1997. Proceedings Oftheeighth Symposium on the Geology Ofthe Bahamas and Other Carbonate Regions

From: Carew, J. L. (ed.). 1997. Proceedings oftheEighth Symposium on the Geology ofthe Bahamas and Other Carbonate Regions. Bahamian Field Station. San Salvador. Pp. 47-58. HYDROGEOLOGY OF THE COCKBURN TOWN AQUIFER, SAN SALVADOR ISLAND BAHAMAS. AND TH.E CHANGE IN WATER QUALITY RESULTING FROM THE ,_ DEVELOPMENT OF A RESORT COMMUNITY Jason S. Erdman and Marcus M. Key, Jr. P.O. Box 1773 Department of Geology Dickinson College Carlisle, PA 17013-2896 R. Laurence Davis Department of Biology and Environmental Science University of New Haven 300 Orange Ave: West Haven, CT 065 I 6 ABSTRACT 17 January 1995. The construction of the Club Med According to many historians, resort led to a 382% increase in groundwater Columbus landed in the New World in 1492, at pumping. There was also a 51% increase in the what is now San Salvador Island in the salinity of the water delivered from the Bahamas. Spurred by this event, five hundred wellfield. At 9 g/1, this salinity renders the years later, a large resort community opened water unusable. Thus, the combination of an on the island near Cockburn Town, the historical quirk that encouraged resort government seat. Water s~pply to both the development, and the island's complex karst town and the resort comes from a I 05 head hydrogeology, has resulted in the degradation wellfield, located nearby. We examined the of the aquifer and the water supply for both hydrogeology and water quality at the well field Cockburn Town and the resort. · to determine what impact the opening of the Ground water elevations did fluctuate resort, with its attendant increase in water with the tides, but their amplitude did not demand, had on the aquifer. dampen with distance from the ocean. The aquifer is in the Cockburn Town Conductivity measurements also showed a Member of the Pleistocene Grotto Beach complex spatial pattern of fluctuations through Formation. The we!lfield is located between the tidal cycle. Although relationships are _the ocean and two inland, hypersaline lakes on weak, it is possible that this complex pattern a platform with an elevation of approximately reflects the influence of dissolucional conduits 3 m above currenr sea level. The ocean is that connect the two lakes to the ocean and roughly 0. 7 km west of the wellfield, Flamingo which probably pass through the aquifer. Pond is roughly 1.1 km east-northeast, and Because of this, and the thinness of ·the Little Lake is roughly 1.6 km southeast. There freshwater lens, the quality of the water is a thin freshwater lens at the wellfield site, supplied to Cockburn Town. prior to the resort and groundwater elevations are tidally development was already marginal. influenced. We obtained 119 weeks of data on. INTRODUCTION pumping rates, rainfall, and salinity, covering the period 9 October 1992 to 20 January 1995 San Salvador Island, Bahamas (formerly from the Bahamian Water and Sewerage known as Watlings Island) is located Corporation and the Bahamian Department of approximately 620 km east-southeast. of Fo:t Meteorology. We also measured groundwater Lauderdale, Florida (Figure l ). The 1sland IS elevations and conductivity in 26 · of ·rhe 1 I km wide (E-W) and 19 km long (N-S) field's l 05 wells over a 25 hour period on 16- (Figure 2). Inland hypersaline lakes occupy 47 .···· ····· ..... ~ . 'Co.. ;_ THE \ BAHAMA ' ISLANDS 0 100mi. I I I 0 100km Balhymetric contours in fathoms '• (cay sat \._ \ Sank ·· .._ ·········100 .... .) _,.·-··· ... ··-····· .. Figure !. Map of the Bahamas and .mrrotmding rc;:imt (after Car<'W and Mylroie. 1995a). the swales between the lithified Pleistocene hope that this information might help the and Holocene dune ripges that dominate the people at Club Med and the Bahamian Water topography of the island. and Sewerage Corporation to better understand According to many historians, San the nature of their water supply and perhaps, Salvador was the site of first landfall in the to develop management plans that would allow "new" world in October 1492 (Gerace, 1987). its continued use in the future. Five hundred years later, in 1992, Club Med's Columbus Isle resort opened. The resort San Salvador Geology created a large increase in the demand for freshwater on the island. The source of water San Salvador Island is located on. a for both the resort and Cockburn Town, the small carbon:1te platform in_ the southeastern island's main town and government seat, is a Bahamas (Figure I). Mullins et al. { 1992) wellfield located 1.5 km north of Cockburn suggest that these small platforms are the Town and 0.5 km _southeast of Club Med products of platform-margin collapse and (Figure 2). The goal of our study was to rt'treat spurrt>d by tectonics along the North determine the impact that Club Med's American and Caribbean plate boundary. The opening had on the water quality in the southt'astern part of the Bahamas consists of wellfield and to examine ·what role tht> many of these small platforms, most of which is.land·s karst hydrogeology might have played :tn.• exposed as single isl::lnds such as San in the changes that took place. In the end, we Salvador (Carew and Mylroie, 1995a,b; Curran 48 Wnl~e Cay Green G.a.ut In D C4~to CAY CAY[;> c7fJ "'\... Graham's Harbour \~ut Cay . '\1 Man Head Cay ATLANTIC Barker's North Po1nt -i Rice Bay OCEAN Point ··-.. ..:=----- Hanna Bay Bone fish 2 Bay Crab 1 Ca Fernandez Bay ATLANTIC OCEAN Long Bay Bay t N High .-otto Cay 112 r {J ;::::::=1•ct crt Owl's ~ uoe u:~a Hole Sandy ~ Gulf Hook Q I l Government Dock low Cay Figure 2. Map of San Salvador Island, Bahamas. #l indicates the location of the Cockburn Town wellfield. #2 indicates the location of Club Med's Columbus Isle resort (after Curran. 1989 ). 49 and White, 1995). in hydrostatic equilibrium on the denser Drilling data from the Bahamian saltwater below. The shape of the lens is Platform suggest that over 5 kilometers of determined by the difference in the density of carbonate sediment have accumulated on pre the fresh and saline waters, the amount of Triassic crystalline bedrock (Sheridan et al., recharge, and the permeability of the aquifer. 1988). The upper units on San Salvador Island If the aquifer is extremely permeable, the consist of Pleistocene and Holocene limestones freshwater will spread out on the surface of including eolian calcarenite, beachrock, fossil the saltwater and form a thin, brackish lens. coral reefrock, and minor paleosol and subtidal However, if the aquifer lacks sufficient lateral facies (Curran, 1985). Lithified eolian dune permeability, water will move more quickly ridges, 10 to 20 m high, dominate the downward than it can move horizontally, topography of the island, with shallow resulting in a mound of freshwater that brackish to hypersaline lakes occupying the develops above the base elevation of sea level. depressions between (Davis and Johnson, The depth of the freshwater/saltwater 1989). The shoreline is characterized by sand interface below sea level will theoretically be beaches, commonly containing Holocene approximately 40 times the height of the beachrock, located between headlands freshwater table above sea level. Field composed of older, eroded eolianites (Curran, investigations and groundwater sampling on 1985). San Salvador, with its karstified limestones of Elevations on San Salvador reach 10m varying permeability, have revealed that this in the area east of Club Med and southward relationship does not always hold true (Davis toward Cockburn Town (Klein et al., 1958). and Johnson, 1989). This area contains several shallow sinkholes In the interior of the island, fresh formed by the dissolution and collapse of the groundwater is found in discontinuous, surface limestone (Klein et al., 1958). isolated, lenses that lie beneath the Drainage of the area is chiefly underground consolidated carbonate dunes (Davis and because the rainfall sinks into cracks and Johnson, 1989). The interior freshwater solution cavities, or collects in sinkhole lenses, fed by infiltrating rain water, drain to depressions, and infiltrates into the subsurface the brackish and hypersaline lakes, where the (Klein et al., 1958 ). water evaporates. There are similar freshwater lenses near the coast that drain to the ocean; San Salvador Hydrogeology however, these are disrupted in many places by flow through caves and conduits. Most of the The climate on San Salvador is semi interior lakes and blueholes are connected to tropical with a rainy season lasting from May the sea by conduits, but some appear to be fed to December. Mean annual precipitation entirely by precipitation and groundwater (I 02.4 em) is exceeded by mean annual seeps. The freshwater lenses on the island tend potential evapotranspiration (142.8 em) (Foos, to be less than 20 m thick, irregular in shape, 1994 ). Almost all precipitation immediately small, and fragmented (Davis and Johnson, infiltrates into the rock so there is virtually no 1989). flowing fresh surface water on the island The ocean tides also play a role in the (Davis and Johnson, 1989). hydrology of San Salvador. Tides range from The volume and geometry of the 0.3 m to nearly 1.0 in, varying with the time of freshwater lenses beneath oceanic islands are day, lunar phases, and seasons (Davis and largely determined by the climatic and Johnson, I 989). This tidal action mixes the hydrologic characteristics of the aquifer fresh groundwater with the sea water and (Vacher, 1988). In homogenous, relatively moves this mixture through the dissolution unconsolidated and porous sediments, the conduits beneath the island's surface (Kunze downward infiltrating, less dense rainwater and Quick, 1994). Phreatic conduits introduce will displace the denser saltwater forming a sea water into the freshwater system with the lens-shaped body in accordance with the incoming tides.

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