Advancing Water Resources Research and Management
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Abstract
Introduction
Purpose and Scope
Description of Study Area and Climate
Methods of Data Collection and Analysis
Results and Discussion
Conclusions
References
Authors
The development potential of ground-water resources in northwestern Puerto Rico, from the Río Camuy to Aguadilla, is uncertain due to the limited knowledge regarding aquifer properties and ground-water occurrence. An investigation was conducted using an analysis of linear terrain features and the geophysical methods of natural potential and gravity to detect the occurrence of conduit-controlled ground-water flow at selected sites in the Río Camuy to Aguadilla area in northwestern Puerto Rico. The results obtained at the Pozo Brujo and the University of Puerto Rico Agricultural Extension Farm sites, 2 of 9 sites tested, are presented in this report. At these two sites the presence of distinctive linear terrain features and the detection of natural potential and gravity anomalies suggest the occurrence of conduit-controlled ground-water flow.
Typical hydrologic features in the Río Camuy to Aguadilla area in northwestern Puerto Rico are the presence of freshwater conduit-type springs, cavernous porosity in some wells, numerous sinkholes, and rivers with entrenched and underground courses. Little is known about the areal extent and exact locations of the conduits feeding the springs. Few deep wells have been drilled in this region and aquifer transmissivity appears to be low between the suspected locations of these major karstic flow components, particularly in the area underlain by the Camuy Formation and Aymamón Limestone. Only two deep exploratory wells have been drilled west of Arecibo (Rodríguez-Martínez and Hartley, 1994).
This report presents results from an investigation conducted by the U.S. Geological Survey (USGS), in cooperation with the Puerto Rico Aqueduct and Sewer Authority (PRASA), to define the occurrence of conduit-controlled ground-water flow in the karst aquifer of northwestern Puerto Rico. Some of the results obtained at Pozo Brujo and the University of Puerto Rico (UPR) Agricultural Extension Farms sites, 2 of the 9 sites tested, are presented in this report (fig. 1).
Figure 1. Map showing aereal extent of the Río
Camuy to Aguadilla area, northwestern Puerto Rico.
The study area is in the North Coast Limestone belt and extends from the Río Camuy to Aguadilla in northwestern Puerto Rico (fig. 1). The climate of the study area is tropical humid. Karst topography, the result of dissolution of the middle Tertiary limestone, prevails in the study area. Three significant surface drainage features in the study area are the Río Camuy, Río Guajataca, and Quebrada de los Cedros.
The study area is underlain by a thick sequence of platform carbonates and minor terrigenous clastics ranging in age from Oligocene to Miocene that is part of the regional sequence of middle Tertiary rocks of the North Coast Limestone belt (Monroe, 1980). These rocks comprise a homoclinal sequence that generally dips northward. Dips range from 2 degrees near the coast to 6 or 7 degrees where these rocks lie in contact with the volcanic core of Puerto Rico. However, in the Aguadilla and Moca area the strike is nearly due north and the dip is as much as 10 degrees toward the west because of a north-northwest plunging anticline. A small dome, which seems to reflect the antecedent topography of deeply buried volcanic basement rocks, similarly disrupts the dip of the strata in an area that includes parts of the municipalities of Quebradillas and Isabela. The geologic units exposed in the study area are, from oldest to youngest, the San Sebastián Formation, Lares Limestone, Cibao Formation, Aguada Limestone, Aymamón Limestone, and Camuy Formation (Monroe, 1980).
The Pozo Brujo is an ephemeral spring located at the base of the cliff in the narrow coastal plain of the municipality of Isabela (figs. 1 and 2). Similar to other springs along the cliff base, it is presumed that the discharge originates from fractures or conduits that extend into the carbonate platform to the south. The UPR site (figs. 1 and 3) is relatively flat, with occasional surface depressions and sinkholes. A test well drilled by the USGS in 1986 at this site revealed the existence of caverns of significant size above and below the water table (Rodríguez-Martínez and Hartley, 1994). The drainage at this site seems to be mostly underground, as is the case in most karst plains.
Figure 2. Location of the Pozo Brujo
"ephemeral spring" site and accompanying NP profile.
The analysis of linear terrain features and surface geophysical surveys were used to detect the occurrence of conduit-controlled ground-water flow in the study area. Linear terrain features, as used in this report, refer to straight stream reaches, ridges, lineaments expressed by contrasts in vegetation types, separation lines between soils of different color tone, and alignment of sinkholes (predominantly the orientation of the long axes) (Lattman and Parizek, 1964). The identification of linear terrain features usually is done through the analysis of aerial photographs, topographic and geologic maps, and satellite imagery. Field reconnaissance is needed to verify their existence and determine that they do not represent cultural artifacts. In this study the greatest emphasis was put on features such as straight river reaches, ridges, and aligned sinkholes. These features can be readily identified and studied in the field which allows for completion of geophysical tests and further geological reconnaissance in the same areas. Lineaments were also identified during this study, but because of their implicit ambiguity and because these can not always be identified in the field, these were given only secondary emphasis.
Geophysical prospecting was done at those sites where the presence of linear terrain features strongly suggests the occurrence of conduit-controlled flow. The two geophysical methods used were natural potential (NP) and gravity. The NP method measures voltage potentials which originate from natural electircal currents (Kilty and Lange, 1991). Voltage readings that are significantly above or below a background value may be indicative of subsurface cavities, which in turn might suggest the presence of conduit-controlled ground-water flow. The NP method used was the fixed and roving electrode method (Kilty and Lange, 1991). A station separation of 10 meters was used. A Fluke model 25 digital multimeter was used in the NP surveys (use of trade names in this paper is for identification purposes only and does not imply endorsement by the U.S. Government). The gravity method measures differences in mass distribution in the subsurface. These differences in mass distribution may be due to the occurrence of cavities. A Worden Master gravimeter capable of resolving approximately 0.1 milligal anomalies was used to collect gravity data. Terrain, tide, and drift corrections were applied to the gravity data collected. Tide and drift rate corrections ranged from 0.03 to 0.29 milligal per hour. Gravity data were also corrected for latitudinal location and were reduced to simple Bougher gravity using station elevation data. Ground-surface elevations were measured within 0.06 meter. Gravity readings were obtained at base stations at intervals not longer than 1 hour. Gravity data was collected at 10-meter intervals.
Linear terrain analysis was completed and NP data collected in the area immediately south of Pozo Brujo (fig. 2). The search for conduits at the UPR site consisted of the identification of aligned sinkholes and the use of NP and gravity surface geophysical methods (figs. 3a and 3b, figs. 4a and 4b).
Figure 3a. Location and extent of the NP lines
at the UPR site in the municipality of Isabela.
Figure 3b. NP profiles obtained at the UPR
site in the municipality of Isabela.
Figure 4a. Location and extent of the gravity
survey lines.
Figure 4b. Gravity profiles obtained at the
UPR site in the municipality of Isabela.
In the southernmost part of the Pozo Brujo area a straight-stream reach was determined to be oriented about N. 1 E. In general, the sinkholes south of Pozo Brujo seem to have an orientation ranging from N. 15 W. to N. 10 E. Some of the sinkholes have orientations similar to that of the straight-stream reach and several of the sinkholes are in alignment with this straight-stream reach.
One of the east-west NP profiles obtained south of Pozo Brujo shows two prominent NP anomalies (fig. 2). The easternmost is a well defined M or "sombrero" shaped anomaly. Similarly, several minor NP anomalies are present in the middle of the profile. Several of these anomalies seem to be expressions of ground-water flow paths in the subsurface. A domestic well with a "high and sustained yield," as reported by its owner, is located about 15 meters east of the M shaped NP anomaly already mentioned.
Several of the NP anomalies at the UPR site, shown in figure 3b as hollow arrows, might be caused by artifacts such as pipelines, concrete drainage pipes, irrigated plots, or electrical conduits. Other anomalies, shown as solid arrows in figure 3b, might result from subsurface conduits or other forms of ground-water flow paths like downward movement of water within the vadose zone through sinkholes and air-filled caverns. A significant NP low value, shown with two solid arrows, coincides with the location of a test well drilled by the USGS with significant cavernous porosity.
Some of the relative Bougher anomalies detected at the UPR site are shown in figure 4b. Short wave length gravity anomalies of 0.10 and 1.0 milligal were measured that might indicate the presence of shallow and relatively large density contrasts.The largest negative anomalies are typically associated with sinkholes or more subtle depressions of the ground surface. The large negative anomaly at the line 2N-2S, shown with a solid arrow in figure 4b, is not associated with a topographic depression and, in fact, corresponds with an elevation maximum. In general, the gravity method also indicates the probable presence of subsurface conduits at the UPR site that might be dry or contain water, corresponding to gravity lows.
The degree of success was variable among the various sites studied. However, at the Pozo Brujo and UPR sites, the two geophysical methods used in combination with sound geological constraints proved to be successful in detecting anomalies that might be caused by conduit-controlled ground-water flow. Some of the sites studied are worth of a more detailed analysis, including a test drilling phase to confirm the presence of water-bearing conduits as suggested by the results obtained.
Kilty, K. T. and A. L. Lange, 1991. Natural-Potential Responses of Karst Systems at the Ground Surface, in Proceedings of the Third Conference on Environmental Problems in Karst Terranes and their Solutions. National Water Well Association, Dublin, Ohio, pp. 369-390.
Lattman, L. H. and R. R. Parizek, 1964. Relationship Between Fracture Traces and the Occurrence of Groundwater in Carbonate Rocks. Journal of Hydrology, vol. 2, pp. 73-91.
Monroe, W. H., 1980. Geology of the Middle Tertiary Formations of Puerto Rico. U.S. Geological Survey Professional Paper 953, 93 pp.
Rodríguez-Martínez, Jesús and J. R. Hartley, 1994. Geologic and
Hydrologic Data Collected at Test Holes NC-6 and NC-11, Hatillo and Isabela, Northwestern
Puerto Rico. U.S. Geological Survey Open-File Report 93-465, 39 pp.
1. Jesús Rodríguez-Martínez
United States.Geological Survey, WRD
GSA Center Bldg.651
Federal Drive Suite 400-15
Guaynabo, Puerto Rico 00965-5703
Phone: (787) 749-4346 ext.: 307
Email: "jrodr@usgs.gov"
USGS Water Resources of Puerto Rico and the U.S. Virgin Islands Web Site: "http://dprsj1.usgs.er.gov/"
2. Ronald T. Richards
United States.Geological Survey, WRD
GSA Center Bldg.651
Federal Drive Suite 400-15
Guaynabo, Puerto Rico 00965-5703
Phone: (787) 749-4346 ext.: 310
Email: "rtrich@usgs.gov"
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