 40 m on the ERT profiles while a continuous and thicker sand sequence is observed at the Sandhill Crane site. The sandy aquifer is underlain by clay-rich silt and glacial till respectively who’s hydraulic leakance controls potential vertical fluxes. The average grain size of the sands was between 0.285-0.33 mm, suggesting fine to medium-grained sands. The average K ranged from 2 × 10-4 to 9 × 10-4 m/s, with generally larger K values found in sands sampled from the thickest lenses. Overall, the correlation of higher K values within thicker sand lenses suggests that in these areas, groundwater would be able to flow more easily, and the aquifer could be more easily contaminated than thinner, less connected sand units. We hope to continue this research and improve K estimates and conceptual models to help devise better plans to protect the groundwater resources and ecosystems of the OOR.

Crystalline basement aquifers are characterized by complex flow pathways controlled by varying overburden stratigraphy and thickness as well as fracture network and connectivity within the crystalline rocks. Understanding the hydraulic connection within the fracture network and the overburden regolith is critical to predicting recharge/discharge and contaminant transport pathways. In this study, we combined geophysical imaging with multiple hydraulic testing to quantify hydraulic connectivity within the crystalline basement aquifers at the Ibadan Hydrogeophysical Research Site (IHRS) in Ibadan, Nigeria. The 50 m × 50 m field experimental site is first of its kind established in 2019 to investigate hydrological dynamics within these complex crystalline basement aquifers in sub–Saharan Africa. We acquired multiple parallel 2D electrical resistivity profiles which were also jointly inverted to obtain multiple 2D and 3D electrical resistivity tomograms of the subsurface. The resistivity tomograms were later constrained with lithological profiles from 4 test wells installed down to depths of 30 m at the site to create a conceptual model elucidating potential flow pathways. We also performed a series of 12 hours pumping tests and a NaCl tracer test to estimate flow and transport parameters including hydraulic conductivity, aquifer storage, yield, and groundwater travel time and to assess connection between the four test wells. The resistivity tomograms show 3 major resistivity zones interpreted as a clay-rich topsoil, a saturated weathered overburden, and a fractured basement rock. The delineated fractured bedrock shows an undulating topography with several primary fracture successions at 9, 14, 16 and 22 m. Hydraulic conductivities from pumping tests range from 2.6 x 10-7 to 1.2 x 10-5 m/s for the fractures and 1.7 x 10-10 to 6.4 x 10-6 m/s for the matrix while specific storage range from 3.5 x 10-8 to 1.8 x 10-3. Preferential flow is also observed with stronger connection between wells A and C. Results of this study provide a basis for detailed numerical study which will be focused on predicting recharge and solute transport under different flow and climate regime. This work will provide a scalable framework for a sustainable management of groundwater resources within the crystalline basements of Nigeria.