Water resources of the Maunabo Valley, Puerto Rico

The Maunabo Valley, in southestern Puerto Rico, consists of a 3.5-square-mile alluvial plain surrounded by hills of metavolcanic and igneous intrusive rocks. The principal source of ground water in the basin is a shallow unconfined aquifer in the valley alluvium. Continuous pumping of the shallow aquifer has induced the flow of saltwater and has caused the chloride concentration to increase from about 30 to 540 milligrams per liter in the lower part of the valley. A study from January 1971 to December 1974 indicated that the hydraulic conductivities of the aquifer materials, in the basin, are less than 1 foot per day for the metavolcanic and igneous rocks. Estimated conductivities range from 10 to 100 feet per day for the alluvium. The average transmissivity of the alluvial aquifer is estimated to be 4,000 feet squared per day and the average specific capacity is 20 gallons per minute per foot of drawdown. In December 1974, the alluvial aquifer contained an estimated 10 ,000 acre-feet of water in recoverable storage. The data suggest that water supplies to meet future needs could be supplemented by the construction of surface-water-control structures and additional wells. Analysis of a digital simulation model suggests that wells be located in the upper reaches of the alluvial aquifer where a sustained yield of 3,000 gallons per minute could be obtained. (Woodard-USGS)