Tidal Water Level Fluctuations in Water Wells on San Salvador Island, Bahamas

Abstract

Water level fluctuations were measured in fifteen water wells near the Cockburn Town airport, in Grahams Harbor lagoon and in Riding Rock marina using custom-built electrical-capacitance tide gages. Results indicate prominent tidal cycles in all wells with tidal ranges between approximately 20 cm and 110 cm. Observed well tides show a similar mixed tidal pattern as predicted ocean tides with tidal lag times ranging from near 0 to 60 min and semidiurnal tide attenuation of 5 percent to 80 percent. Measured marine tides in the Riding Rock marina occur at the predicted times but are higher than predicted. Marine tides recorded in Graham9s Harbor correlate poorly with tide table predictions, have about half the predicted range and occur approximately one hour later than predicted. Lag times and tidal attenuations in wells show no clear correlation with distance from the coast and are largely incompatible with the tidal attenuation model of Ferris (1951). Well water salinities are inversely correlated with relative tidal ranges. Diurnal tide components in most wells are enhanced with respect to predicted ocean tides. Aquifer transmissivities calculated from observed semidiurnal tidal lag times, tidal range attenuations, and inland distances are surprisingly high and indicate great aquifer thickness. These results may be largely attributable to a highly cavernous, inhomogeneous subsurface to a depth of several kilometers, major tidal conduits near several wells, irregular coastlines, atmospheric pressure fluctuations, evapotranspiration cycles and unmodeled precipitation events.