Professional Paper 1012
U.S. Geological Survey

You can DOWNLOAD THIS REPORT (7.512 MB) and PLATES 1 and 2 (806 KB) in Portable Document Format (PDF)
The Adobe PDF Reader program is available for free from Adobe.

Hydrogeology of the Karst of Puerto Rico

By Ennio V. Giusti

ABSTRACT

About one-fifth of Puerto Rico is covered by a tropical karst formed on a series of six limestone formations ranging in age from middle-Oligocene to middle Miocene. These formations strike east to west and crop out over the north coast of the island. Structurally, the rocks form a simple wedge abutting southward against a mountain chain of volcanic origin and thickening northward to about 1,400 meters by the seashore. All stages of karstification are present: from the incipient, found at the western end of the belt to the residual, found at the eastern end. Maximum development of sinkholes occurs on theAguada Limestone and upper part of the Aymamón Limestone. These formations have a CaCO₃, content range from about 85 to 95 percent. The semi-impermeable Cibao Formation has developed a fluvial drainage. An analysis of stream channel orientations indicates that the present topographic drainage oriented toward the northeast is super imposed on a former drainage system oriented toward the northwest. Transition from the northwestern to the northeastern drainage orientation is ascribed to Pleistocene eastward tilting of the Puerto Rican platform. This tilt is thought to have affected the subterranean drainage pattern as well, so that springs are found mainly on the western wall of northward-oriented valleys. Estimates of the water budget indicate that the karstic stream basins behave on an annual basis much as other stream basins that are not on limestone terrane. Average incoming solar radiation (expressed as evaporated water) and rainfall (2,900 mm and 1,750 mm, respectively) result in an evapotranspiration of about 1,100 mm (millimeters) annually and a discharge of 650 mm. This discharge is accommodated fluvially in areas underlain by the Cibao Formation and by the lower part of the Lares Limestone and subterraneally through the karst elsewhere. Base flow of streams in limestone in Puerto Rico is less than in streams in volcanic terrane, owing to fast disposal of rainfall through networks of subterranean solution channels. Ground water is found under water-table conditions in the Aymamón and Aguada and under artesian conditions in parts of the Cibao and the Lares. The unconfined ground-water discharges along a strip near the shoreline into swamps and lagoons; the artesian water discharges through a submarine face an unknown distance from the coast and possibly, in part, along a presumed fault near the coast. In the western part of the belt, ground-water discharges through the sea bottom, possibly as springs. Permeability is found to decrease exponentially with stratigraphic depth. Except for lake waters resting on terra rossa, most waters of the limestone belt are saturated or super saturated with respect to calcite, and as much as 86 percent of the solution is computed to arise mainly from enrichment of rainwater with CO₂ in the soil from the decomposition of organic acids. The denudation rate of the limestone belt through solution is computed as 0.070 mm per year with some evidence that abrasion may increase the denudation rate locally by as much as 40 percent. Calculations based on a projected initial limestone surface and the computed solution rate reveal that the limestone belt emerged from the sea about 4 million years ago and that the eastward tilt of the Puerto Rican platform, reported in the literature, occurred about 1 million years ago. Of the factors pertinent to karst development, aquifer permeability, both vertical and lateral, and primary rock porosity are thought to be the basic control for the existence and morphology of the karst. Assuming sufficiently pure limestone, climate is considered of secondary importance.

The citation for this report, in USGS format, is as follows:

Giusti, E.V., 1978, Hydrogeology of the karst of Puerto Rico: U.S. Geological Survey Professional Paper 1012, 68 p., 2 pls.