USE OF FIELD WATER QUALITY DATA TO CONSTRAIN HYDRAULIC CONDUCTIVITY OF OCEAN BEACH SANDS

In June 2000 I supervised an undergraduate research project that involved groundwater-level and water-quality monitoring over a 5-day period at Little St. George Island, a barrier island in northwest Florida. The project involved installation of temporary monitoring tubes (small piezometers) in two transects perpendicular to the shore, one on the bay side and the other on the ocean side of the island. The ocean-side (Gulf of Mexico-side) transect consisted of nine monitoring tubes typically inserted about 0.5 m below the water table. Several monitoring tubes were placed near one another on the high tide berm, in the back-berm depression, and then at intervals farther inland. During high tide, swash from larger waves moves through cuts in the first berm to fill the narrow (approximately 2-m wide), linear, back-berm depression with salt water. This water then infiltrates during the backwash until the next set of large waves breaks on the shore. Groundwater levels fluctuate the most at the monitoring tubes closest to the shoreface. During high tide, groundwater levels are highest at the berm and cause the water-table gradient to be reversed. The gradient reversal extends inland approximately 15 m from the high-tide berm (12 m from the back-berm depression). Field measurements of salinity, pH, and temperature identify the landward extent of salty groundwater at the monitoring tubes. These chemistry data can be used to constrain the estimate of hydraulic conductivity. This is obtained using Darcy's Law and the observed landward water-table gradient from the back-berm depression to calculate a maximum inland saltwater travel distance. Hydraulic conductivities measured by slug tests are on the order of 10-2 cm/s and appear to be consistent with the hydraulic conductivity required by the gradient analysis.