Climatic control of hydraulic conductivity of Bahamian limestones

Data from 244 single-well pumping tests of the karstified Lucayan Limestone on 14 different islands distributed through the Bahamian Archipelago, demonstrate a strong log-linear relationship between annual rainfall and the effective mean island effective hydraulic conductivity, which averages over two orders of magnitude higher in the wetter northern Bahamas (mean annual rainfall 1550 mm) than in the more arid southern islands (mean annual rainfall 810 mm). This relationship is independent of the saturated depth of the boreholes tested, although the hydraulic conductivity of the lower part of the formation is significantly greater (about 0.6-0.7 order of magnitude) than the upper unit, reflecting progressive secondary porosity generation with time. The direct effect of increasing ground-water flux on rates of dissolutional porosity generation in the wetter northern islands is supplemented by the greater geochemical potential for carbonate dissolution resulting from higher primary vegetative production. This both increases soil pCO2 via root respiration (a process augmented by higher soil moisture permitting more microbial activity), and the availability of organic matter for in situ oxidation within the aquifer. Effective hydraulic conductivity may also be directly related to island size. Islands in the north of the Archipelago are larger than those in the south, thus the fresh-water lens and fresh-/salt-water mixing zone are greater in size and expose more bedrock to dissolution