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Abstract:

Over 500 km of horizontally extensive submerged cave passages have been explored within the density-stratified carbonate aquifer of the Caribbean coast of the Yucatan Peninsula. These drain the large inland recharge area to the coastal margin and represent important regional-scale anisotropic permeability features. However, conventional theory is inadequate to explain this aquifer where conduit flows dominate within a high permeability matrix. Cave diving and surface observations have been used to develop a regional and multi-seasonal dataset of the density-stratified water column, and the circulation within the fresh and saline waters. A sharp mixing zone (MZ; 0.3 - 4 m thick) separates a cool fresh water lens (FWL) from a warmer saline water zone (SWZ) within the conduits. The SWZ temperature is highest at the coast (~ 28.0 oC) and decreases to ~ 25.5 oC at 10 km inland, while the FWL temperature is spatially constant (25 ± 0.2 oC, n = 33). The MZ has subtle isostatic response (2 - 10 cm) to the semi-diurnal coastal tides, however it remains at an almost uniform depth (within 1 m) despite significant water table variations (> 1 m). Thus, the FWL volume changes in direct proportion to the water table elevation. Previous Yucatan water budgets indicate that 15 % of precipitation recharges the aquifer, however this study shows that > 30 % is required to provide the observed coastal discharge. At least two flow regimes occur within the SWZ. Shallow SWZ flow (to ~ 5 m below the MZ) circulates with alternating periods of net inflow and outflow depending on the mean sea level. This regional reversing flow is substantiated by the inflow of warm marine water at the coast and a pattern of decreasing temperature along the flowpath with distance inland. At greater depth below the MZ (~ 5 - 45 m), saline water flows continuously inland irrespective of sea level. It is proposed that deeper saline flows may represent a cross-platform saline circulation, driven principally by a large east-west ocean head difference (~ 20 - 40 cm).