The Lajas Valley, located in the southwestern region of Puerto Rico, has relatively extensive aquifers that produce a lesser quantity of water that contains minerals, due to the climate, geology and agricultural practices. The valley covers an area of 84.5 square miles from Guánica to the Hormigueros-Cabo Rojo region. The central plain, where the main aquifer is located, covers approximately 35 square miles (Figure 20).
The valley’s topography is relatively flat. It has a maximum elevation of 45 feet and drops to sea level in the eastern and western zones. The average annual precipitation in the valley is 45 inches, with a potential annual evapotranspiration of 37.8 inches. No permanent rivers or streams originate in the valley due to the scarcity of rain and the high evapotranspiration. There are two important lagoons, however: Cartagena and Guánica.
The Cartagena Lagoon is fed by mineralized subterranean water from the western zone of the valley and from the Lajas Canal. The Guánica Lagoon is also fed by mineralized subterranean water, from the eastern zone of the valley, and from floods from the Loco River. The lagoon is dry most of the time because it is drained by a canal, built in 1955, that connects it with Los Negros Canyon, which empties into Guánica Bay. Except for the water imported through the Lajas Canal, subterranean water, which is scarce, is the only source of water in the valley.
The Lajas Valley has two main aquifers: the non-confined alluvial aquifer and the confined alluvial aquifer. In addition to these aquifers, there are limestone rocks underneath the alluvium that may contain moderate quantities of water. The surface geology of the valley consists mainly of non-consolidated deposits of clay and silt, intermixed with layers of thick sedimentary material carried in by temporary rivers and streams. These sediments rest on a volcanic base with small blocks of limestone of indeterminate age in the southern and southwestern parts of the valley.
The igneous and volcanic formations that surround the Lajas Valley have a low permeability. Wells built around the perimeter of the valley generally penetrate the meteorized part of the rocks and yield about 15 gpm. Irregularly distributed Tertiary cretaceous limestone formations are exposed in the hills in the north of the valley and presumably extending to the south, where they may be covered by the clay and alluvial deposits in the valley. The thickness of the limestone rocks in the central part of the valley may reach 50 feet and the two alluvial deposits a maximum thickness of 300 feet.
Mineralized water predominates in the aquifers of the Lajas Valley, with pockets or areas of fresh water. Until the end of the Tertiary period (65-70 million years ago), the valley was covered by the sea. When the ocean receded, sea salt accumulated on the ground. Later, the marine deposits changed and decomposed and mixed with alluvial deposits. The salts were retained in the soil due to the scarcity of rain and because the low permeability of the valley’s soils did not allow it to dissolve and disappear.
Wells drilled into the limestone rocks can produce up to 2,000 gpm, but the water soon becomes mineralized as the salts in the soil dissolve. In modern times, the rain that infiltrates through the alluvium has dissolved the salt in the aquifers and mineralized the water. In some sectors, the recharging by rain and surface flows through high permeability zones has resulted in the removal of the salts and the accumulation of pockets of fresh water.
The most significant sources of subterranean fresh water in the Lajas Valley are found in the western zone. The most important pockets of fresh water occur in fractures of the limestone formations. The most extensive one is probably a semi-confined limestone formation in the northeastern part of the valley. Recharging of the water in the aquifers comes from the rain on the perimeter of the valley, surface flows from temporary rivers and streams and, since 1955, the application of water provided by the Lajas Valley Irrigation District.
In the western zone of the valley, up to 2.5 mgd of fresh water are extracted for the Cabo Rojo municipal water supply. In the eastern sector of the valley, toward Guánica, 1.0 mgd are extracted for the public water supply and for agricultural irrigation. In the 1940s, up to 18 mgd of underground water was extracted from the valley. The progressive mineralization of the water as the salts dissolve limits the extraction and the current total is no more than 5 mgd.
The yield of wells in the valley varies from 5 to 690 gpm. Despite the reduced production, the fresh water has a high concentration of dissolved solids. As a result, there are no significant wells. The existing wells are located mainly in the central and northeastern parts of the valley, where the potential for developing water resources is greater.
Author: María A. Juncos Gautier
Published: August 27, 2014.
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