HYDROLOGICAL AND CHEMICAL PROCESSES IN GROUNDWATER WITHIN YUCATAN KARST

The fresh-water lens in the unconfined limestone surface aquifer of the northern Yucatan Peninsula generally has a constant vertical composition, ranging from 900 mg/l in the center of the peninsula to >1,400 mg/l tds five km from the coasts. At locations and depths unaffected by surface heating, the water column temperature is constant down to the halocline where the temperature generally increases rapidly by one to three degrees Centigrade. Apparently, fresh-water convection is occurring, induced by upward heat transfer across the halocline which destabilizes the bottom fresh-water and causes it to rise, mix, and sink. The process introduces and mixes dissolved salts from the halocline, dissolved oxygen from the water table, and pollutants entering at the top and bottom of the lens. The resulting fresh-water column is aerobic, with a relatively high salt content, uniform pollutant composition and lacks sulfate reduction. The salt composition is that of rainwater charged with carbon dioxide by decay processes, modified by carbonate-rock interactions, and mixed with modified seawater.

Yucatan sinkholes that collect organic matter and extend in depth below the halocline generally have ubiquitous sulfate reduction within the anoxic saline water, accompanied by anaerobic sulfide oxidation by photosynthetic purple and green bacteria within the halocline. The Eh drops below –100 mv and the pH spikes as much as a pH unit in the halocline. The source of the sulfate is seawater, making up the saline water in the northern Yucatan Peninsula. Gypsum dissolution in both the fresh and saline groundwater zones is an additional sulfate source in southern Yucatan and Quntana Roo.