Karst hydrology of San Salvador.

Alternative Title

Proceedings of the Fourth Symposium On The Geology of Bahamas

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Publication Date

1-1-1989

Abstract

San Salvador is a typical carbonate platform island in the eastern Bahamas. Its major featu res include arcuate, consolidated dune ridges up to 40 meters high, and shallow, hypersaline lakes. These occupy the low areas between the dunes and cover a substantial portion of the island's inter­ ior. The subsurface contains large numbers of caves, conduits and vertical shafts. Despite the nearly 120 cm of annual rainfall, there is no flowing surface water. The ground water system is poorly defined and fresh water lenses are discon­ tinuous and sporadic. Data collected during two years of field work has led to the construction of a general model of the island's hydrologic regime. The system consists of six elements: I) Precipitation; 2) Evapotranspiration; 3) Ground Water; 4) Inland Lakes; 5) Conduits and Blue Holes; and 6) Ocean tides. Investigations included an examination of rock and water geochemistry, rock permeability, and physical hydrology. Over 430 water samples and 16 rock samples were analyzed. ca++/Mg++ and Mg++/Ci- ratios were used to trace water sources. Water chemistry was used to pinpoint the locations of conduits and seeps on the inland lakes. Ground water elevations were mapped in several locations and conduit flows were mea­ sured. Pump and laboratory tests were performed in order to estimate rock permeabilities. Tide gauges were used to measure ocean tides and water level fluctuations in caves and in the hypersaline inland lakes. This field data was used to construct the general model. Groundwater in the interior of the island is found in discontinuous, i$olated, fresh water lenses beneath the consolidated carbonate dunes. The lenses, which are fed by infiltrating rain water, drain into the hypersaline lakes where the water evaporates. Near the coast there are similar lenses, but these are disrupted in many places by flow through caves and conduits. Some of the interior lakes are connected to the sea by con­ duits, but most appear to be fed directly by ground water seeps and precipitation. The height of the tides varies considerably from day to day. The elevation of many of the inland lakes is above that of the lower high-tides. Therefore, conduit flow into them only occurs during the higher high-tides. At these times, the flow is especially vigorous and can contribute to the disruption of the fresh water lenses in the vicin­ ity of the conduit. The rock has relatively low primary permeability and the direct influence of ocean tides on the fresh water lenses near the shore appears to be small. This model can be used to locate new freshwater resources for San Sal­ vador and other similar islands. It may also be useful in examining the paleohydrology of the island and its impact on cave formation.

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Conference Proceeding

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