Advancing Water Resources Research and Management
|
|
Abstract
Introduction
Methods
Description of the Study Area
Hydrogeologic Setting
Land Use
Results and Discussion
Conclusions
References
Authors
The upper aquifer contained within the Manatí-Vega Baja area has undergone a degradation in water quality. At some locations, nitrate concentrations have exceeded the maximum contaminant level of 10 milligrams per liter for nitrate as nitrogen in drinking water. A water-quality assessment was conducted between 1992 and 1995 to define the areal concentration of nitrate in the aquifer, identify the principal sources of the nitrate, and estimate the nitrate loads from major sources identified. The study was concentrated on an area of about 33 square kilometers which constitutes the part of the upper aquifer with drainage to the Laguna Tortuguero. The study indicated that nitrate concentrations within the Laguna Tortuguero ground-water drainage basin were close to or above 10 milligrams per liter. The investigation confirmed that the sources of nitrate are derived primarily from fertilizer used in the cultivation of pineapples and from septic tank effluent at rural communities. The nitrate concentrations down-gradient the pineapple fields and un-sewered rural communities are between 6.8 and 10.0 milligrams per liter. The potential nitrate load from these land uses to the sub-surface was estimated at 760 kilograms of nitrogen per hectare per year from pineapple fields and 200 kilograms of nitrogen per hectare per year from un-sewered rural communities. Mass balance calculations indicate that all the estimated nitrogen load from septic tank effluent may reach the upper aquifer, whereas the load from fertilized fields may be only 45 kilograms of nitrogen per hectare per year.
The Manatí-Vega Baja area in north-central Puerto Rico (fig. 1) has been subjected to extensive agricultural, industrial, and urban development. Three main hydrogeologic units compose the limestone aquifer system in the area: an unconfined upper aquifer, a middle confining unit, and a confined lower aquifer. The unconfined upper aquifer is the principal source of water in the area, supplying approximately 37,000 cubic meters per day (m3/d) for public supply, 4,900 m3/d for industrial self-supplied use, and an estimated 900 m3/d for agricultural use (Conde-Costas and Rodríguez-Rodríguez, 1996).
Figure 1. The Manatí quadrangle area and the Laguna
Tortuguero ground-water drainage basin in the north-central Puerto Rico (drainage basin as
defined by Bennet and Giusti, 1972).
Nitrate concentrations exceeding the maximum contaminant level of 10 milligrams per liter (mg/L) permitted by the Puerto Rico Department of Health at public-water supply wells have been documented at wells in the Manatí-Vega Baja area by Gómez-Gómez and Guzmán-Ríos (1982); Guzmán-Ríos and Quiñones-Márquez (1984); and Román-Más and Ramos-Ginés (1988). Between 1988 and 1990, nitrate concentrations equal to or exceeding 10 mg/L were detected at three public-water supply wells in the area (Puerto Rico Environmental Quality Board, 1992). The findings led to the closure of these wells, resulting in a production loss of 3,900 m3/d. The use of fertilizers in pineapple fields (the main agricultural activity), septic tanks, livestock facilities discharges, and illegal dump sites were suggested as the most probable causes of the high nitrate concentrations detected at the affected public water-supply wells (Puerto Rico Environmental Quality Board, 1992).
The U.S. Geological Survey (USGS) in cooperation with the Puerto Rico Environmental Quality Board (PREQB), conducted a ground-water quality assessment of the upper aquifer in the Manatí-Vega Baja area between 1992 and 1995. Only the upper aquifer was included in the assessment since nitrate concentrations exceeding 10 mg/L have not been detected from the lower aquifer. The purpose of the study was to define the areal distribution of nitrate, identify the principal potential sources of the nitrate, and estimate the nitrate loads from major sources identified. The study focuses on an area of about 33 square kilometers that constitutes the Laguna Tortuguero ground-water basin as defined by Bennett and Giusti (1972; fig. 1).
An areal water-quality assessment was conducted between August 25 and November 12, 1992, in order to define existing water-quality conditions and the areal distribution of nitrate. Water samples were collected at 29 wells, one spring, and one creek (fig. 2). A monthly sampling program to monitor seasonal changes in nitrate concentrations was established from June 1994 to May 1995 at eight active wells (well sites 9, 13, 14, 16, 17, 25, 29, and 36 in fig. 2). Six observation wells were drilled to obtain ground-water quality data within agricultural areas and to supplement existing potentiometric data (well sites 7, 8, 10, 11, 18, and 24 in fig.2). Land-use data and the stable-isotope ratio of nitrogen-15 to nitrogen-14 results were examined for assessing nitrate contamination sources.
Figure 2. Location of wells, springs, and surface-water
sites sampledin the Manatí quadrangle, north-central Puerto Rico.
The relative contribution of nitrate to the upper aquifer from fertilizer use at pineapple fields and from domestic wastewater disposal in rural communities was obtained from mass balance calculations. Current nitrogen fertilizer application rates and procedures were provided by the Puerto Rico Land Authority, Pineapple Program (W. Gandía-Torres, written commun., 1995). The household wastewater discharge to septic tanks was estimated from domestic water-use data available for 1982 for public-water supply un-sewered customers (Torres-Sierra and Avilés, 1986). The per capita total nitrogen excreted by humans, which averages 17 grams per day (g/d) (Kaplan, 1987), was used in calculating the maximum potential nitrate load from septic tanks to the subsurface.
The Manatí-Vega Baja area is located within the North Coast Limestone belt geographic region of Puerto Rico. A well defined tropical karst topography characterizes most of the land surface. Sinkholes are the most relevant karst features and represent the most important mode of aquifer recharge. Near the coast, the most important physiographic feature is Laguna Tortuguero, the main natural discharge area of the upper aquifer in the Manatí-Vega Baja area (fig. 1).
The unconfined upper aquifer is contained in the early Miocene Aymamón and Aguada Limestones, and consists of a wedge of freshwater floating above saline water. This wedge thins toward the coast. The thickness of the freshwater lens ranges from about 33 meters (m) near the eastern shore of Laguna Tortuguero to as much as 130 m near highway PR-2 (Rodríguez-Martínez and others, 1992). The depth to the water table is about 65 m below land surface where the freshwater lens possibly has its maximum thickness within the Aguada Limestone. Transmissivity in the freshwater portion of the upper aquifer ranges from 14,000 meters squared per day (m2/d) within the Aymamón Limestone to 60 m2/d in the Aguada Limestone (Renken and Gómez-Gómez, 1994). Recharge to the upper aquifer occurs throughout the outcrop areas of the Aymamón and Aguada Limestones as direct rainfall infiltration and as direct inflow of surface runoff draining into sinkholes.
Land use within the approximately 7,900 hectares (ha) of Laguna Tortuguero ground-water drainage basin, as defined by Bennett and Giusti (1972), was distributed as follows in 1995: 570 ha consisting of Laguna Tortuguero and hydric soils on the periphery of the Laguna Tortuguero, 4,300 ha in pasture or fallow plain; 600 ha in cultivated pineapple fields; 170 ha in urban community (sanitary sewer); 255 ha in rural community (no sanitary sewer); 277 ha in industrial use, and 1,728 ha of rugged terrain (mogotes and cone karst) with minimal or no agricultural use and mostly native vegetative cover (fig. 3).
Figure 3. Selected land-use within the Laguna Tortuguero ground-water
drainage basin
in north-central Puerto Rico.
Results of the water-quality assessment conducted between August and November 1992 are listed in Conde-Costas and Rodríguez-Rodríguez (1997). The survey confirmed concentrations of nitrate as nitrogen close to or exceeding 10 mg/L at wells south of highway PR-2 and north of the pineapple fields along highway PR-670. Six active wells (well sites 9, 13, 14, 17, 25, and 36 in fig. 2) and four of the observation wells (well sites 8, 11, 18, and 24 in fig. 2) had concentrations of nitrate as nitrogen above 10 mg/L on at least one sampling date. The sampling program implemented at selected wells (well sites 9, 13, 14, 16, 17, 25, 29, and 36 in fig. 2) from June 1994 to May 1995 indicated that concentrations of nitrate as nitrogen remained consistently close to or above 10 mg/L throughout the year only at wells 13, 17, 25, 29, and 36 (fig. 2). The highest nitrate concentration occurred at well 17 (18 mg/L), an agricultural-use well located at a fertilizer warehouse (fig. 2). The concentration of nitrate at well 17 ranged from 15 to 18 mg/L. At well 8, an observation well located within pineapple fields (fig. 2), the concentration of nitrate ranged from 14 to 16 mg/L. As indicated by selected wells (well sites 13, 14, and 16 in fig. 2) the nitrate concentrations in the upper aquifer down-gradient the pineapple fields and un-sewered rural communities are between 6.8 and 10.0 milligrams per liter.
In the Manatí-Vega Baja area, non-point sources of contamination such as cultivated farm land and septic tank leachate in rural communities have the potential for increasing nitrate concentrations in the upper aquifer. These are the predominant land use and contamination sources within recharge areas of the upper aquifer having high-nitrate-concentrations. The use of fertilizers in the cultivation of pineapples and disposal of domestic waste water in areas not serviced by sanitary sewers are considered the most important nitrate contributors to the upper aquifer.
The stable-isotope concentration of nitrogen-15 in nitrate ( d15 N) was obtained at selected sites in the study area to infer the source of nitrate (fig. 4). The average d15 N value of 2.2 obtained at well 17, was used as the representative signature for fertilizer affected areas. The d15 N value of 8.1 obtained from the average of five wells (well sites 15, 26, 31, 33, and 34) and a spring (site 35) was used as the representative signature for organic wastes affected areas. The five wells and the spring are located in the eastern part of the study area in an area of similar hydrogeologic conditions where organic wastes derived primarily from domestic wastewater sources constitute the only significant source of nitrate contamination. These representative d15 N signatures in conjunction with nitrate concentrations, land-use and potentiometric-surface data indicate that fertilizer is the primary source of the nitrate in the upper aquifer throughout the central part of the Manatí quadrangle northward of the land area under pineapple cultivation. At areas to the east and west of the pineapple fields, nitrate in the upper aquifer may be derived primarily from animal or human organic wastes.
Figure 4. 15dN of NO3 -N
at selected wells in the Manatí quadrangle, north-central Puerto Rico.
The potential maximum nitrate load to the sub-surface was estimated at an average of 760 kilograms of nitrogen per hectare per year (kg-N/(ha-yr)) from pineapple fields and 200 kg-N/(ha-yr) from un-sewered rural communities with no sanitary sewer. Of the average 760 kg-N/(ha-yr) accounted in fertilizer used at pineapple fields it is possible that an average of 550 kg-N/(ha-yr) is not incorporated into vegetative matter or mineralize in the soil and may be available for volatilization or leached to the subsurface. Mass balance calculations made for a part of the aquifer impacted by septic tanks effluents indicates that all 200 kg-N/(ha-yr) estimated in domestic wastewater may result in nitrate load to the upper aquifer. The calculated nitrate load to the upper aquifer from fertilized pineapple fields may be only 45 kg-N/(ha-yr). The mass balance calculations indicate that the bulk of the difference between the 550 kg-N/(ha-yr) estimated as available for volatilization or leaching to the subsurface and the 45 kg/(ha-yr) accounted in ground water is probably in transient storage within the vadose zone, which has a thickness between 65 and 140 m at pineapple fields.
The results of the study indicate that the portion of the upper aquifer with nitrate concentrations close to or above 10 milligrams per liter is contained primarly within the Laguna Tortuguero ground-water drainage basin. Sources of nitrate in the study area are derived primarily from fertilizer used in the cultivation of pineapples and from septic tank effluents from rural communities. On an areal unit basis the maximum potential nitrate load from agricultural land under pineapple cultivation (760 kg-N/(ha-yr)) is about four times that of rural communities without sewerage connections (200 kg-N/(ha-yr)). The total nitrate load estimated in domestic wastewater discharge at un-sewered communities may reach the upper aquifer according to mass balance calculations. In addition, the mass balance calculations indicate that much of the un-accounted nitrate load from fertilizer use at the pineapple farms (550 kg-N/(ha-yr)) is probably in transient storage within the vadose zone since only about 45 kg-N/(ha-yr) is accounted in the upper aquifer.
Bennett, G. D., and E. V. Giusti, 1972. Ground water in the Tortuguero area, Puerto Rico, as related to proposed harbor construction. U.S. Geological Survey Water-Resources Bulletin 10, 25 pp.
Conde-Costas, Carlos and Gilberto Rodríguez-Rodríguez, 1996. Potentiometric surface and hydrologic conditions of the upper aquifer in the Manatí-Vega Baja area, north-central Puerto Rico, March 1995. U.S. Geological Survey Water-Resources Investigations Report 96-4184, 1 pl.
Conde-Costas, Carlos and Gilberto Rodríguez-Rodríguez, 1997. Reconnaissance of ground-water quality in the Manatí quadrangle, Puerto Rico, September-November 1992. U.S. Geological Survey Open-File Report 96-628, 28 pp.
Gómez-Gómez, Fernando and Senén Guzmán- Ríos, 1982. Reconnaissance of ground-water quality throughout Puerto Rico, September-October 1981: U.S. Geological Survey Open-File Report 82-332, 1 pl.
Guzmán-Ríos, Senén and Ferdinand Quiñones-Márquez, 1984. Ground-water quality at selected sites throughout Puerto Rico, September 1982 - July 1983. U.S. Geological Survey Open-File Report 84-058, 1 pl.
Kaplan, O.B., 1987. Septic systems handbook. Lewis Publishers, Chelsea, Michigan, 290 pp.
Py, C., J. J. Lacoevilhe and C. Teisson, 1987. The pineapple cultivation and uses. Techniques Agricoles et Productions Tropicales, editions GP, Maisonneuve & Larose, Paris.
Puerto Rico Environmental Quality Board, 1992. Informe sobre pozos con presencia de nitrato en los municipios de Manatí y Vega Baja. Puerto Rico Environmental Quality Board, 15 pp.
Renken, R. A. and Fernando Goméz-Goméz, 1994. Potentiometric surfaces of the upper and lower aquifers, North Coast Limestone aquifer system, Puerto Rico. U.S. Geological Survey Open-File Report 93-499, 16 pp., 4 pls.
Rodríguez-Martínez, Jesús, R. A. Scharlach and Arturo Torres-González, 1992. Geologic and hydrologic data collected at test holes NC-4 and NC-14, Manatí and Vega Baja, Puerto Rico. U.S. Geological Survey Open-File Report 92-126, 32 pp.
Román-Más, Angel and Orlando Ramos-Ginés, 1988. Compilation of water-quality data for the North Coast Limestone aquifers, Puerto Rico, 1951 to 1987. U.S. Geological Survey Open-File Report 87-533, 133 pp.
Torres-Sierra, Heriberto and Ada Avilés, 1986. Estimated water use in Puerto Rico, 1980-82. U.S. Geological Survey Open-File Report 85-557, 77 pp.
U.S. Environmental Protection Agency, 1995. Drinking water
regulations and health advisories. Washington, D.C., U.S. Environmental Protection
Agency Report, Office of Water, 11 pp.
1.Carlos Conde-Costas, Hydrologist
United States.Geological Survey, WRD
GSA Center Bldg.651
Federal Drive Suite 400-15
Guaynabo, Puerto Rico 00965-5703
Phone: (787) 749-4346 ext.: 291
Email: "ceconde@usgs.gov"
USGS Water Resources of Puerto Rico and the U.S. Virgin Islands Web Site: "http://dprsj1.usgs.er.gov/"
2. Fernando Gómez-Gómez, Supervisory Hydrologist
United States.Geological Survey, WRD
GSA Center Bldg.651
Federal Drive Suite 400-15
Guaynabo, Puerto Rico 00965-5703
Phone: (787) 749-4346 ext.: 268
Email: "fggomez@usgs.gov"
home page