GROUND- AND SURFACE-WATER INTERACTIONS IN A COASTAL SAND AQUIFER, CLATSOP PLAINS, OREGON

Interactions between ground water and interdunal lakes and impacts on lake eutrophication were investigated in the Clatsop Plains aquifer, Clatsop County, Oregon. The aquifer lies within shore and dune facies that parallel the Pacific coast. Sand accretion during late Pleistocene and Holocene sea level rise, temporal distribution of shore, dune, and soil-forming environments, erosion and deposition produced by uplift and coseismic subsidence, variation in sand supply from the Columbia River, and precipitation of cement at redox fronts impact hydraulic properties within the aquifer. Water well and piezometer logs and GPR surveys reveal buried paleosols, heavy mineral lag deposits, clay and gravel lenses, and around lakes and in wetlands up to 2.5 m of peat. The highly permeable sand deposits (k=0.02 to 0.09 cm/s) overlie low permeability Tertiary marine silt- and mudstone.

Water levels within the shallow aquifer and hydrogeochemistry were analyzed between October 2002 and December 2003. Each lake and stream has a unique annual hydrograph. The February 2003 potentiometric surface is highest beneath the highest dune ridge system and slopes both landward and seaward at approximately 7m/km and 12m/km respectively. Distribution of organic carbon in peat, paleosols, and buried woody debris influence redox conditions and is consistent with variable iron concentrations in the shallow aquifer (below detection up to 72 ppm). Mineral precipitation at redox boundaries may locally impact hydraulic properties leading to isolation of lakes from ground water. Nutrient distribution in the shallow aquifer varies with redox potential and by season. Br:Cl, conductivity, and cation/anion ratios indicate sea spray contribution near the coast with decreasing influence landward. Seasonal shifts in Br:Cl and distinct departure from seawater ratios (Br:Cl=0.0034) due to increased Cl (Br:Cl=0.0024 to 4x10-4), particularly in the rainy season suggests flushing of anthropogenic contamination into the shallow aquifer from the vadose zone during winter months.