CLIMATIC CONTROL OF HYDRAULIC CONDUCTIVITY OF BAHAMIAN LIMESTONES

Data from 244 single-well pumping tests of the karstified Lucayan Lime stone on 14 different islands distributed through the Bahamian Archipe lago, demonstrate a strong log-linear relationship between annual rain fall and the effective mean island effective hydraulic conductivity, w hich averages over two orders of magnitude higher in the wetter northe rn Bahamas (mean annual rainfall 1550 mm) than in the more arid southe rn islands (mean annual rainfall 810 mm). This relationship is indepen dent of the saturated depth of the boreholes tested, although the hydr aulic conductivity of the lower part of the formation is significantly greater (about 0.6-0.7 order of magnitude) than the upper unit, refle cting progressive secondary porosity generation with time. The direct effect of increasing ground-water flux on rates of dissolutional poros ity generation in the wetter northern islands is supplemented by the g reater geochemical potential for carbonate dissolution resulting from higher primary vegetative production. This both increases soil pCO(2) via root respiration (a process augmented by higher soil moisture perm itting more microbial activity), and the availability of organic matte r for in situ oxidation within the aquifer. Effective hydraulic conduc tivity may also be directly related to island size. Islands in the nor th of the Archipelago are larger than those in the south, thus the fre sh-water lens and fresh-/salt-water mixing zone are greater in size an d expose more bedrock to dissolution.