The northernmost part of the fresh-water aquifer of Yucatan, Mexico, is confined near the coast by a thin, flat, nearly impermeable calcareous layer. We think it probable that this confining layer is now developing by a process of sedimentation and precipitation of porefilling cement on and near the surface of older limestone at the landward margin of the swamp that extends virtually continuously along the north Yucatan coast. It is our interpretation that this CaCO3-cemented layer, which is 0.5 to 1.4 m thick, develops in the zone of discharge of the fresh-water aquifer. The presence of 14C (6.0% ±0.4% modern carbon in a sample of aquitard) supports the hypothesis that cementation is an ongoing process. Further support comes from the remarkable regularity between modern mean sea level and the elevation of the landward boundary between the confining zone and normal karst along 250 km of coast.

Confinement of the aquifer produces an elevation of the piezometric surface to about 0.5 m above mean sea level and a concurrent depression of the fresh-water/salt-water interface to an estimated depth of about 18 m below mean sea level at the coast. Breaching of the confining layer, implicit in some development schemes for the region, could dramatically decrease the thickness of the fresh-water lens, a valuable water resource. The mixing zone beneath the confined part of the aquifer is a chemically active volume that may be vigorously pumped by tides (as evidenced by increased salinity of ground water near the coast), thus making this zone a likely place for rock-water interaction including, perhaps, dolomite formation.

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