Groundwater Surface (GWS) Mapping by Ground Penetrating Radar (GPR) For Use in Protecting Freshwater Habitats, Water Quality, and Active Dune Landscapes, In the Florence Coastal Dune Sheet, Oregon, USA

Ground penetrating radar (GPR) profiling was performed in the Florence (FLOR) coastal dune sheet to test relations between remotely-sensed groundwater surface (GWS) trends, measured groundwater phreatic surfaces, and overlying freshwater features/habitats. Following preliminary GPR testing, the GWS trend mapping was employed in the north FLOR dune aquifer (17 km in length and 5 km in width), in anticipation of increasing development pressures on aquifer groundwater withdrawal by the City of Florence, Oregon. Several available technologies, including continuously-towed GPR profiling (5-8 km/hr), real-time GPS positioning (±2 m horizontal), Lidar elevation control (±0.5 m NAVD88), and GIS mapping/surface trend analyses permitted upscaling to the large management area (40 km2) in the north FlOR dune aquifer. Totals of 95 km of GPR track-line, including 943 averaged shot points at 100 m track-line intervals (total ~100,000 shot points), were collected during a three-week field effort. The remotely sensed GWS, ranging from 1 to 14 m depth subsurface and 0 to 57 m elevation NAVD88, was ground-truthed in ponds, gaining-stream reaches, and monitored water wells. An area wide groundwater surface map confirmed a modeled dune-ramp aquifer, sloping (0.5-2.0 % gradients) to the Pacific Ocean shoreline and the dividing Siuslaw River valley. The continuous GPR profiles connected large dune barrage lakes, interdune valley window lakes, anadromous fish passage streams, and sensitive bog habitats to the locally-variable GWS (0.98 R2 correlation coefficient). These elevated freshwater features were shown not to be developed on perched dune soil aquitards or lake bottom mud seals, but rather they are directly dependent on the mounded, variably sloping, and very-shallow GWS in the regional dune aquifer. Shallow GWS depths also promote colonization of active dune surfaces by non-native invasive dune grasses. The freshwater lakes and ponds were shown to be susceptible to contaminant transport by down-gradient GWS flows from surrounding residential and resort development.