Reconnaissance of the ground-water resources of Vieques Island, Puerto Rico

Abstract

A reconnaissance of the ground-water resources of the Island of Vieques was conducted during 1982-1984. The island, located about 9 miles southeast of Puerto Rico, is formed by volcanic and intrusive rocks overlain by thin alluvial deposits along portions of the north and south coasts. The Esperanza valley, in the south-central part, contains the principal alluvial aquifer. The area of the valley is about 10 square miles and the alluvial deposits are about 60 feet thick. Transmissivity of the aquifer ranges from 200 to 2,000 square feet per day. Previous to 1982, pumpage of as much as 400,000 gallons per day of water from the aquifer resulted in saline water intrusion throughout the coastal zone of the valley. Ground water withdrawals from the aquifer (1982 to 1985) were nearly zero. Most of the wells sampled (1982-1984) exhibited chloride ion concentrations of less than 100 milligrams per liter. However, sodium absorption ratios ranged from 2.9 to 4.8 units, indicating a potential salinity hazard for some crops. Relatively important alluvial deposits also occur in Resolucion valley, on the northwest coast, and near Playa Grande, to the southwest. Pumpage from the Resolucion aquifer by the U.S. Navy through 1977 was as high as 200,000 gallons per day. In the Playa Grande area, as much as 50,000 gallons per day were pumped during 1965. A 2-dimensional finite-difference model was developed and calibrated for the Esperanza alluvial aquifer. Digital model tests indicate that about 300,000 gallons per day of water can be withdrawn from the aquifer during the wet season before a reversal of the gradient of the water table occurs indicating potential saltwater encroachment. During the dry season, withdrawals would be limited to about 200,000 gallons per day. INTRODUCTION Vieques is the largest offshore island belonging to Puerto Rico, with a surface area of about 51 mi 2 (fig. 1). It is located about 7 mi east-southeast of Puerto Rico and has a population of 7,662 (1980 census). Water resources on the Island of Vieques are limited due to its relatively small basins which receive an average of about 45 inches per year of precipitation and experience high evapotranspiration. There are no perennial streams on the island, and the known aquifers common yield is usually less than 100 gal/min (Gomez-Gomez and Heisel, 1980). Well fields in the Esperanza and Resolucion valleys were developed in the early 1960's, with total withdrawals of about 600,000 gal/d (written commun., Anderson, 1972). Ground-water withdrawals have been stopped in Resolucion Valley, and operation of the well field in Esperanza Valley was discontinued in 1978 as a result of increasing salinity and maintenance problems. A pipeline was constructed in 1977 between eastern Puerto Rico and Vieques to provide water to supply the town of Isabel II and the Esperanza valley area. Currently about 500,000 gal/d of drinking water are pumped through the pipeline from the Rio Blanco filtration plant to Vieques. In 1982, ruptures in the pipeline reduced the amount of freshwater available to Vieques to less than 250,000 gal/day. Water had to be shipped by barge from Puerto Rico to supply the deficit. At the same time, plans for agricultural development of Esperanza Valley were drafted by the Puerto Rico Land Administration and the Puerto Rico Department of Agriculture. Although the pipeline was later repaired, the reliability of this supply system remains in question. The potential demands for agriculture greatly exceed the pipeline capacity. The U.S. Geological Survey began an investigation in 1982 to define the ground-water resources of Vieques. The project was conducted in cooperation with the Puerto Rico Land Administration (PRLA), the Puerto Rico Department of Natural Resources (PRDNR), and the Puerto Rico Aqueduct and Sewer Authority (PRASA). Purpose The principal purposes of the investigation were as follows: 1. To describe the potential optimum yield and the occurrence, availability, and movement of water in the principal aquifers throughout the island of Vieques. 2. To define the areal and vertical extent of the Esperanza alluvial aquifer and the general characteristics of the Resolucion aquifer. 3. To define the principal chemical and physical characteristics of ground water throughout Vieques. A. To determine the effects of ground-water withdrawals on the Esperanza alluvial aquifer and the potential for saline-water intrusion. 5. To design, construct, and calibrate a two-dimensional ground-water flow model of the Esperanza alluvial aquifer. Although the investigation included the entire island of Vieques, most efforts were concentrated in the Esperanza and Resolucion valleys (fig. 2). 67«00' 66°00' 65°oo'