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.

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.

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Extensive use of cement and concrete is envisaged in the construction of geological disposal facilities for radioactive wastes. The hyperalkaline porefluids typical of groundwaters that have reacted with these materials have the potential to react chemically with other engineered barrier components such as bentonite, potentially degrading their performance. Analcime, NaAlSi2O6.H2O, has been identified from previous modelling and experimental studies as a potential alteration product of bentonite.Laboratory experiments to investigate the stability of analcime under hyperalkaline porefluid conditions have been performed. Experiments used both batch and fluidized bed equipment at 25, 50, 70 and 90°C in K-based pH buffer solutions, both under- and over-saturated with respect to analcime. Results from dissolution experiments demonstrate that release of Na was greatly enhanced (by up to a factor of thirty) over that for Si and Al, particularly at pH 10 and 11. However, enhanced release of both Na and Al occurred in the batch experiments at pH 12–13. Near stoichiometric dissolution was observed in fluidized bed experiments under steady-state conditions at 70°C. Sodium was removed from the analcime structure by ion exchange for K, without involving dissolution and re-precipitation of the analcime framework. Scanning electron microscopy of reacted analcime grains showed that some grains had pronounced cracks parallel to original cleavage traces. These cracks were a result of volume decrease due to the substitution of K for Na ions and water molecules in the analcime structure to form leucite, KAlSi2O6.Synthesis of the dissolution data shows that the rate of dissolution increased with increasing temperature in the range 25–70°C and with pH at each temperature. Absolute rates of dissolution ranged from 10−10 mol m−2 s−1 at pH 9.5 at 25°C to 10−7 mol m−2 s−1 a pH 12 at 70 and 90°C. The rate of dissolution at any temperature was pH-dependent, such that the rate could be described by k (aH+)n, where k is the rate constant and n is −0.3 at 25°C, −0.4 at 50°C, −0.6 at 70°C and −0.7 at 90°C. Attempts to measure the growth rate of analcime in supersaturated solutions at 70 and 90°C were unsuccessful, although a limiting rate at 70°C, pH 10 was calculated to be 4 × 10−11 mol m−2 s−1, roughly 100× less than the rate of dissolution under the same conditions.These results imply that any trace amounts of analcime in bentonite will be converted to leucite by reaction with cement fluids with a high K/Na ratio. In some instances, leucite may thus incorporate K+ in preference to other phases (e.g. illite, K-feldspar) during alteration of bentonite by cement porefluids.

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