KARST HYDROLOGY ON SAN SALVADOR ISLAND, BAHAMAS: A SUMMARY OF THE RESULTS OF 31 YEARS OF RESEARCH

My students and I first came to San Salvador Island, Bahamas, on January 1, 1986. In 1989, we published our first paper on the island's karst hydrology. It presented our model of island's karst hydrology. Thirty-one years later, after almost 100 more trips, that model still stands. However, we have been able to refine it, adding detail on freshwater lenses, subsurface transport, the mechanisms driving the hydrologic cycle, water chemistry, subsurface "conduits" and much more. Freshwater lenses are discontinuous, subsurface transport is tidally driven, the cycle is evaporation dominated, the water chemistry has enough variation to allow tracing of sources and connections, subsurface "conduits" are almost certainly zones of higher hydraulic conductivity, not tubes or tunnels. Recent work has focused on an abandoned well-field (Line Hole) on the north coast of the island. We have a ten-year record of salinity profiles from 13 wells, begun two weeks after the well field closed and repeated 3-4 times per year since then. Salinity was high after closure in December 2006. By April, 2007, salinity had decreased substantially and it has remained the same since. We also installed water level recorders in all wells simultaneously. Tidal lags were expected to increase and amplitudes to decrease with distance from the shore. This was generally true but some wells showed anomalies (which were consistent over several years). The tidal signal arrived sooner than expectedand the wells had higher than expected salinities. Pump tests also indicated higher hydraulic conductivities. This suggests that these wells may intersect a "conduit" that is allowing more transport of sea water inland at higher velocities than the surrounding rock.

Despite the increase in our knowledge and the refinement of our model, many mysteries still remain. For example, flow in and out of the small ponds at the south end of the island exhibits some strange physical and chemical behavior. We have not been able to propose even a conceptual model to explain our observations. So, while we have learned much, there is far more work to do. I have said that there is work for "fifty people for fifty years." While this may be an exaggeration, things on San Salvador do not behave in textbook fashion and the island gives up its secrets very reluctantly, so my statement may actually have a ring of truth.