HYDROGEOLOGY AND CONCEPTUAL MODEL OF THE KARSTIC COASTAL AQUIFER IN NORTHERN YUCATAN STATE, MEXICO

The coastal zone of northern Yucatan Peninsula (YP) is mainly constituted by Tertiary limestones, covered by Pleistocen limestones, where there exist swamps and estuary systems, locally called “rias”, with mouths connecting them to the sea and hence being a way for an important amount of groundwater to discharge, like in Ria Lagartos and Celestun. These limestones have karstic layers located at depths from 8 to 16 meters below terrain surface. It is in these layers where groundwater mainly flows toward coast, passing below the sand dune and discharging in the sea in the form of submarine springs which in many cases manifest themselves on the marine surface depending on the hydraulic or piezometric fresh water head. The width of the superficial limestone within this coastal fringe, called “caliche”, varies from 5 to 10 kilometers in the study zone (Chuburna-Progreso-Chicxulub). Its permeability is extremely low, so it constitutes a confining layer that impedes superficial waters to percolate toward groundwater. The hydraulic head of the groundwater below this confining layer is over the mean sea level and also over the swamp water level, coastal lagoons and estuaries. There are two important hydrological phenomena that occur in this coastal fringe: 1) There is no recharge to the aquifer (groundwater ) due to limestone rock outcrops is impermeable or semipermeable ; and 2) groundwater pressure is not lost, nor saline interfase is rised if the superficial layer is broken. The groundwater pollution vulnerability within this coastal fringe is less than that for the superficial saline waters of swamps and estuaries, because of caliche’s low intrinsic permeability that impedes percolation.