No abstract

Dissolution of carbonate minerals in the coastal halocline is taking place in the karst terrain along the northeastern coast of the Yucatan Peninsula. The dissolution is being accelerated in cenotes (sinkholes) where sulfate reduction and oxidation of the produced sulfide is occurring. Hydrogen‐sulfide concentrations ranged from 0.06 to 4 mmolal within the halocline in two sinkholes. Relative to concentrations expected by conservative mixing, fluids with high hydrogen‐sulfide concentrations were correlated with low sulfate concentrations, high alkalinities, low pH values, and heavy sulfur isotope values for sulfate. Hydrogen‐sulfide concentrations were less than those predicted from sulfate reduction, calculated from deficiencies in measured sulfate concentrations, indicating mobility and loss of aqueous sulfide. Fluids with low hydrogen‐sulfide concentrations were correlated with very high calcium concentrations, high strontium and sulfate concentrations, slightly elevated alkalinities, low pH values, and sea‐water sulfur isotope values for sulfate. Gypsum dissolution is supported by the sulfur isotopes as the major process producing high sulfate concentrations. However, oxidation of aqueous sulfide to sulfuric acid, resulting in carbonate‐mineral dissolution is needed to explain the calcium concentrations, low pH values, and only slightly elevated alkalinities. The halocline may trap hydrogen sulfide that has been stripped from the underlying anoxic salt water. The halocline can act as a stable, physical boundary, holding some of the hydrogen sulfide until it is oxidized back to sulfuric acid through interaction with the overlying, oxygenated fresh water or through the activity of sulfide‐oxidizing bacteria.

No abstract

No abstract

Ground‐water velocities within fractures and boreholes, hydraulic heads, and depth profiles of conductivity were measured along a 70 km section of the northeastern coast of the Yucatan Peninsula, Mexico. Hydraulic heads ranged from 40 to 60 cm above mean sea level between 2 and 4 km from the coast. Fluid velocities estimated from point‐dilution tests, in the dual‐porosity rock in a borehole several kilometers from the coast, were 0.021 cm/sec in the fresh‐water lens and 0.082 cm/sec near a fracture in the underlying sea‐water zone. Velocities in large fractures increased from 1 cm/sec 10 kilometers inland to 12 cm/sec near discharge points along the coast. This increase is attributed to the decrease in thickness of the fresh‐water lens. The thickness of the fresh‐water lens is approximately 40% less than the Ghyben‐Herzberg relation predicts for a static system, providing the potential to drive fresh water through fractures into the sea‐water zone below the halocline. Overall, the halocline appears to be in a steady‐state position due to the rapid flow of fresh water and brackish water towards the coast combined with rising sea water in corivectional flow.