The Hydrogeology of Puerto Rico
Eric Harmsen1 and Ingrid Padilla2
Groundwater is defined as the water in the saturated pores of soil and geologic
material beneath the water table (Freeze and Cherry, 1979). Groundwater flow processes
in Puerto Rico are very complex. Aquifer systems receive recharge from rainfall and
numerous anthropomorphic activities. For example, irrigation water is commonly
applied in access of crop water needs, which results in deep percolation to underlying
water table aquifers. Line and point source recharge can also occur from streams and
rivers, and from injection of water through injection wells and agricultural drainage
wells. Groundwater is released from aquifer systems in a variety of ways including:
discharge to streams, rivers, lakes and the ocean; pumping by municipalities, industry and
agriculture; seepage from hillsides; discharge of water from springs originating from
confined aquifers; and direct evaporation when the water table when in close proximity to
the surface. Water also moves in complex ways between and within aquifers systems,
especially in areas of Puerto Rico characterized by karstic limestone.
Setting
Puerto Rico is an island between the Caribbean Sea and the North Atlantic Ocean,
east of the Dominican Republic. It is located at geographic coordinates: 18 15 N, 66 30
W and has an area of 9,104 sq km, slightly less than three times the size of Rhode Island.
The highest elevation in Puerto Rico is 1,338 m above mean sea level at Cerro de Punta,
PR (U.S. CIA, 2002).
Many of the islands of the West Indies archipelago share similar characteristics with
respect to climate. This is due in part to the influence of the trade winds and the islands’
mountainous topography (Kent, 2002). Generally, for these islands, rainfall is greatest in
the northeast and interior mountain areas. Due to orographic effects, the leeward side
may be quite dry and even semi-arid, as in the case of southwest PR. The rainy season
tends to be from June to November and the dry season from December to May. For a
given location, air temperature variations throughout the year are small; however, air
temperatures are highly correlated with elevation. An exception to this occurs within
interior mountain valleys where warm air can become trapped. In some cases average
temperatures within interior mountain valleys may be higher than coastal areas at lower
elevations (Capiel and Calvesbert, 1976).
1
Associate Professor, Department of Agricultural and Biosystems Engineering, University of Puerto Rico,
P.O. Box 9030, Mayagüez, PR 00683. email: eric_harmsen@cca.uprm.edu
2
Assistant Professor, Department of Civil Engineering and Surveying, University of Puerto Rico, P.O. Box
___, Mayagüez, PR 00681. email: ingrid@ce.uprm.edu
Review of Previous Studies
The U.S. Geological Survey (USGS) has divided Puerto Rico into five
groundwater areas: the North Coast, South Coast, West Coast, East Coast, and East
Central areas (Veve and Taggart, 1996). The North Coast area is underlain by limestone
aquifers as much as 5,600 ft thick. The highly productive aquifer system consists of two
limestone aquifers separated by an intervening confining unit (Olcott, 1997). The
Southern Coast is underlain by an alluvial aquifer that averages 3 mile wide and is
typically from 200 to 300 feet thick. West of Salinas to Ponce, the alluvium aquifer
ranges in thickness from about 100 ft to more than 1,000 ft near the coast (Olcott, 1997).
The West Coast area includes several alluvial valleys that are separated by ridges
composed of volcanic rock. The Lajas Valley, although included in the Western area is
different in that it is believed that block faulting formed the aquifer systems. The East
Coast area consists of a discontinuous narrow coastal plain, which includes several
alluvial river valleys. The East Central area is characterized by mostly mountainous
terrain and is underlain by volcanic rocks, intrusive rocks, a few interbedded limestones,
and isolated alluvial deposits in river valleys (Veve and Taggart, 1996).
For many years, the USGS has been conducting studies to characterize the
groundwater resources of Puerto Rico. These reports contain information about location
and major geographic features, population and groundwater use, geologic setting,
hydrogeology, groundwater levels and movement, and aquifer permeability. Frequently,
the USGS performed studies in areas to try to quantify the groundwater resources
available for human activities (e.g., agriculture, municipal and industrial uses). The
following list represents a few of the many reports prepared by the USGS in Puerto Rico:
Ground-water resources of the Caguas-Juncos Valley, Puerto Rico (Puig and RodríguezMartínez, 1993); Hydrogeology and ground-water/surface-water relations in the Bajura
area of the Municipio de Cabo Rojo (Rodríguez-Martínez, 1996); Ground-water
resources of alluvial valleys in northeastern Puerto Rico: Río Espíritu Santo to Río
Demajagua area (Pérez-Blair and Carrasquillo-Nieves, 1994, Pérez-Blair, 1997);
Groundwater resources in Lajas Valley, Puerto Rico(Graves, 1991); Detection of conduitcontrolled ground-water flow in northwestern Puerto Rico using aerial photographs
interpretation and geophysical methods (Rodríguez-Martínez and Richards, R. T., 2000);
Surface-water, water-quality, and ground-water assessment of the Municipio of Comerío,
Puerto Rico (Rodríguez-Martínez, 2001); and Effects of changing irrigation practices on
the ground-water hydrology of the Santa Isabel – Juana Díaz area, South Central Puerto
Rico (Ramos-Ginés, 1994).
A number of studies have also been performed in which computer models were
used to simulate local or regional groundwater flow in Puerto Rico. No studies have
attempted to simulate groundwater flow over the entire island, nevertheless, the following
studies, when combined, contain valuable information which can be used for the
development of an island-wide groundwater model. The locations of hydrogeologic
studies and groundwater computer modeling studies conducted in Puerto Rico are shown
in Figure 1.
Figure 6. Locations of Groundwater Resource and Modeling Studies in Puerto Rico
Robison and Anders (1973) constructed a three-layer, electrical analog model to
study the alluvial aquifer in the Yabucoa Valley located along the southeast coast of
Puerto Rico. The purpose of the model was to assess the groundwater flow patterns
under steady and non-steady-state conditions, and to assess maximum and practical
sustained yields from the aquifer. The study also considered the extent to which pumping
would cause inflow of sea water to the aquifer. Heisel and González (1979) constructed
an analog model to evaluate groundwater conditions on the south coast of Puerto Rico.
This works was based on an earlier regional-analog model of the South Coast Water
Province by Bennet (1976). Groundwater levels were evaluated during normal and
drought conditions. The model was also used to evaluate the injection of treated
wastewater by means of infiltration basins, injection wells, and irrigation. The study
concluded that, owing to the limited vertical hydraulic conductivity, irrigation is the most
practical method of wastewater use.
Torres-González (1985) developed a two-dimensional, steady-state groundwater
flow model of the limestone aquifer near Barceloneta, in north central Puerto Rico. The
objective of the model was to estimate the effects of additional groundwater withdrawals
on the aquifer water levels, and to determine whether hydraulic gradients relative to mean
sea level would be reversed as a result of the additional pumping. The MODFLOW
digital groundwater flow model developed by McDonald and Harbaugh (1984) was used.
The steady-state model was calibrated to February 1982 groundwater levels. Model
results were found to be highly sensitive to aquifer recharge. Calibrated values of aquifer
recharge were as high as 25 inches in some locations. During drought periods,
Groundwater discharge to the Caño Tiburnones and Río Grande de Manatí were found to
be significantly reduced.
Quiñones-Aponte (1986) developed a two-dimensional groundwater flow model
of the Río Yauco near Yauco, PR. The groundwater flow model used was MODFLOW.
The purpose of the model was to evaluate the effects of artificial or natural stresses on the
aquifer water levels. The model was calibrated for steady-state and transient conditions.
The steady-state calibration utilized October 1960 water budget information. The
transient calibration used data from 1979 through 1983. A sensitivity analysis was
conducted which determined the most sensitive model parameter was the streambed
vertical hydraulic conductivity to thickness ratio. The model was relatively insensitive to
aquifer recharge and evapotranspiration rates. Type curves showing the response of the
aquifer to various stream flow and pumpage conditions were developed.
A numerical model of groundwater flow (MODFLOW) in the Aguadilla to Río
Camuy area of Puerto Rico was developed by Tucci and Martínez (1995). The purpose
of the model was to quantify the components of the groundwater budget and evaluate the
aquifer as a possible public water supply. The three-dimensional, steady-state, two-layer
model covered approximately 200 square miles. The area consists of an uplifted rolling
plain to the north that lies 200 to 400 ft above seal level, and is heavily forested karst
upland to the south. The confining layer separating the upper water table aquifer and the
lower confined aquifer was simulated by means of a vertical leakance term. A basic
assumption of the model was that the karst rocks could be treated as equivalent porous
media (EPM), a limiting assumption in the governing flow equation used in MODFLOW.
A calibrated value of 22-in/yr aquifer recharge was used in the model in the karst uplands
to the south. Because of limited available data, the authors considered the model to be
only partially, or incompletely, calibrated. Despite the limitations of the model, the
results provide evidence that the aquifer could provide significant amount of water for
public water supply, and that additional groundwater development may be possible. The
report included a list of additional data needed to more completely define the
hydrogeologic system of the Augadilla to Río Camuy area.
Quiñones-Aponte et al. (1996) modeled the groundwater flow in the Salinas to
Patillas area of Puerto Rico using MODFLOW. The three vertical layer model covered
an area of 196 square miles. The purpose of the model was to test the sensitivity of the
aquifer system to changes in various hydrologic and geohydrologic factors. The model
was calibrated under steady-state conditions for March 1986, and also for transient
conditions coving the period from 1900 to 1986. The sensitivity analysis considered
hydrologic variables, hydraulic coefficients, and boundary hydraulic coefficients. Within
each of these groups respectively, the model was most sensitive to recharge from
irrigation applications, hydraulic conductivity of model layer 2, and conductance of the
sea-face boundary.
Saltwater intrusion is a problem in the shallow aquifers along the coast, especially
in the North Coast and South Coast areas. Olcott (1997) reported on the use of a
computer model to evaluate movement of the fresh-water/salt-water interface in the Río
Humacao Valley. The model results indicated that with large increases in pumping, over
1984 withdrawal rates, saltwater could potentially move more than 6000 ft inland.
Sepúlveda (1999) simulated groundwater flow and solute transport in the water
table aquifer in Vaga Alta, Puerto Rico. The three-dimensional groundwater flow and
solute transport model HST3D developed by Kipp (1987) was used. Although HST3D
can simulate variable fluid density, as occurs at the salt-water/fresh-water interface, this
option was not used. Heads measured between 1983 and 1992 were used to calibrate the
transient flow model. The root mean squared error of the residual heads (i.e., simulated
minus measured heads) was 0.81 ft. The calibrated groundwater flow model was used to
predict the hydraulic head and velocity distribution between 1992 and 2022. The
velocities from the flow model were utilized by the solute transport model to estimate
trichloroethylene (TCE) concentrations under three remedial alternatives.
.
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