GEOMORPHOLOGICAL AND HYDROLOGICAL CONTROLS ON FRESH WATER WETLANDS, SAN SALVADOR ISLAND, BAHAMAS

San Salvador is dominated by large, shallow inland lakes bounded by lithified carbonate dunes. It has a tropical savanna climate with a winter dry season. The water budget is negative which, when combined with the island's low elevation and extensive karst, creates conditions that favor saline to hypersaline water chemistry in the lakes. However, careful exploration of the island's interior has shown that, despite these conditions, there are many freshwater wetlands. The purpose of this study is to account for their presence and to determine how they fit into the island's hydrologic regime.

The island is composed entirely of young carbonates. There are three recognized stratigraphic units, two Pleistocene and one Holocene. The Pleistocene units are capped by micritized paleosols which form hard, impervious crusts. These crusts may play a role in the formation of the fresh water wetlands.

Five wetlands ranging from fresh to slightly brackish were examined in 2002-2003, at the beginning of the dry season and again near its end. Rock thin sections, water chemistry, and water level data were collected at each site. During this interval, water levels declined and chloride concentrations increased. This suggests that the wetland system is evaporation dominated. Wetland areas were at least partially underlain by micritic paleosols. Short time interval data logging of water levels in the wetlands did not show a clear tidal signal. The wetlands themselves are located in shallow or deep depressions.

We hypothesize that the geology and geomorphology of the areas surrounding the fresh water wetlands work like a natural "catch" basins. Rain water first infiltrates only the thin soil layer, is then transported by interflow along the impervious paleosol, pools in the lowest areas, perched on the paleosols, and finally slowly evaporates during the dry season.