Advancing Water Resources Research and Management
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Abstract
Introduction
Methods
Results
Discussion
Conclusions
References
Authors
To describe predevelopment conditions in the San Juan Bay estuary system, chronostratigraphic sediment units sampled throughout the estuary system were analyzed for naturally occurring trace elements and anthropogenic contaminants. Estimates of sedimentation rates at six sediment-core sites were made by analyzing sediment layers for the activity of cesium-137, a radioisotope from nuclear testing fallout. Sedimentation rates ranged from 0.24 centimeter per year in an isolated area of a mangrove-bordered lagoon to 3.9 centimeters per year in a sediment-filled, natural tidal channel. Based on the calculated average sedimentation rates, additional sediment cores from the six sites were sampled and homogenized to represent depositional periods from 1925 to 1949, from 1950 to 1974, and from 1975 to 1995. Sediment deposited during the three time intervals were analyzed for trace elements, pesticides, and PCB's. Concentrations of lead, mercury, and arsenic in sediment strata deposited before 1950 averaged 20, 0.05, and 11 micrograms per gram respectively for the six coring sites. These concentrations are similar to those for sediments sampled in streambeds in undisturbed uplands. At the most contaminated site in the Caño de Martín Peña, concentrations of lead in the sediments increased from 30 to 745 micrograms per gram; mercury increased from 0.16 to 4.7 micrograms per gram; and PCB's increased from 12 to 450 micrograms per kilogram. DDT, measured at 0.48 microgram per kilogram (sum of DDT, DDD, and DDE) for the period 1925 to 1950, increased to 46 micrograms per kilogram for the period 1950 to 1975, and decreased to 14.6 micrograms per kilogram for the period 1975 to 1995.
Coordination between the Puerto Rico Environmental Quality Board (PREQB) and the U.S. Environmental Protection Agency (USEPA) in 1993 succeeded in establishing a San Juan Bay Estuary Program (SJBEP) under the auspices of the National Estuary Program. The SJBEP is charged with creating a Comprehensive Conservation and Management Plan (CCMP) for the estuary system. The CCMP will include actions to restore estuarine functionality to as close to predevelopment conditions as possible while balancing the needs of continued development. Water quality in the estuary system began to be measured with regularity in the early 1970's, well after the system had been severely degraded by decades of intense agriculture and industry. Therefore, the Puerto Rico Environmental Quality Board and the U.S. Environmental Protection Agency requested that the U.S. Geological Survey (USGS) complete a synoptic survey of water and sediment quality for the estuary system. A major part of the sampling program was directed at deriving temporal and spatial changes in historical water quality by analyzing constituents in chronostratigraphic units identified in the sediments deposited on the estuarine floor. This paper details the results of the temporal and spatial changes observed in the sediments. The results of the synoptic survey of water quality and temporal changes in the water quality from 1970 through 1995 are presented in Webb and Gómez-Gómez (1998).
The economy and associated land use of Puerto Rico, including the San Juan metropolitan area that borders most of the estuary, was mostly agrarian prior to 1925 (Birdsey and Weaver, 1987). Development of urban areas increased after 1925, and a rapid industrial transformation began in the late 1940's and early 1950's. The changes in land use and increasing population pressures resulted in specific changes in water quality and sediment quality in the estuary.
The San Juan Bay estuary watershed covers 240 km2 and consists of limited uplands draining north across a broad, flat coastal plain (fig. 1). The subtidal portion of the system consists of the Bahía de San Juan, connected by a series of natural and dredged channels to four lagoons: Laguna del Condado, Laguna San José, Laguna La Torrecilla, and Laguna de Piñones. In total, the inundated areas, including San Juan Bay and lagoons, cover 25 km2. Quaternary deposits, which cover 147 km2, include marine and riverine terrace deposits, alluvium, beach and swamp deposits, blanket deposits, and artificial fill. Volcanic and intrusive rocks cover 55 km2 and are limited to exposures in the uplands to the south. Limestone formations cover 13 km2. These deposits are located mostly in the western part of the basin and are actively mined to provide raw material for the island's construction industry.
Figure 1. Drainage basin of the San Juan Bay
estuary system, sediment coring sites, chronostratigraphic units and concentrations of
pesticides, PCB’s, and trace elements for strata corresponding to time periods
1925-49, 1950-74, and 1975-95. Note that small differences observed using the logarithmic
scale represent large differences in the absolute values.
Denudation of the uplands is enhanced by the warm temperatures and abundant rainfall. The normal range of mean monthly temperatures in San Juan is only 3.2°C between the warmest month, August (27.1°C), and the coolest months, January and February (23.9°C) ( Calvesbert, 1970). Mean-annual rainfall averages from 1,500 mm near the coast in the San Juan area to 2,100 mm in the uplands defining the southern boundary of the drainage basin. The mean annual runoff to the estuary system is estimated to be 185x106 m3/yr. Since 1977, extensive areas of pasture and forest in the uplands of the Río Piedras watershed have been urbanized with housing and commercial development coincident with suspended-sediment yields exceeding 15,000 Mg/km2/yr (Díaz and others, 1996).
The sediments deposited in the San Juan Bay Estuary system reflect sediments transported from the uplands mixed with anthropogenic pollutants. In addition to the surface water discharge by streams to the estuary system, there are discharges from urban "flood control" drainage pumps. Combined sanitary and fluvial connections and raw sewage discharges have direct impacts on the water quality and on the concentrations of pollutants scavenged from the water column and deposited in the sediments. Quebrada Blasina, which discharges into Laguna La Torrecilla, receives discharge from the backwash of the Puerto Rico Aqueduct and Sewer Authority (PRASA) Sergio Cuevas Water Treatment Plant.
The water and sediment quality of the bay, lagoons, and interconnecting natural and man-made channels has been significantly altered from its natural state not only by land-use activities, but also by modification of the hydraulic properties of the interconnecting waterways by dredging and placement of fill ( Ellis, 1976; Seguinot-Barbosa, 1983). Sediment contamination in the estuary is increased in areas with poor tidal flushing. The average tidal range is 48.8-cm. Over a 25-hour tidal cycle approximately 10x106 m3 of seawater from the Atlantic Ocean enter and leave the estuary system. Full-range oscillations occur at Laguna La Torrecilla, Laguna del Condado, and in the Bahía de San Juan. Tidal oscillations at Laguna San José and Laguna de Piñones are limited to about 5 cm because the flood and ebb flows are throttled by narrow connecting channels.
For this study, estimates of average sedimentation rates at six sites were determined from Cs-137 activity levels measured in sediment layers from sediment cores collected in December 1994. The estimated average sedimentation rates were then used to define strata containing sediments deposited during the time periods corresponding approximately to 1925-49, 1950-74, and 1975-95. In July 1995, sediments representing each of these time periods were sampled, homogenized and analyzed for concentrations of selected pesticides, PCB's, and trace elements.
Bottom sediments were cored and samples collected from six sites in December 1994: Bahía de San Juan (SJB600); Caño de Martín Peña (MPN500); Laguna Los Corozos (CRZ400); Laguna San José (SJS300); Laguna La Torrecilla (TRC200); and Laguna de Piñones (PNN100). The sites were chosen in areas of fine-grained sediment deposits and away from dredged or filled areas. In addition, wherever possible, the coring sites were located in the central areas of water bodies to document contamination introduced from various sources around the perimeter of the water body and not just the influence of a local point source. To obtain enough material for radioisotope analysis, three cores from each site were recovered and homogenized in 5-cm intervals. The only exception was the Caño de Martín Peña site. Sedimentation rates were expected to be high there, and so only two cores were obtained and sampled in 10-cm intervals. Sampling proceeded from the least contaminated sites to the most contaminated sites.
Sedimentation rates at the six sites in the estuary system were estimated using Cs-137 techniques. Cs-137 is a by-product of atmospheric nuclear-bomb testing with a half-life of 30 years. The radioisotope does not occur naturally and is distributed throughout the atmosphere after sub-aerial detonations. The radioisotope eventually settles out on the surface of the earth where it binds firmly to sediment particles. Fluvial processes transport the sediment and adsorbed Cs-137 through the drainage system to depositional areas (McHenry and others, 1980). Strata in the sediments with high Cs-137 activity correlate to periods of increased activity of sub-aerial nuclear detonations. The most distinct Cs-137 time horizons, or peaks, usually identified are 1952, the first year of significant nuclear testing and subsequent fallout of Cs-137, and 1963, the most active year with frequent testing by both the United States and the former Soviet Union. The peaks may lag by a year or two after 1963 if there is any delay in the fluvial transport from the land surface to the depositional site ( Crusius and Anderson, 1995).
Cs-137 activities in the bottom sediments were determined using a multichannel analyzer and germanium lithium-drifted solid-state crystal detector. Measurements were calibrated using reference materials certified by the National Institute of Standards and Technology. Eight-hour counting periods were used.
For all sites, except for the one in Caño de Martín Peña (MPN500), the average vertical accretion rates were estimated by assigning the year 1954 to the base of the lowest strata that had values of Cs-137 greater than the minimum reporting limit of 1 mBq/g. This assumes that approximately 2 years passed after the initial widespread testing of thermonuclear weapons in 1952 ( Crusius and Anderson, 1995) before significant quantities of Cs-137 began accumulating in the estuarine sediments. The Caño de Martín Peña site displayed significant activity of Cs-137 even in the deepest samples recovered during the December sampling. For this site, the base of the layer with the highest activity was assigned the year 1965, two years after 1963 (the year of maximum fallout). Sedimentation rates vary from year to year based on changing precipitation patterns and changing sediment yields responding to changing land use. However, for purposes of defining the chronostratigraphic units, a constant sedimentation rate from 1925 through present was assumed and the horizons representing the time intervals from 1925 to 1949, 1950 to 1974, and 1975 to 1995 were defined. Because a decadal scale sampling unit was used, only longer-period fluctuations in precipitation and basin sediment yields would invalidate this assumption as yearly fluctuations would be averaged out.
Sediment cores from the six sites were homogenized by intervals representing the time periods 1925-49, 1950-74, and 1975-95. The longest core obtainable at the Caño de Martín Peña site was 185 cm. Therefore, the pre-1950 strata (178 to 185 cm) would represent 1948-1949, not 1925-1949 as was the case for all other cores. Each homogenized sample was analyzed for 17 organochlorine compounds including total PCB's and PCN's, and the trace elements As, Ba, Cd, Cr, Pb, Hg, and Se. Specific methods and discussion of data quality are included in Webb and Gómez-Gómez (1998).
The estimated sedimentation rates were similar at all sites with the exception of Caño de Martín Peña. Sediments are depositing at the other five sites at rates ranging from 0.24 cm/yr in the sheltered Laguna de Piñones to 0.61 cm/yr at the Bahía de San Juan and the Laguna La Torrecilla sites. In contrast, storm runoff deposits large amounts of sediment into Caño de Martín Peña resulting in a sedimentation rate of 3.9 cm/yr. Sedimentation rates exceeding 10 cm/yr are probable where terrestrial runoff enters the low-energy waters over deep dredged areas in Bahía de San Juan, Caño de Martín Peña, Laguna San José, and Laguna La Torrecilla. The peak activity of Cs-137 measured in the strata sampled at site SJN600 was 3.6 mBq/g, in the top 5 cm of the sediment core; Cs-137 activity was not detected above the analytical reporting limit of 1 mBq/g at depths below 15 cm. The Caño de Martín Peña site, MPN500, receives significant loads of sediment from the Río Puerto Nuevo in addition to numerous combined sewer outfalls located along its southern and northern banks. The peak Cs-137 activity measured in the strata sampled at the Caño de Martín Peña site was 31.2 mBq/g, in the sediments sampled from 110 to 120 cm deep, which was similar to activities measured in Lago Loíza, a nearby reservoir. Maximum Cs-137 activity in the sediments cored from Laguna Los Corozos (site CRZ400) was 11.9 mBq/g, in the strata between 5 and 10 cm deep. In southern Laguna San José (site SJS300) the maximum Cs-137 activity was 12.9 mBq/g, in the strata between 10 and 15 cm deep. Circulation is restricted in Laguna San José (and its northern embayment, Laguna Los Corozos) and whatever loads do enter the lagoon from sources around its perimeter are likely to be deposited there. This explains why the sedimentation rates were calculated to be higher at the two remotely located sites in Laguna San José than they were for the site in the Bahía de San Juan. The remaining two core sites in Laguna La Torrecilla (TRC200) and Laguna de Piñones (PNN100) receive only minimal amounts of fine-grained terrigenous sediments, probably deposited in the wake of major floods. Maximum Cs-137 activities for these sites (4.5 mBq/g for TRC200 and 1.5 mBq/g for PNN100) were measured in the top 5 cm of the sediment cores. The lowest sedimentation rate was 0.24 cm/yr in an isolated area of a mangrove-bordered lagoon and the greatest, 3.9 cm/yr in a sediment-filled, natural tidal channel. The sedimentation rates provided excellent guidance in describing chronostratigraphic units with similar shifts in the concentrations of the contained pollutants.
At the most contaminated site, in Caño de Martín Peña, concentrations of Pb increased from 30 to 745 µg/g; Hg increased from 0.16 to 4.7 µg/g; and PCB's increased from 12 to 450 µg/kg. DDT, which was used from the 1940's through the end of the 1960's, decreased from 46 µg/kg (sum of DDT, DDD, and DDE) in the period 1950 to 1975, to 14.6 µg/kg in the period 1975 to 1990. DDT and metabolites were measured at 0.48 µg/kg for the period 1925 to 1950.
PCB's are ubiquitous throughout the system and were measured at concentrations of almost 20 µg/kg even in Laguna de Piñones. The concentrations of PCB's generally increased from the oldest strata (core segment representative of the 1925-49 depositional period) to the most recent strata (representative of 1975-95). The Los Corozos site was second to that of Caño de Martín Peña with 360 µg/kg of PCB's in the most recent strata (1975-95), and had the highest concentration of DDE (62 µg/kg) in the strata of sediments deposited during the approximate period 1950-74.
Of the seven trace elements (As, Ba, Cd, Cr, Pb, Hg, and Se) measured at discrete depth intervals in bottom sediments, only Pb and Hg showed significant enrichments in recent sediments above the concentrations measured for the period 1925-49. Concentrations representative of natural predevelopment conditions are assumed here to be similar to the concentrations measured in the deepest chronostratigraphic unit, 1925-49. Arsenic concentrations generally declined at all core sites from a maximum in sediments corresponding to the 1925-49 depositional period. Concentrations of As for the 1925-49 period were 18 µg/g at the Bahía de San Juan site; 16 µg/g at the Laguna San José and Laguna La Torrecilla sites; and about 10 µg/g elsewhere. Barium also averaged about 20 µg/g for all sites except for enrichments of up to 50 µg/g at the Caño de Martín Peña and Laguna de Corozos sites. Core samples from these sites were the only ones to contain Cd above the minimum reporting limit of 1 µg/g (maximum of 3 µg/g). Chromium concentrations ranged from a minimum of 30 µg/g at the Laguna de Piñones core site to almost 50 µg/g at the Caño de Martín Peña and Los Corozos core sites. The average concentration for Pb within the estuary system for the 1925-49 period was 20 µg/g for Pb, and between 0.01 to 0.05 µg/g for Hg. Selenium was not detected above the reporting level of 1 µg/g at any site for any strata.
Anthropogenic effects on sediment quality are most evident for constituents used in the uplands of the watershed that are subsequently adsorbed onto sediments and transported into the tidal portions of the estuary. Among these are PCB's (from electrical transformers, high-pressure lubricants, and insulations of wires and cables), dieldrin (from application to sugar cane crops), Pb (from leaded gasoline and paints), and Hg (from electrical and mechanical components).
Organochlorine compounds entered the commercial market during the 1940's (PCB's, dieldrin, and DDT and metabolites). Their concentrations in surficial bottom sediments are expected to decrease in response to microbial degradation and dilution as their use has been discontinued except for limited use of PCB's in sealed transformers. Also, it is possible that sources of PCB's within the Caño de Martín Peña drainage basin are still active. These results merit more detailed sampling of surficial bottom sediments within Bahía de San Juan, Caño de Martín Peña, and Laguna San José. In addition, sampling and analyses of surficial bottom sediments are merited at Laguna del Condado considering that a thermoelectric power plant existed within the eastern shoreline of the lagoon until about 1960.
The major source for As in the bottom sediments of the estuary system is most likely weathering of As-enriched rocks in the uplands. Bed-material samples from small undisturbed basins in the uplands had As concentrations of almost 20 µg/g ( Marsh, 1992a; March, 1992b).
As with the organochlorine compounds, both Pb and Hg concentrations are expected to decline in surficial bottom sediments as a results of discontinued use of leaded gasoline in 1985 and the banned use of Hg in manufactured products. The decline in their concentrations in bottom sediments is not yet evident in the chronostatigraphic core samples because of the large time intervals (25 years) for the composited-sample analysis. However, the concentrations of total recoverable Pb measured in water samples from the Río Piedras and Quebrada Blasina have declined from generally more than 10 µg/L before 1985 to less than 5 µg/L for recent samples (Webb and Gómez-Gómez, 1998). Since Pb is principally transported in the suspended-sediment load of streams (as is typical for most water insoluble contaminants), this indicates that the flux of contaminants from the watershed to the estuary system is relatively rapid (years instead of decades).
Cesium-137 was shown to be a useful tool in estimating sedimentation rates in a coastal environment. Predevelopment concentrations of naturally occurring elements in the estuarine sediments (Pb, 20 µg/g; Hg, 0.05 µg/g; and As, 11 µg/g) were similar to those presently measured in undisturbed streambeds in the mountainous uplands. Anthropogenic sources of Pb, Hg, and certain organochlorine compounds have resulted in contamination of the upper 25 cm of sediment where sedimentation rates are low and of the upper 1 to 2 meters where sedimentation rates are high. The most contaminated areas of the San Juan Bay estuary were the Caño de Martín Peña and Laguna San José reflecting both intense population stress and the limited circulation of these water bodies
Birdsey, R. A. and L. Weaver, 1987. Forest Area Trends in Puerto Rico. U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station, Research Note SO-331, February 1987, pp. 1-5.
Calvesbert, R. J., 1970. Climate of Puerto Rico and U.S. Virgin Islands, in Climatography of the United States No. 60-52; Climate of the States: Washington, D.C., U.S. Department of Commerce, Environmental Science Services Administration, Environmental Data Service, 29 pp.
Crusius, John, and R. F. Anderson, 1995. Evaluating the mobility of 137Cs, 239+240Pu, and 210Pb from their distributions in laminated lake sediments. Journal of Paleolimnology, v. 13, pp. 119-141.
Díaz, P. L., Zaida Aquino, Carlos Figueroa-Alamo, R. J. Vachier and A. V. Sánchez, 1996. Water Resources Data, Puerto Rico and the U.S. Virgin Islands, Water Year 1996. U.S. Geological Survey Water-Data Report PR-96-1, 564 pp.
Ellis, S. R., 1976. History of dredging and filling of lagoons in the San Juan area, Puerto Rico. U.S. Geological Survey Water-Resources Investigations Report 38-76, 25 pp.
Marsh, S. P., 1992a. Analytical results for stream sediment and soil samples from the Commonwealth of Puerto Rico, Isla de Culebra, and Isla de Vieques. U.S. Geological Survey Open-File Report 92-353A, 8 pp.
Marsh, S. P., 1992b. Analytical results for stream sediment and soil samples from the Commonwealth of Puerto Rico, Isla de Culebra, and Isla de Vieques. U.S. Geological Survey Open-File Report 92-353B, data on a 5.25 inch diskette.
McHenry, J. R., J. C. Ritchie and C. M. Cooper, 1980. Rates of recent sedimentation in Lake Pepin. Water Resources Bulletin, v. 16, no. 6, pp. 1,049-1,056.
Seguinot-Barbosa, José, 1983. Coastal modification and land transformation in the San Juan Bay area, Puerto Rico. Department of Geography and Anthropology, Louisiana State University, Baton Roque, Ph.D. dissertation, 302 pp.
Webb, R. M. T. and Fernando Gómez-Gómez, 1998. Synoptic Survey
of Water Quality and Bottom Sediments, San Juan Bay Estuary System, Puerto Rico, December
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1. Richard M.T. Webb, Hydrologists
United States.Geological Survey, WRD
GSA Center Bldg.651
Federal Drive Suite 400-15
Guaynabo, Puerto Rico 00965-5703
Phone: (787) 749-4346 ext.: 278
Email: "rmwebb@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, Hydrologists
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"
3. Sherwood C. McIntyre, Ecologist
United States Department of Agriculture
Agricultural Research Service
Tuskegee University
Milbank Hall, Room 102
Tuskegee, AL 36088
Phone: (334) 727-8444
Fax: (334) 727-8552
Email: "mcintyre@acd.tusk.edu"
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