How coastal karst aquifers work:

Insight from the eastern Yucatan Peninsula, Mexico

Dr. Patricia Beddows

Department of Earth and Planetary Sciences

Northwestern University

ABSTRACT:


   Nearly 900 km of horizontally extensive flooded cave passages have been explored along the Caribbean coast of the Yucatan Peninsula, mostly within a narrow 9-12km-wide coastal band. Typical of all karstified carbonate aquifers, these high-permeability cave systems provide for >99% of the aquifer flux, while 97% of the aquifer water is held in matrix porosity. Cave diving via sinkhole ‘’cenotes” now provides exceptional access for direct observation and monitoring of the meteoric and intruding marine waters.

  

    The Caribbean coast caves are formed by coastal zone fresh water/saline water mixing processes. They comprise cross-linked anastomosing networks and canyon-shaped fracture guided passages. Multiple phases of development are associated with glacio-eustatic changes in sea level and all conduits are heavily modified by sediment and speleothem infill, and extensive collapse. Speleogenesis is also guided by bank-marginal fracturing, and low-porosity and low-permeability “caliche” subaerial exposure surfaces. The pattern of caves differs from the flank-margin eogenetic mixing chamber caves, the dendritic and rectilinear patterns of epigenetic continental (telogenetic) caves, and from the deeply penetrating vertical cenotes common in the NW of the peninsula.


    Entrainment of saline water is of limited significance in the conduit networks. Instead, decoupled fresh and saline flows span the sharp 0.3-4 m thick halocline. The temperature of the shallow saline water zone decreases exponentially with distance inland, and approaches thermal equilibrium with the fresh waters at ~10km. Flow-metering and dye tracing show the tidal modulation of the shallow saline circulation, with alternating periods of net inflow and outflow, and point velocities comparable to those of the overlying coastward-flowing fresh water (~1-5cm/s). Caribbean seawater actively circulates inland directly below the halocline, a flow direction opposite to that of both the conventional freshwater flow entrainment and geothermal convection models of coastal aquifer circulation. At greater depth below the halocline (~5-45 m), deeper saline water flow continuously inland and may traverse the platform driven by an east-west ocean head difference (~20-40 cm).


    The thickness of the halocline does not increase with proximity to the coast, and its depth level remains near stationary despite significant water table variations (>1m) during rainy seasons and hurricanes. The lens volume changes in direct proportion to the water table elevation, negating the value of the simple Dupuit-Ghyben-Herzberg relationship in calculating high-frequency responses to head variations. Long term monitoring of coastal discharge indicates that more than half of the >1m annual precipitation recharges the aquifer, in contrast to the commonly suggested 15% recharge. Mixing of saline water into the fresh water lens occurs where obstacles spanning the halocline induce turbulence, however salinization is limited at sites >4 km inland since most conduits lie above or below the halocline.


    This research provides an improved understanding of eogenetic coastal karst aquifer systems particularly in response to changing boundary conditions.  Applications of the findings include towards ecological and water and waste management along this coastline facing the Meso-American Barrier Reef System, a coastline where development plans include urbanization and dredging of navigable channel networks to create the “new Miami”. Salinization of the coastal aquifer is a greater threat in the Yucatan Peninsula compared to southern Florida peninsula given the conduit development, the role of extensive caliche as aquitards, and the thin fresh water lens measuring less than 25m thickness at 10km inland.