Environ. Sci. Technol. 1997, 31, 3232-3237
Long-Term Variations of PCB in the Water of a River in Relation to Precipitation and Internal Sources GUDRUN BREMLE* AND PER LARSSON Ecotoxicology, Department of Ecology, Lund University, 223 62 Lund, Sweden
In this study, the PCB concentration in river water was continuously monitored with weekly samples for 2 years. The aim was to study in what way PCB concentration was related to water discharge, temperature, concentration of suspended matter, and other limnological variables. One hypothesis examined was that at high water discharge PCB will approach “background” levels found in precipitation, whereas at low water discharge internal sources will determine the PCB concentration in the river.
Material and Methods PCB concentration in the water of a south Swedish river was followed for 2 years. The concentration of the pollutants varied seasonally, approaching levels in precipitation at high water discharge. Principal component analysis (PCA) revealed that PCB congener distribution in river water at high flow was similar to that found in precipitation. At low water discharge, and coinciding with high water temperatures during summer, concentration of PCB increased. The relationships indicated desorption of PCB from sediment, a conclusion supported by PCA of the congener distribution in the sediment and water. The results, consequently, revealed two sources of PCB to river water, atmospheric dominance during high flow and internal, sediment recycling during low flows explaining seasonal variation.
Introduction Few investigations report on the concentration of polychlorinated biphenyls in low-contaminated freshwater. For a mass balance study Harrad et al. (1) used values from a study by Baker et al. (2), measured in Lake Superior (the least contaminated of the Great Lakes in North America and stated to most likely represent “background contamination”) to estimate PCB concentrations of freshwater (1.2 ng/L) in United Kingdom. PCB investigations of rivers almost exclusively report values from either highly contaminated ones (3, 4) or rivers in highly populated urban areas (5). Time series of background PCB levels in rivers, with simultaneous measurements of limnological variables, are few. Sediment records have often been used to reflect the pollutant burden of a river (6). The major input of PCB to most freshwater systems is the atmosphere (7). Except for the more direct transfer of PCB from air to water, for example as rain on the water surface, PCB deposited on soil and vegetation can also reach the waterbody as a result of washout with precipitation from the catchment area. Soil and vegetation were calculated to hold a substantial part of the PCB in a mass-balance study of United Kingdom (1). The type of soils and land use in the catchment areas will influence the amount of leakage to freshwater. PCB content in water can also be influenced by PCB point sources, from towns, etc, and by recycling of PCB within the watercourse. Although PCB is a mixture of very lipophilic compounds, it will dissolve in water and is transported in this phase. Suspended particles and organic compounds in the water will, however, facilitate the transport (7). Variables, such as water discharge, temperature, and precipitation will also influence the concentration of PCB in the water, as well as biotic factors such as uptake in phytoplankton and fish, and withdrawal by sedimentation. * Corresponding author fax:
[email protected].
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 11, 1997
Location. Emån River, situated in the southeastern part of Sweden, has a catchment area of 4460 km2. The river may be classified according to its biogeography (8) as belonging to rivers in the southern mixed coniferous forest (group 2). These streams have low gradient, and flow through mixed coniferous and deciduous forests, interspersed with clear or humic lakes, ponds, and wetlands. The typical group 2 river has a dominance of lakes along the longitudinal profile which results in considerable influence of lake processes and an appreciable runoff all year. Emån River, though, has few lakes and is known to have large fluctuations in water discharge with high flow during winter and spring (mean discharge at river mouth 30 m3/s; estimated maximum 270 m3/s). The sampling station was situated 115 km from the river mouth (57°24′ N, 15°32′ W). At this site, the river is about 25 m wide and about 0.5 m deep. The river bed is hard, with sand, gravel, and stones. The vegetation is sparse with Epilobium plants, some moss (Fontinalis antipyretica) and Scirpus lacustris, and a few Salix along the bank. The benthic fauna shows high diversity indicating good water quality (9). Sampling and Analysis of PCB. The concentration of PCB was continuously measured in the river water from April 1, 1993, to April 30, 1995. The sampling method has been described earlier (10). For each sample, about 100 L of water was pumped through polyurethane columns (PUC) at a flow rate of 10 mL/min and represented an integrated 1 week sample. Both dissolved PCB and PCB adsorbed to particles in the water phase were collected by the columns. Samples were frozen until analysis which normally took place within 1 week. Detailed methods for sample preparation and analysis are described in Bremle et al. (11). In short, PUCs were Soxhlet extracted in hexane:acetone and evaporated in a vacuum centrifuge. Concentrated extracts were purified on acid/basic double layer silicagel column. Eluates were evaporated in a vacuum centrifuge and redissolved in iso-octane. Samples were then analyzed for PCBs by capillary-gas chromatography/ECD [Shimadzu (GC-14A) with split/splitless injector, 20 m DB 5 quarts capillary column (id 0.18 mm)]. PCB components were identified and quantified according to Mullin et al. (12) and Schulz et al. (13). As a chromatographic standard (to check retention and response), pentachlorobenzene was used. The analytical performance was regularly checked with PCB standards such as Aroclor 1242 and Clophen A60. Concentration of total PCB was calculated from the sum of concentrations of 53 identified peaks. Polyurethane filters were precleaned by Soxhlet extraction with acetone and hexane and found to be uncontaminated after this treatment as were unexposed filters returned from the sampling area. Extraction efficiency of the surrogate standard octachloronaphthalene was determined for this method in Agrell et al. (14) and recoveries for river samples found to be 97 ( 26%. Samples were not corrected for extraction efficiencies.
S0013-936X(97)00217-4 CCC: $14.00
1997 American Chemical Society
TABLE 1. Water Discharge and the Concentration of PCB in River Water and the Correlation to Other Variables (Pearson Correlation, Pairwise, P Values from SYSTAT)a PCB concentration water discharge correlation variable
n
water discharge temperature turbidity fixed residue concn susp. matter concn TOC concn TotP concn TotN pH alcalinity conductivity absorbance color
109 109 109 109 109 106 105 105 53 53 53 50 53
p
coefficient
-0.75 0.05 0.41 0.03 0.67 0.20 0.53 -0.37 -0.84 -0.82 0.88 0.77
