IMPLICATIONS FROM STUDIES OF CARBONATE AQUIFER SYSTEMS IN MEXICO

The University of Texas has conducted research in two unique Mexican karst systems - Cuatrociénegas in Coahuila with a very wide areal extent and Sistema Zacatón in Tamaulipas, one of the world's deepest karst systems. This has many implications for Cuatrociénegas, which contains groundwater-dependent ecosystems hosting over 70 endemic species threatened by groundwater pumping in adjacent valleys. Few hydrogeologic data exist; the location and timing of recharge are poorly constrained; and climate change effects must be estimated. We use sparse hydrogeologic data, catchment delineation, surface geophysics, and geologic maps to infer a wide regional flow system. Isotope data suggest old, regional groundwater flow predominates over recent, local recharge and that recharge occurs at elevations between 1,500 and 2,300 m. Because the springs discharge from both regional and local flow systems, management must also be regional in scope. Zacatón developed as a result of deep-seated, hypogenic processes, triggered by Pleistocene volcanism, within an immense hot spring Quaternary travertine and by collapse of large phreatic sinkholes (pozas). One of the pozas, El Zacatón, is one of the deepest phreatic sinkholes (~329 meters below the water table as mapped by DEPTHX, which is a NASA autonomous submersible robot that uses a suite of sonar transducers, inertial navigation, and a Doppler velocity meter to navigate in, map, and sample the physical underwater environment simultaneously).“Lids” of calcium carbonate have isolated large underlying water-filled voids that have been detected by electrical resistivity. Waters in the deepest sinkholes are reducing and warm with measurable levels of carbon dioxide and methane and anoxic conditions. These systems host a diverse microbial habitat that may contain new kingdoms of archea and bacteria identified by DNA sequencing; it is possible that unique microorganisms may have evolved in the sealed sinkholes. These studies suggest that long-term models of deep karstic systems may have to extend wider and deeper than is commonly considered.