M.I. Martinez | Geology Reports

USGS1995

Surface electromagnetic geophysical exploration of the ground-water resources of Isla de Mona, Puerto Rico, a caribbean carbonate island

Two electromagnetic surface geophysical techniques were used to explore the ground-water resources of Isla de Mona, Puerto Rico—a 55-square-kilometer island located between Puerto Rico and Hispañiola, Isla de Mona is a tectonically uplifted carbonate plateau of Neogene age that has an average elevation of about 50 meters above mean sea level. This plateau is bounded by vertical cliffs except on the southwest where there is a narrow, 3-square-kilometer coastal plain. The coastal plain is composed of Quaternary carbonate deposits, and has a maximum elevation of less than 10 meters above mean sea level. No large-scale surface-water features are found on the plateau or on the coastal plain. To better understand the aquifer characteristics of the island, terrain conductivity and transient electromagnetic data were collected on the coastal plain and on the plateau. Computer programs were used to analyze quantitatively the electromagnetic data. Geoelectric models were produced to approximate the depth below land surface of the saline-freshwater interface underlying both the coastal plain and the plateau. Because the geophysical methods could not discern the water-table, it was assumed that it was near sea level. The thickness of the freshwater lens was estimated by subtracting the elevation of the land surface above mean sea level from the depth to the saline-freshwater interface as determined from the geophysical data. Results from the geophysical methods and water-level observations indicate that a freshwater lens with a maximum thickness of about 10 meters exists under the coastal plain. This lens thins towards the ocean, and it also thins away from the ocean toward the plateau. The model produced from the transient electromagnetic data indicates that the freshwater lens under the plateau has a maximum thickness of about 14 meters, which is a much thinner lens than previously estimated. A freshwater lens thickness of 14 meters is similar to direct measurements by divers in Cueva de Agua at Punta los Ingleses (located on the southeast coast), and observations by divers of freshwater seeps into the ocean at depths of 8 to 10 meters along the north coast cliffs after a major rainstorm. Ground-water flow in the coastal plain appears to be radial from the center of the freshwater mound. At the intersection between the coastal plain and the plateau, the flow is parallel to the coastline. The direction of flow on the rest of the plateau could not be determined accurately with the available data.

Carbonates and Evaporites↗

USGS1995

Hydrology and simulation of ground-water flow in the Aguadilla to Rio Camuy area, Puerto Rico

The aquifers of the Aguadilla to Rio Camuy area, in the northwestern part of Puerto Rico, are the least developed of those on the north coast, and relatively little information is available concerning the ground-water system. The present study, which was part of a comprehensive appraisal of the ground-water resources of the North Coast Province, attempts to interpret the hydrology of the area within the constraints of available data. The study area consists of an uplifted rolling plain that is 200 to 400 feet above sea level and a heavily forested, karst upland. The only major streams in the area are the Rfo Camuy and the Rio Guajataca. Most water used in the area is obtained from Lago de Guajataca, just south of the study area, and ground-water use is minimal (less than 5 million gallons per day). Sedimentary rocks of Tertiary age, mainly limestone and calcareous clays, comprise the aquifers of the Aguadilla to Rio Camuy area. The rocks generally dip from 4 to 7 degrees to the north, and the total sedimentary rock sequence may be as much as 6,000 feet thick near the Atlantic coast. Baseflows for the Rio Camuy are 58 cubic feet per second near Bayaney and 72 cubic feet per second near Hatillo. The ground-water discharge to the Rio Camuy between these stations is estimated to be 15 cubic feet per second, or 2.6 cubic feet per second per linear mile. The flow of the Rio Guajataca is regulated by the Guajataca Dam at Lago de Guajataca. Ground-water discharge to the Rio Guajataca between the dam and the coast is estimated to be about 17 cubic feet per.second, based on the average ground-water discharge per linear mile estimated for the Rio Camuy. Both water-table and artesian aquifers are present in the Aguadilla to Rio Camuy area; how-ever, most ground water occurs within the watertable aquifer, which was the primary focus of this study. The top of the confining unit, below the water-table aquifer, generally is within the unnamed upper member of the Cibao Formation; however, it is within the Los Puertos Formation in the eastern part of the study area. The water-table aquifer primarily is composed of rocks of the Aymam6n Limestone and the Los Puertos Formation. The estimated saturated thickness of the water-table aquifer ranges from zero at the southern limit of the aquifer to more than 600 feet south of Isabela. Hydraulic conductivity of the Aymam6n Limestone, based on specific-capacity test data for seven wells, ranges from about 1 to about 25 feet per day and averages 7.5 feet per day. Hydraulic conductivity of the Los Puertos Formation, based on specific-capacity test data for four wells, generally was less than 7 feet. per day. The average hydraulic-conductivity value for both the Aymam6n Limestone and the Los Puertos Formation, based on specific-capacity test data, is estimated to be about 6.0 feet per day. These hydraulic-conductivity values are much less than average values for the water-table aquifer reported for other parts of the North Coast Province. Transmissivity values, based on the average hydraulic-conductivity value for the aquifer derived from specific-capacity tests, range from zero to about 4,000 feet squared per day; however, these values were adjusted upward during model calibration. Ground water generally moves from the highlands in the south toward the sea to the north and west, and locally, to streams. A major groundwater divide extends from the southeastern corner of the study area to the northwest, and separates flow north and east into the study area from flow to the southwest toward the Rio Culebrinas. Nearly all recharge to the aquifer is from infiltration of rainfall into the karst uplands. Discharge from the aquifer primarily occurs as leakage to streams and to the sea, and to a lesser degree as flow to wells. A two-layer, three-dimensional, steady-state, numerical model was constructed to simulateground-water flow in the water-table aquifer between Aguadilla and the R/o Camuy area. A basic a

Water-Resources Investigations Report↗