TOTAL PHOSPHORUS AND TOTAL NITROGEN MASS BALANCES FOR LAGO DE CIDRA, CENTRAL PUERTO RICO

Orlando Ramos Ginés⁽¹⁾

Table of Contents

Abstract
Description of Study Area and Climate
Methods and Procedures
Nutrient Loads and Export Coefficients for Major Land Uses
Total Phosphorus and Total Nitrogen Mass Balances
References
Author

Abstract

Total phosphorus and total nitrogen loads in Lago de Cidra were estimated using a network of 14 data-collection sites to monitor rainfall, surface-water flow, reservoir stage, water quality, and reservoir withdrawals during a 1-year period extending October 1992 to September 1993. Five sub-basins, each with distinct land use characteristics, were instrumented to obtain discharge and samples during storm-runoff events. The total phosphorus and total nitrogen loads entering Lago de Cidra for the 1-year period were calculated at 6,530 and 18,700 kilograms, respectively. Of the total load, about 5,700 kilograms of total phosphorus and 10,200 kilograms of total nitrogen were retained in the reservoir by bottom sediments, aquatic plants, or within the reservoir water column. The total phosphorus export coefficients, in kilograms per hectare, for five monitored sub-basins having distinct land-use characteristics were estimated to be 0.37 for a watershed consisting primarily of secondary-growth forest and fallow lands; 1.49 and 1.98 for two rural agricultural watersheds; 2.55 for an urban sewered community; and 7.06 for an urban unsewered community. The export coefficients for total nitrogen, in kilograms per hectare for the same areas were estimated to be 2.72; 6.91 and 8.59; 6.62; and 17.1, respectively.

Description of Study Area and Climate

The study area is located in central Puerto Rico, 24 km (kilometers) south of the town of Bayamón (fig. 1). Lago de Cidra had an original storage capacity of 6.2 million cubic meters. It was impounded in 1946, principally to augment the streamflow of the Río de Bayamón during low-flow conditions to meet the downstream water requirements of the Guaynabo public water-supply filtration plant operated by the Puerto Rico Aqueduct and Sewer Authority (PRASA). The reservoir, at present, also serves as the sole source of water for the PRASA water-supply filtration plant serving the town of Cidra and part of the town of Aguas Buenas. Since its impoundment, the reservoir was susceptible to eutrophication problems fostered by runoff from the cultivation of sugarcane which lasted until about the mid 1960's. The level of eutrophication, great proliferation of aquatic plants, predominantly water hyacinths, and frequent fish deaths in the Lago de Cidra became issues of great concern to the Commonwealth of Puerto Rico, the Municipality of Cidra, and local environmental groups in 1992.

Figure 1. Lago de Cidra and data-collection sites in the study area drainage basin in central Puerto Rico.

The drainage basin of Lago de Cidra consists of volcanic rocks, primarily volcanic breccia and metamorphic rocks of Tertiary age (Pease and Briggs, 1960; Rogers, 1979). Soils in the study area are clayey, moderately-deep to deep (356 to 1520 mm (millimeters)), moderately- to very-steep (12 to 60 percent slope), and well drained (Boccheciamp, 1978). Principal land uses in the watershed include secondary-growth forests and fallow lands 1,030 hectares (ha), agriculture 641 ha, rural 194 ha, urban 128 ha, and industrial 42 ha.

The Río de Bayamón, Río Sabana, and Quebrada Prieta are the three major streams entering Lago de Cidra. The total combined drainage area of these streams is about 850 ha, or about 42 percent of the reservoir's watershed. A pump station in the Río de la Plata Basin provides a substantial amount of the surface water contribution to Lago de Cidra, estimated to be 19 percent of the total water budget for water year 1993 (Ramos-Ginés, 1997).

Climate in the study area is humid. The mean 30-year (1951-80) annual rainfall in the study area is 1,570 mm (U.S. National Oceanic and Atmospheric Administration, 1982). The mean 30-year monthly rainfall is highest during the months of August to October, ranging from 160 mm to 210 mm per month, and the lowest mean-monthly rainfall is about 70 mm during February.

Methods and Procedures

A network of 14 data-collection sites (fig. 1) was established to collect data for rainfall, streamflow upstream and downstream of Lago de Cidra, inter-basin water transfer from the Río de la Plata, reservoir stage, and withdrawals from the reservoir for public water-supply. Stations 1 to 5 were established as index stations representative of land use practices within the Lago de Cidra watershed. These monitoring stations were equipped with automatic water samplers (AWS). Discharge-weighted composite samples were prepared from discrete samples collected by the AWS during each storm-runoff event for determination of total phosphorus and total nitrogen concentrations. Discharge-weighted sub-sample volumes were computed using the mid-interval method of subdivision (Porterfield, 1972, p. 49-52). Grab low-flow and discharge-weighted composite samples were preserved with a solution of mercuric chloride and maintained at a temperature of no more than 4 degrees Celsius. The samples were analyzed at the USGS National Water Quality Laboratory in Arvada, Colorado.

Total nutrient loads determined for storm-runoff events and low-flow periods were added to obtain the total nutrient loads for water year 1993 at each of the monitoring sites. Total nutrient export coefficients, expressed as kilograms per hectare per year, were determined for each of the land use index stations --secondary forest and fallow lands, rural agricultural, urban sewered, and urban unsewered. The total nutrient loads from surface runoff into Lago de Cidra were estimated by multiplying the total nutrient export coefficients obtained at the index stations for specific land-use categories by the drainage area of sub-basins with similar land uses. Total nutrient loads from direct rainfall were estimated using a mean concentration value of 0.02 mg/L for total phosphorus and 0.36 mg/L for total nitrogen for bulk precipitation (an integration of wet and dry deposition) in samples collected within the Caribbean National Forest in eastern Puerto Rico (Bruijnzeel, 1990, p. 52-53). Total nutrient loads from predominantly light-industrial land-use areas were estimated using the total nutrient export coefficients determined for runoff from the urban sewered area. Total nutrient loads from a wastewater treatment plant that receives the effluent from a 174-unit housing area were estimated by using in the calculation the outfall loads of 0.95 kg/day of total phosphorus and 2.35 kg/d of total nitrogen during weekdays and outfall loads of 10.95 kg/d of total phosphorus and 10.15 kg/d of total nitrogen during weekends throughout water year 1993. The weekday and weekend total nutrient load values were determined from composite samples obtained during a weekday and a weekend monitoring at the wastewater treatment-plant outfall. A major nutrient source identified in the study was commercial plant nurseries. The nutrient loads from plant nurseries were estimated by multiplying the average concentration of total phosphorus (6.9 mg/L) and total nitrogen (7.9 mg/L) in storm-runoff grab samples collected between October 1, 1992 and March 31, 1993 at one commercial plant nursery. The average nutrient concentrations from the plant nursery were multiplied by the estimated nursery annual rainfall runoff, which was estimated at one-half the annual rainfall. Total nutrient loads from ground water flow to Lago de Cidra were estimated using the average total phosphorus and total nitrogen concentrations in water samples collected at two wells near the reservoir, and multiplying the values by the estimated amount of ground-water flow to Lago de Cidra. Total nutrient loads in water withdrawn for public water-supply were estimated from flow meter readings and samples collected at the raw water intakes. Nutrient loads leaving Lago de Cidra in water releases, over-dam flows, and seepage at the dam were estimated from flow and nutrient concentration in samples collected at a gaging station downstream from the dam.

Nutrient Loads and Export Coefficients for Major Land Uses

Total phosphorus and total nitrogen loads calculated for monitoring sites in sub-basins having distinct land use characteristics ranged from 27.8 to 393 kg/yr (kilograms per year) for total phosphorus and from 72.1 to 1,710 kg/yr for total nitrogen (table 1). The total phosphorus and total nitrogen export coefficients for the predominant land uses in the study area were predominantly greater than average for temperate-climate zones as obtained by the U.S. OECD (Organization for Economic Cooperative and Development; Rast and Lee, 1978). Among the most important findings of the Lago de Cidra watershed study was that, on a unit area basis, urban areas contribute greater amounts of total phosphorus and similar or greater amounts of total nitrogen than the agricultural-rural areas.

Table 1. Total phosphorus and total nitrogen export-coefficient values for major land uses in the Lago de Cidra basin in central Puerto Rico as determined from subbasins monitored during water year 1993.

Total Phosphorus and Total Nitrogen Mass Balances

Total phosphorus and total nitrogen loads entering Lago de Cidra were calculated at 6,619 and 18,863 kg, respectively (table 2), and leaving the reservoir about 839 and 8,610 kg, respectively. A substantial amount of total phosphorus and total nitrogen entering Lago de Cidra remained within the reservoir. Although transient storage in the water column and aquatic plants can account for part of the difference, the most likely sink is the absorption on fluvial sediments and subsequent settling within the reservoir. The net mass balance between the input and output nutrient load values were about 5,700 kg of total phosphorus and 10,170 kg of total nitrogen. Total phosphorus concentrations in reservoir bottom-sediment samples collected during July 1992 had concentrations that ranged from 218 to 2,570 mg/kg and averaged 669 mg/kg; total nitrogen concentrations ranged from 430 to 3,500 mg/kg and averaged 2,185 mg/kg. The bottom-sediment concentrations for total phosphorus are predominantly higher than those in other Puerto Rico reservoirs. The total nitrogen concentrations in bottom sediments were lower in samples from Lago de Cidra than in samples from eight other reservoirs in Puerto Rico (U.S. Geological Survey, unpublished data) (fig. 2).

Figure 2. Comparison of the (a) total phosphorus and (b) total nitrogen concentrations in bottom-sediment samples collected in Lago de Cidra, during July 27-28, 1992, and data from eight other reservoirs in Puerto Rico.

Table 2. Total phosphorus and total nitrogen loads entering and leaving Lago de Cidra, Puerto Rico during water year 1993 [kg/yr, kilograms per year]

References

Bruijnzeel, L. A., 1990. Hydrology of moist tropical forests and effects of conversion; a state of knowledge report. UNESCO International Hydrological Programme, A Publication of the Humic Tropics Programme, Faculty of Earth Sciences, Free University, Amsterdam, 224 pp.

Pease, M. H. and R. P. Briggs, 1960. Geology of the Comerío quadrangle, Puerto Rico. Miscellaneous Geologic Investigations Map I-320, 1 pl.

Porterfield, George, 1972. Computation of fluvial-sediment discharge. Techniques of the Water-Resources Investigations of the U.S. Geological Survey, Book 3, Chapter C3, 66 pp.

Ramos-Ginés, Orlando, 1997. Water balance and quantification of total phosphorus and total nitrogen loads entering and leaving the Lago de Cidra, Central Puerto Rico. U.S. Geological Water-Resources Investigations Report 96-4222, 28 pp.

Rast, W. and G. F. Lee, 1978. Summary analysis of the North American (US portion) OECD eutrophication project: Nutrient loading-lake response relationships and trophic state indices. U.S. Environmental Protection Agency, EPA-600/3-78-008, Corvallis Environmental Research Laboratory, Corvallis, Oregon, 1978, 454 pp.

Rogers, C. L., 1979. Geologic map of the Caguas quadrangle, Puerto Rico. U.S. Geological Survey Miscellaneous Investigations Map I-1152, 1 pl.

U.S. National Oceanic and Atmospheric Administration, 1982. Monthly normals of temperature, precipitation, and heating and cooling degree days 1951-80, Puerto Rico. 12 pp.

Boccheciamp, R. A., 1978. Soil survey of the San Juan area of Puerto Rico. U.S. Department of Agriculture, 141 pp.

Authors

1.Orlando Ramos Ginés, 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.: 305
Email: "orgramos@usgs.gov"
USGS Water Resources of Puerto Rico and the U.S. Virgin Islands Web Site: "http://dprsj1.usgs.er.gov/"

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