Authors: John W. Lane Jr, Martin A. Briggs, Pradip K. Maurya, Eric A. White, Jesper B. Pedersen, Esben Auken, Neil Terry, Burke Minsley, Wade Kress, Denis R. LeBlanc, Ryan Adams and Carole D. Johnson
Abstract: The hydrogeology below large surface water features such as rivers and estuaries is universally under-informed at the long reach to basin scales (tens of km+). This challenge inhibits the accurate modelling of fresh/saline groundwater interfaces and groundwater/surface water exchange patterns at management-relevant spatial extents. Here, we introduce a towed, floating transient electromagnetic (TEM) system (i.e. FloaTEM) for rapid (up to 15km/h) high-resolution electrical mapping of the subsurface below large water bodies to depths, often a factor of 10 greater than other towed instruments. The novel FloaTEM system is demonstrated at a range of diverse 4th through 6th-order riverine settings across the United States including 1) the Farmington River, near Hartford, Connecticut; 2) the Upper Delaware River near Barryville, New York; 3) the Tallahatchie River near Shellmound, Mississippi; and, 4) the Eel River estuary, on Cape Cod, near Falmouth, Massachusetts. Airborne frequency domain electromagnetic and land-based towed TEM data are also compared at the Tallahatchie River site, and streambed geologic scenarios are explored with forward modelling. A range of geologic structures and pore water salinity interfaces were identified. Process-based interpretation of the case study data indicated FloaTEM can resolve varied sediment-water interface materials, such as the accumulation of fines at the bottom of a reservoir and permeable sand/gravel riverbed sediments that focus on groundwater discharge. Bedrock layers were mapped at several sites, and aquifer confining units were defined at a comparable resolution to airborne methods. Terrestrial, fresh groundwater discharge with flow paths extending hundreds of meters from shore was also imaged below the Eel River estuary, improving on previous hydrogeological characterizations of that nutrient-rich coastal exchange zone. In summary, the novel FloaTEM system fills a critical gap in our ability to characterize the hydrogeology below surface water features and will support more accurate predictions of groundwater/surface water exchange dynamics and fresh-saline groundwater interfaces.
Science of the Total Environment, 740(10)
