WATER QUALITY CHANGES IN A KARST AQUIFER FOLLOWING ABANDONMENT OF A WELL FIELD: LINE HOLE, SAN SALVADOR ISLAND, BAHAMAS

San Salvador, Bahamas is a carbonate island with a complex, karst ground water system. Water supply for the island’s 1400 people has been problematical because of the presence of subsurface conduits that connect the sea to the island’s interior hypersaline lakes. The tides force saline water into the rocks, disrupting any freshwater lenses that may exist beneath the lithified dune ridges. The Line Hole well field served the United Estates Settlement from the mid 1990’s until December, 2006. While the water coming from the 10 open bore hole wells was initially of good quality, it quickly became brackish with pumping. Because of this, pumping stopped on 15 December 2006 and the field was abandoned. To determine whether the poor water quality was the result of saltwater upconing caused by pumping or the direct migration of seawater into the wells via conduits that might intersect them, we conducted field studies that began in January 2007 (right after closure). So far, we have collected six sets of field measurements during both wet and dry seasons. To assess water quality and to determine the presence or absence of a halocline in the water column (and its position, if present) we completed specific conductivity profiles at each well. The tidal signal at each well was also recorded and compared to published tide predictions. It was expected that lag times would correlate to distance from the ocean. “Early” lags would suggest the presence of conduits. In an attempt to gather visual evidence of conduits, we also used a down hole camera to record videos of the rock in each well.

The conductivity profile data showed an increase in brackishness with depth immediately following well field closure and a strong trend of improving water quality later. These data support the hypothesis that, in most of the wells, over-pumping caused an upconing of saline groundwater and was therefore the primary cause of poor water quality. However, two wells continued to show consistently brackish water. The tide data for these two wells showed accelerated tidal lags compared to the predicted tide times. This supports the hypothesis that the two wells are connected to the sea via conduits in the rock. While the down hole videos clearly showed the nature and degree of dissolution in the limestone, they were inconclusive when it came to showing evidence of conduits.