CENOTES OF THE YUCATáN PENINSULA: AN AQUEOUS GEOCHEMICAL STUDY

Groundwater is the only readily available source of fresh water for the Yucatán Peninsula in México. The unconfined aquifer underlying the Yucatán consists of a mature karst system, which develops collapse features referred to as cenotes. Cenotes expose the water table to the surface, enable direct access to the aquifer, and also serve as sites for tourism. The primary source of economic growth for the area is tourism which has major impacts on the environment, including the aquifer. The high permeability of the karst system make the groundwater exceptionally susceptible to contamination. By creating an aqueous geochemical profile of 5 cenotes over 7 kilometers along the Ruta de los Cenotes (in Puerto Morelos, Quintana Roo, Mexico), potential connections between the cenotes and the aquifer can be identified. Time series sampling of 1 cenote used by tourists and 1 not used for recreational purposes provide a view into the potential differences in geochemistry between these two types of uses. Potential sources of groundwater recharge (e.g. surface runoff) were also characterized to see how they may contribute to the underlying aquifer. Analyses of ammonia, nitrate, nitrite, sulfate, sulfide, phosphate and chlorine were conducted using spectrophotometry and major cations and anions were analyzed using ion chromatography. Ammonia concentrations range from 0.02 - 0.13 mg/L. Nitrate ranges from 0.06 - 4.2 mg/L, with the highest values occurring in the tourist cenotes. The same is true for sulfate, which ranges from 6 to 30 mg/L. Phosphate ranges from 0.03 - 0.24 mg/L, with no discernible pattern between the tourist and non-tourist cenotes. Metals (arsenic, barium, cadmium, chromium, lead, selenium, and silver) were analyzed using inductively coupled plasma mass spectrometry and all were below detection limits. These initial results indicate little impact to the cenotes from potential sources of contamination such as surface runoff or wastewater. The major ion chemistry (analyses ongoing) will be used to create Piper and Stiff diagrams which will allow for comparisons to be made between the various cenotes. Using published data on the direction of groundwater flow in the area, it may be possible to generate simple mixing models to examine the evolution of the groundwater chemistry along the flow paths.