Estimation of the change in freshwater volume in the North Coast Limestone upper aquifer of Puerto Rico in the Rio Grande de Manati-Rio de la Plata area between 1960 and 1990 and Implications on public-supply water availability

Abstract

Ground water in the upper aquifer of the North Coast Limestone aquifer system historically has been the principal source of public-supply and self-supplied industrial water use in north-central Puerto Rico. Development of the aquifer for these two major water-use categories began in about 1930; however, withdrawals did not become an important water-supply source for sustaining local development until the 1960s. Ground-water withdrawals averaged about 6 million gallons per day from 1948 to the mid-1960s and peaked at about 33 million gallons per day in the 1980s. Withdrawals have since declined, averaging about 11.5 million gallons per day in 2002. Aquifer contamination by industrial chemical spills and by nitrates from agricultural and domestic sources initially reduced pumpage for public-supply use within localized areas, leading eventually to increased withdrawals at unaffected well fields. The long-term effect of unconstrained ground-water withdrawals has been a regional thinning of the freshwater lens in an area encompassing 50,600 acres between the Río Grande de Manatí and Río de la Plata, generally north of latitude 18º25’. The effects of aquifer overdraft have been documented in the regional thinning of the freshwater lens, with an increase in dissolved-solids concentration in ground-water wells. Dissolved-solids concentration in public-supply wells were generally between 250 and 350 milligrams per liter during the 1960s, but increased to greater than 500 milligrams per liter in virtually all of the wells by 2000. Depletion of fresh ground water was estimated at 282,000 acre-feet: 103,000 acre-feet in the Río Grande de Manatí to Río Cibuco area between 1960 and 1995, and 179,000 acre-feet in the Río Cibuco to Río de la Plata area between 1960 and 1992. Thus, aquifer freshwater volume depletion below mean sea level datum may have contributed as much as 38 percent (7.5 million gallons per day) of the 20-million gallons per day average withdrawal rate during the stated time periods. The calculated depletion of aquifer freshwater volume is equivalent to an average long-term rate of 8,400 acre-feet per year. Aquifer withdrawals can be anticipated to decline to about 10 million gallons per day by 2010 at the projected trend of well closures. The lost supply would have to be compensated from surface-water sources because the part of the North Coast Limestone aquifer system south of latitude 18º25’, although less vulnerable to saline-water encroachment, is not as productive.