In Puerto Rico, a variety of aquifers make up a valuable and important system of water resources. These store rain water that seeps down through the subsoil – through porous soils, holes, and cracks in the surface. The water in an aquifer continues on its course to the sea, and in the process can feed other bodies of water, such as streams, lagoons, and rivers. The aquifers of Puerto Rico are formed of three types of geological material: rocks made up of limestone sediment of varying porosity, alluvial deposits of high porosity, and rocks of volcanic origin whose porosity depends on the abundance of fractures.
The principle aquifers of Puerto Rico are classified by hydrological zone: those in limestone and alluvial deposits in the northern zone; those in the alluvial deposits of the coastal valleys in the southern zone; the alluvial aquifers in the inland valleys of Caguas, Cayey and Cidra (the interior zone); and the lesser aquifers in the river valleys in the eastern and western zones. In the interior zone, aquifers of lesser importance have formed as a result of cracks in rocks of volcanic origin and alluvial deposits in the valleys of the rivers. On the islands of Vieques, Culebra and Mona, there are also alluvial aquifers of lesser importance. The most important aquifers in Puerto Rico appear in Table 1 and are illustrated in Figure 1.
|Description: Indicator of three types of aquifer: limestone, alluvial, and volcanic.(DRNA, 2004)|
|Description: Hydrogeological map of Vieques and Culebra. (USGS, 2004 DRNA, 2004)|
The most important of Puerto Rico’s aquifers are in the northern zone, in an area called the Karst Region (also referred to as the North Coast). This extends from Luquillo to Aguadilla and occupies an area of approximately 905 square miles. The zone runs from the coast to the foothills of the central mountain range in the middle of the island and the Sierra de Luquillo in the east. In this zone, there are two principle systems of aquifers: the Upper (Flat or Freatic) Aquifer and the Lower (Deep or Artesian) Aquifer (Figure 2) :
1. The Upper (Flat or Freatic) Aquifer is formed of sand and gravel deposited by rivers (alluvial matter) and marine deposits of low porosity placed over segments of limestone formations (some older and some more recent). These formations are those of San Sebastián, Montebello, Lares, Cibao, Aguada, Bayamón and Camuy. The aquifer extends as a whole from the area of Campanilla in Toa Baja to the alluvial valley of the Río Grande of Arecibo.
|Description: Aquifers in the north of Puerto Rico.(UGS 1998, DRNA 2004).|
The upper aquifer of the karst region is the most productive and has the greatest capacity for storage of water in Puerto Rico. It is an important source of supply for human consumption, industrial use and agricultural activities, and in 2002, approximately 52 million gallons per day (mgd) were extracted from it.
The alluvial deposits are in the form of fans located in the coastal river valleys that cross the zone, principally from Toa Baja to Arecibo. This fan-shaped alluvial system is part of the river system that originates in the central mountains and flows north toward the ocean, draining the northern zone. These rivers include the Río Grande of Arecibo, Río Grande of Manatí, Río Cibuco, Río de la Plata, Río Bayamón and Río Hondo, though others cross the zone on the eastern and on the western sides of the aquifer.
The thickness of the aquifer varies from zero to 1,640 feet, with individual averages in the individual valleys from zero to 500 feet. These characteristics lead to hydraulic conductivity values from 300 to 3,000 feet per day, among the highest in Puerto Rico. Hydraulic conductivity is the degree to which the pore spaces or fractures allows the water to pass through. Together with abundant rainfall in the recharging areas of the aquifer, in the foothills of the central mountain range and the karst region, these properties yield a high level of productivity from the aquifer.
Prudent long-term use of an aquifer requires that the extraction of underground water not exceed the amount of water that recharges it naturally, unless the recharging is augmented using artificial methods. Planned use contributes to preventing or minimizing such problems as significant reduction in the water levels, saline intrusion with increases in the solids dissolved in the water, and deterioration of the quality of the water due to migration of contaminants from dispersed sources such as served waters from domestic septic tanks or industrial spillage. Documented and estimated extraction for each of the regions is presented in Table 2.
|Description: Extraction of underground water from upper aquifer in the north of Puerto Rico in 2002.(DRNA, 2004)|
2. The Lower (Deep or Artesian) Aquifer includes layers of rock from the limestone formations of San Sebastián, Lares and Montebello, confined by stata of clay and silt from the upper part of the Cibao layer (or confining layer). The aquifer extends from Aguadilla to the metropolitan area of San Juan, though the areas with greatest water production capacity are found between Manatí and Arecibo.
The lower aquifer is the principal source of water in Puerto Rico for industrial use, as well as supplying moderate amounts for human consumption. In the coastal areas of the municipalities from Barceloneta to Manatí, in 2002 approximately 5.3 mgd was extracted from the Aartesian aquifer for industrial pharmaceutical use. The water level in this aquifer allowed the wells in the zone to flow without needing to be pumped, but due to excessive extraction in the recharging areas, as well as leaks from deep wells, the water pressure of the aquifer has declined.
The lower aquifer of the northern zone, which runs from Camuy to Vega Baja, is commonly referred to as the artisan or deep aquifer. The flow of underground water in this aquifer is primarily toward the north-northeast, with discharge points along the coast and in the sea. This aquifer, in limestone from the Lares and Montebello formations, lies below the Upper or flat fquifer, separated from it by the confining stratum.
It is estimated that the aquifer receives an annual water recharge of 7 inches, on average, principally from areas nearby, from the town of Florida to the Guajataca Reservoir. The zones of greatest water production in the aquifer run from Manatí to Barceloneta. In that zone, there are 28 industrial, domestic, and agricultural wells drilled to depths of up to 1,500 feet that extract an average of 5.3 mgd from the aquifer.
Extraction began in 1970 to supply water to the pharmaceutical industry that has developed in the area. Water extraction, together with reductions in the recharging of the aquifer due to interception by the wells of the Autoridad de Acueductos y Alcantarillados (AAA, Aqueduct and Sewer Authority) in the municipality of Florida; have resulted in drastic reductions in the water pressure in the aquifer. Defects in the construction of the wells allow leakage from the lower aqueduct to the upper aqueduct. This leakage of water (estimated at more than 2.0 mgd) occurs primarily in the area of the upper aquifer, where most of the water is briny or saline, and so is not potable. These changes in recharging and filtering have lead to a decrease in the water pressure in the area of Cruce Dávila in Barceloneta from 440 feet above sea level, in 1970, to approximately 290 feet above sea level in 2005 (Figure 4) .
|Description: Changes in artesian pressure in the lower aquifer near Barceloneta, between 1970 and 1992.(USGS,DRNA, 2003).|
The upper aquifer in the northern zone receives an annual recharging that varies by place and over the year from 10 to 12 inches. The recharging varies with the permeability of the materials in which the aquifer has formed, the most critical factors in maintaining the level being the maintenance of the height of the freatic stratum and the level of the water. This, in turn, controls saline intrusion in the coastal zone and allows for continuing careful extraction of underground water using wells. In general, recharging is greater in areas where the Upper Aquifer is in limestone (to the south of State Road PR-670), rather than in alluvial valleys. In limestone areas, natural recharging is up to 28 inches, while in the alluvial valleys of rivers, it rarely exceeds 8 inches.
Maximum Extraction Capacity
Though the upper and lower aquifers of the northern zone are the most important in Puerto Rico, their capacity to produce water is not infinite. The Department of Natural Environmental Resources estimates that the maximum production capacity of the Upper or freatic aquifer is approximately 50 mgd. This value is derived from studies conducted by the U.S. Geological Survey and is based on a sustainable level of extraction that does not lead to significant decreases in the freatic levels of the aquifer. This value includes several zones that have been affected by saline intrusion and chemical contamination. Current levels of extraction exceed the sustainable production of the aquifer. In several areas of the aquifer, principally between Dorado and Vega Baja, the freatic levels will continue to go down unless extraction is reduced immediately.
Author: Sigfredo Torres González
Published: August 27, 2014.
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