GROUND PENETRATING RADAR (GPR) PROFILING OF GROUNDWATER SURFACE TRENDS IN HOLOCENE FOREDUNE-RIDGE STRATA OF THE CLATSOP BEACH PLAIN, OREGON, USA

We have used ground penetrating radar (GPR) to map groundwater surface trends in the Clatsop Plains coastal aquifer (30 km in length) in northwestern Oregon. Abandoned foredune-ridges (5-20 m in height) are developed above upper-shoreface strata (10-30 m thick) that prograded 2.5 km seaward in late-Holocene time (5-0 ka). Some of the small, linear lakes (1.0-5.0 km in length) that have formed in the interdune-valleys (0.1-0.5 km in width) are undergoing early stages of eutrophication. It is not known whether septic systems or other potential sources of nutrients (N, P, Fe, etc.) are contributing contaminants to the lakes by groundwater migration through the porous sand aquifer. To help establish shallow groundwater flow directions in the aquifer we are using regional GPR profiling to map seasonal groundwater slopes between the foredune ridges and interdune-valleys (lakes). Shore-normal GPR profiles (1-2.5 km in length) were collected with a Sensors and Software Digital pulseEKKO 100 system using 400V and 1000V transmitters, and 100 MHz antennae, at 0.5 m step spacing. Penetration ranged from 5-20 m subsurface. The GPR profiles are superimposed by ArcView-GIS onto a regional high-resolution DEM (± 1 m error) compiled from LIDAR swath coverages. Summer groundwater surfaces, typically 2-7 m subsurface, vary regionally by as much as five meters in elevation throughout the aquifer. Importantly, the groundwater surface trends (0.001-0.01 gradients) drop from dune-ridges toward the interdune valleys. Annual precipitation (>1OO cm) and limited creek runoff (shore-parallel along the interdune valleys) results in deeper underflow. However, septic system loading on dune ridge flanks is likely to reach adjacent interdune valleys (lakes) along down slope flow paths. Seasonal GPR profiling is now underway to calibrate the surface trends with installed pieziometers and groundwater sampling.