Abstract
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.
Highlights
The north Florence coastal dune sheet (130 km2 surface area) occurs within a larger complex of paleo-dune sheets (280 km in combined along-coast length and 500 km2 surface area) that exist between the Pacific Ocean and the temperate rainforest-covered foothills of the south-central Oregon coast (Figure 1) (Cooper, 1958; Reckendorf, 1975; Peterson et al, 2007a)
We present applications of available technologies, including continuous ground penetrating radar (GPR) profiling, GPS positioning, Lidar elevation control, and geographic information system (GIS) surface trend analyses to map the regional groundwater surface (GWS) in coastal dune aquifers
A subsurface remote sensing instrument, ground penetrating radar (GPR), is shown to be useful in imaging the groundwater surface (GWS) of shallow aquifers in large coastal dune sheets. Such imaging and associated ground-truthing established that contamination of Cleawox Lake, and other barrage lakes, in the large FLOR dune sheet is not primarily from in-lake recreation users but rather from non-point sources of nitrogen and phosphorus contamination to the surrounding dune aquifer, which connects to the dune barrage lakes through shallow groundwater flows
Summary
The north Florence coastal dune sheet (130 km surface area) occurs within a larger complex of paleo-dune sheets (280 km in combined along-coast length and 500 km surface area) that exist between the Pacific Ocean and the temperate rainforest-covered foothills of the south-central Oregon coast (Figure 1) (Cooper, 1958; Reckendorf, 1975; Peterson et al, 2007a). Many of the lakes and ponds occur in upland coastal dune settings, m above mean sea level (MSL), so they were assumed to represent ‘perched’ lakes, such as the world heritage McKenzie Lake, located at 75 m elevation in the large dune sand complex of Fraser Island, Queensland, Australia (Arthington and Hadwen, 2003). Paleo-beach sand was blown landward by eolian transport across the emerged middle- and inner-shelf to ramp-up against the foothills of the Coast Range, building the late-Pleistocene dune sheets (70–18 ka in age) This semi-contiguous late-Pleistocene coastal dune complex thins to the north (~ 100 km distance) and to the south (~ 100 km distance) of the SPH low-stand depocenter, which was centered offshore of the FLOR dune sheet segment. Interstratified paleosol Fe/Al accumulation zones or B soil horizons (Birkeland, 1999) and coastal loess layers (Peterson et al, 2014) form localized hardpans in the late-Pleistocene dune deposits
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