Puerto Rico has a variety of aquifers that constitute a hydrological resource of great value and importance. Aquifers store the rain water that filters into the subsoil. This filtration occurs through porous soils, cavities and sinkholes in the surface of the earth. The water in an aquifer continues its course to the sea and, in the process, may emerge into other bodies of water such as streams, lagoons or rivers. The island’s aquifers are formed from three types of geological materials: sedimentary limestone rocks of variable porosity; alluvial deposits with high porosity; and volcanic rocks with limited porosity but with abundant fractures.

The main aquifers in Puerto Rico are divided into hydrological regions, which are: those formed by limestone rocks and alluvium in the northern region; the alluvial aquifers of the coastal valleys in the southern region; the alluvial aquifers in the interior valleys of Caguas, Cayey and Cidra (interior region); and the minor aquifers in the river valleys of the eastern and western regions. The most important aquifers in Puerto Rico are summarized in Table 1 and illustrated in Figure 1.

The southern zone of Puerto Rico has various independent alluvial aquifers: Patillas to Salinas, Coamo (Santa Isabel and Coamo), Juana Díaz to Ponce, Tallaboa (Peñuelas) to Guayanilla, Yauco and Guánica. The southern region of Puerto Rico is defined as extending from the Grande de Patillas River to the Loco River in Guánica, including the southern slopes of the Central Mountain Range and the coastal alluvial valleys.

The alluvial aquifers of the southern region are erroneously referred to as the “Great Southern Aquifer.” In hydrological terms, there is no continuous aquifer in the coastal valleys of the region, but rather a series of alluvial valleys separated by the rivers that flow from the Central Mountain Range. The region is divided into two main segments: the first extends from Patillas to Ponce and the second from the Peñuelas zone to Guánica.

Each alluvial valley in the two segments of the southern region have their own characteristics in terms of the extraction of water, and the recharging of water resources in one does not directly affect the other. Similarly, the extraction of water, the presence of pollution and salt intrusion in one does not directly affect the other.

The main geological strata or deposits that form the coastal aquifers of the band that runs from Patillas to Ponce in the southern region consist mainly of sand, gravel and rocks (a mix called alluvium). The alluvium has been carried in by the rivers from the mountains over millions of years (a period of approximately 1.6 million years). These alluvial deposits lie on limestone rocks of marine origin and volcanic rocks, both of which have low porosity.

In the zone that extends from Ponce to Peñuelas, there are no significant alluvial deposits and the surface consists mostly of limestone rocks of marine origin from the Tertiary geological period, of approximately 70 million years ago. The hills of limestone rocks that can be observed from Highway 2 to the west of Ponce to the eastern border of Guayanilla are not a productive aquifer.

Structurally, the coastal aquifers of the southern region of Puerto Rico are extensive, considering the limited size of the valleys that form them. The thickness of the alluvium in the aquifers varies from 100 feet in the Ponce zone to approximately 3,000 feet in the sectors near Santa Isabel. As indicated above, these deposits of alluvium lie on older limestone or volcanic rocks that do not have a high level of permeability like the alluvium.

The underground water flows from north to south in this region. This occurs because the water that recharges the aquifers comes from the rivers that flow from the southern slopes of the Central Mountain Range located to the north of the alluvial valleys, as well as the irrigation canals in the northern parts of the valleys. Once the water penetrates the subsoil, it flows vertically, due to the force of gravity, to the saturated zone, forming the aquifer level. Eventually, if the water is not extracted from the aquifer through wells, it discharges on the coast in the form of springs, wetlands and lagoons, or directly into the sea.

The coastal lagoons and swamps of the southern region are areas where the underground water surfaces. When extractions exceed the recharging, the aquifer level drops below that of the lagoons, swamps or the sea, and the natural flow of water ceases. This occurs in various zones of the aquifers in the southern region, with the greatest effect seen in Guayama, Salinas, Santa Isabel-Coamo and Ponce. The reduction in the recharging of the aquifers in these zones has led to drops of up to 40 feet in the levels of the aquifers, along with an increase in the concentration of dissolved solids. This renders the water part of the aquifer useless.

Author: María A. Juncos Gautier
Published: August 27, 2014.

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