Environ. Sci. Technol. 1999, 33, 2953-2957
Silver in Sediments from the St. Lawrence River and Estuary and the Saguenay Fjord CHARLES GOBEIL* Maurice Lamontagne Institute, Department of Fisheries and Oceans, P.O. Box 1000, Mont-Joli, Quebec, Canada G5H 3Z4
The concentration of Ag in sediment cores from the St. Lawrence River and Estuary and the Saguenay Fjord reaches values 2-15 times higher than its crustal abundance, indicative of widespread dispersion of anthropogenic Ag. The direct discharge of wastewaters is likely the most important pathway for the introduction of anthropogenic Ag into these environments, but the input from the Great Lakes can also be important as suggested by very high Ag concentrations in a core collected in the river near Lake Ontario. The distribution of Ag in 210Pb-dated cores reveals that the Ag contamination increased markedly after 1930, reached a maximum during the 1980s, and has been diminishing since. Estimates of the total burden of anthropogenic Ag deposited in the sediments are on the order of 43 tons in the lower St. Lawrence Estuary, 12 tons in the St. Lawrence River, and 2.7 tons in the Saguenay Fjord. The average annual deposition since 1930 is equivalent to 0.3% of the total annual anthropogenic flux of Ag into United States waters.
Introduction Anthropogenic Ag enters the aquatic environment mainly via the discharge of wastewaters (1). Ag can disperse as dissolved or colloidal species, but ultimately it accumulates in bottom sediments (2-4). The measurements of Ag in sediments can therefore be used to delineate the impacted zones of sewage discharges into the aquatic environment and to assess changes in environmental quality associated with these discharges. Sedimentary records of Ag have been reported for a few locations intensively affected by urban activities, for instance the San Francisco Bay, the Puget Sound, and the Massachussetts Bay (4-6). To document further the contamination of the sedimentary environment, this study reports the depth distribution of Ag in sediment box cores collected downstream from the Great Lakes in the St. Lawrence River and Estuary, and in the Saguenay Fjord, a tributary to the St. Lawrence Estuary. The vertical profiles of 210Pb and acid volatile sulfide (AVS) in some of the cores have also been determined in order to support the interpretation of the Ag profiles. These results are the first to be reported for Ag in these environments where wastewaters have been discharged for a long time without treatment.
Methods Sampling and Chemical Analysis. Undisturbed sediment box cores were collected at 9-24 m depth in the St. Lawrence * Phone: (418) 775-0591; fax: (418) 775-0718; e-mail: GobeilC@ dfo-mpo.gc.ca. 10.1021/es981322u CCC: $18.00 Published on Web 07/29/1999
Published 1999 by the Am. Chem. Soc.
River and its upper estuary, 301-358 m depth in the lower St. Lawrence Estuary, and 258-274 m depth in the Saguenay Fjord (Figure 1). The cores were sectioned on board into horizontal layers using equipment especially designed to subsample box cores (7). The sediments were kept frozen (-20 °C) in acid-cleaned polyethylene bottles. In the laboratory, sediment aliquots were freeze-dried, homogenized by grinding, and digested in a microwave oven with a mixture of nitric, hydrochloric, fluoric, and perchloric acids (8). The Ag concentrations were then determined by atomic absorption spectroscopy, using a pyrolytic graphite furnace equipped with a L’vov platform. The overall analytical procedure was verified with the reference sediment BCSS-1 of the National Research Council of Canada. The precision, expressed as the coefficient of variation of replicate analyses of the reference material, was 5%. The accuracy was within 9%. Acid volatile sulfides (AVS) were determined on wet sediment following the protocol described by Allen et al. (9). The detection limit was 0.01 µmol g-1, defined as twice the standard deviation of the blank. The analytical precision, as determined by replicate analyses of one sample, was 8% at 0.2 µmol g-1. Finally, 210Pb was determined via its granddaughter 210Po according to the procedure of Eakins and Morrison (10) with an analytical precision of 7% at 8 dpm g-1. Sediment Dating. The vertical profiles of 210Pb were determined in 4 of the 13 cores analyzed for this study. Excess 210Pb activities were estimated assuming that 210Pb supported by 226Ra is equal to 0.8 dpm g-1 in the areas under investigation (11, 12). At St. 800, which is located in the upper estuary zone of maximum turbidity, the excess 210Pb activity is almost constant as a function of depth in the sediments (Figure 2), which agrees with previous observations at this site (13). Rather than indicating a very high sedimentation rate, the almost homogeneous 210Pb activity down to the bottom of the core at this station is interpreted as an indication of the intense mixing resulting from periodic resuspension/resedimentation (13-15). At the other stations, however, the profiles are characterized by an exponential decrease of the 210Pb activity as a function of depth in the sediment. Assuming a constant sedimentation rate and a constant input flux of unsupported Pb, the sedimentation rate, ω, can be calculated using the following relationships: A ) A0e-βz and β ) γ/ω, where A and A0 are the activities of the excess 210Pb at cumulative mass z and at the top of the core, respectively, β is the slope of ln A versus z, and γ is the decay constant for 210Pb (16). Sedimentation rates thus determined are 0.16, 0.44 and 0.56 g cm-2 year-1 at St. 865, St. 23A, and St. S6, respectively.
Results and Discussion The Ag content of the sediment varied from 0.05 to 1.1 µg g-1 (Figure 3), the highest concentrations being found at St. 865, St. 845, and St. 800 in the St. Lawrence River and its upper estuary. At one site, St. 800, there was no discernible depth trend. This was expected given that the sediments are intensively mixed at this location. At all other stations, however, the concentrations increased upward from values similar to the crustal abundance near the bottom of the cores (0.07 µg g-1) (17), reached values 2-15 times the background at a depth varying between 5 and 15 cm, and then decreased more or less regularly toward the sediment surface. At most stations, this decrease was about 10-25%. Concentrations of Ag in excess of its natural level in all cores examined constitute evidence of the wide dispersion of anthropogenic Ag in the regions under investigation. The levels of Ag in the cores from the St. Lawrence River and its VOL. 33, NO. 17, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Map showing the sampling sites in the St. Lawrence River and Estuary and the Saguenay Fjord. their high levels of metallic contamination (19). Although most of the metals entering the Great Lakes are retained in the sediments, it has been concluded that there is a significant export of some metals via the St. Lawrence River (20, 21). Historical Trends in Ag Contamination. The relationship between the distribution of a metal in a sediment core and the historical evolution of contamination is often obscured by the effects of diagenesis. In fact, a subsurface concentration maximum, as observed for Ag in this study, can be created by the diffusion of a metal into the sediments and the subsequent precipitation below the sediment surface. It has been demonstrated, for instance, that diagenesis contributes to the formation of subsurface peaks of Zn, Cu, and Ni in acid lake sediments and subsurface peaks of Cd in coastal marine sediments (22-25). It is therefore necessary to examine the possibility of diagenetic alteration before interpreting the Ag profiles.
FIGURE 2. Excess 210Pb activity vs total mass accumulation at St. 865, St. 800, St. 23A, and St. 6. upper estuary are comparable to levels found in sediments from environments severely impacted by human activities (4, 5). The discharges of urban effluents, which can be more than 200 times enriched in Ag over uncontaminated sediments (6), is the most likely pathway for the introduction of anthropogenic Ag. The drainage basin of the St. Lawrence River and Estuary excluding the Great Lakes is inhabited by about 6 million people living in more than 300 riverine municipalities, and until 1984 sewage from most of these, including the City of Montreal, were discharged without treatment (18). Untreated urban effluents are also released in the Saguenay River and Fjord at a rate of about 185 000 m3/day, mostly from Chicoutimi and Jonquie`re, two midsized industrialized municipalities with an aggregate population of about 150 000. In addition to the local direct discharge of sewage, contaminant Ag originating in the Great Lakes regions can also be significant. This is suggested by the presence of very high Ag concentrations at St. 865 located near the junction of Lake Ontario and the St. Lawrence River. To the best of our knowledge, Ag in the Great Lakes has as yet to be studied, but the lower Great Lakes are notorious for 2954
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The presence of detectable amounts of AVS at or just below the sediment surface in our cores (Figure 4) is one reason to suspect possible diagenetic alteration of the sedimentary record of Ag. AVS is produced during the oxidation of organic matter by sulfate reducing bacteria, and the main product of this reaction, hydrogen sulfide, can react with iron and other metals to form the AVS (26). Since AVS increases sharply downward from the sediment surface, at St. 865, St. 845, and St. 800, and from 2 to 3 cm depth, at St. 23A, St. 22A, and St. S6, the possibility exists that dissolved Ag, either in the water column or remobilized in the oxic surface sediments, can diffuse into the sediment and be precipitated as a sulfide. As a class B metal, Ag has a strong tendency to form strong insoluble sulfides in anoxic environment (27, 28). To assess the influence of the diagenesis on the sedimentary record of Ag, we assume that the concentration of Ag in the interstitial water of the oxic surface sediments and in the overlying water is on the order of 100 pM and that Ag has to diffuse only 1 cm before it is precipitated in the sediment. This assumption relies principally on the work of Rivera-Duarte and Flegal (29), who found an average Ag concentration of 120 pM in 55 pore water samples from 5 different cores from the San Franciscco Bay, but also on other studies of the Ag concentration in riverine and coastal waters (3, 30). At steady state, the flux of dissolved Ag into the interior
FIGURE 3. Distribution of Ag in the sediments at all sampling sites. of the sediment can be estimated using Fick’s law of diffusion: J ) -φ2Ds ∂[C]/∂x), where J is the flux, φ the porosity, ∂[C]/∂x the Ag concentration gradient (0.1 pmol‚cm-4), and Ds the whole sediment diffusion coefficient. The latter is assumed to be equal to φ2D0 for high porosity sediments (31); D0 is the molecular diffusion coefficient in water (10-5 cm-2 s-1) (32). Given these assumptions, the rate of downward diffusion of Ag into the interior of the sediment is 2.5 ng cm-2 year-1. This is only 2-4% of the present deposition rate of Ag at our St. 865, St. 23A, and St. 6, which is determined by multiplying the Ag concentration in the surficial sediment by the respective sedimentation rate. It thus appears that diffusion of dissolved Ag into the sediments is not sufficient to induce important variations of the depth distribution of Ag. Although we cannot exclude completely the possibility of diagenetic alteration of the sedimentary record, it is likely that the Ag profiles in these sediments reflect the temporal changes in the deposition of Ag. In conclusion, and with application of the 210Pb geochronology at our St. 865, St. 23, and St. 6 sites the Ag contamination in the St. Lawrence and the Saguenay increased markedly after 1930. It is possible that this was a result of the expansion of the photographic industry, known to be one of the most important sources of anthropogenic Ag (1). The presence and the position of a subsurface Ag
peak at those stations indicates that the contamination reached a maximum in the 1980s and has diminished slightly since. This recent reduction of Ag deposition is consistent with the introduction of wastewater treatment facilities in the most important riverine municipalities along the St. Lawrence River during the past 10-15 years (33). Ag Burden in Sediments. The Laurentian Trough in the lower St. Lawrence Estuary is an important site for the accumulation of contaminants. This 350 m deep basin, which covers an area of about 5000 km2 below the 200 m isobath, is the main site of fine-grained sediment deposition with sedimentation rates of a few millimeters per year (16). The burden of anthropogenic Pb in this basin has been estimated at 13 000 metric tons, based on the assumption that the results obtained for sediment cores collected along the trough can be extrapolated to the whole area below the 200 m isobath (34). The calculations were made assuming a sediment density of 2.65 g cm-3 and using measured porosity values. Using this approach for Ag, the total burden of Ag in excess of its natural background (0.07 µg g-1) in the lower St. Lawrence Estuary amounts to 43 tons (Table 1). Similar calculations for the approximately 100 km2 zone of sediment deposition in the Saguenay Fjord reveals an accumulation of 2.7 tons of anthropogenic Ag in this region (Table 1). VOL. 33, NO. 17, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Distribution of acid volatile sulfide (AVS) as a function of depth in the sediments.
TABLE 1. Total Burden of Anthropogenic Ag Deposited Below 200 m Depth in Lower St. Lawrence Estuary and the Saguenay Fjord station 24A 24 23A 23 22A 22 21A total S7 S6 S5 total
inventory (mg m-2)
area (km2)
Lower St. Lawrence Estuary 12 280 30 290 18 320 15 490 9.8 630 4.5 960 4.2 1810 4780 27 24 28
Saguenay Fjord 33 40 28 101
total burden (103 kg) 3.3 8.7 5.6 7.4 6.1 4.4 7.7 43 0.92 0.96 0.79 2.7
The retention of anthropogenic Ag in the St. Lawrence River and its upper estuary is more difficult to assess. According to the literature there is no net accumulation of sediment in the upper estuary (14, 15). A few such sites do exist in the St. Lawrence River, as revealed both by the results of this study (St. 865) and a recent study on sediment cores (18). Overall, it seems reasonable to assume that 6-8% of the 5 × 106 tons of suspended solids carried annually by the St. Lawrence River is retained within the St. Lawrence River and lake system (35). Adopting 0.5 µg g-1 as the concentration of anthropogenic Ag in the sediments that have accumulated since 1930 (suggested by our St. 865 results), the total burden of anthropogenic Ag within the St. Lawrence river system over the past 65 years is about 12 tons. The total amount of anthropogenic Ag accumulated in the sediments of the whole area investigated has thus been about on the order of 58 tons, or 0.9 ton/year since 1930. This is equivalent to 0.3% of the current estimate of the total annual flux of anthropogenic Ag into the waters of the United States (267 tons year-1) (1), confirming the role of the St. Lawrence River as an important pathway for the export of contaminants to the coastal marine environment. 2956
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Acknowledgments This research was supported financially by the Department of Fisheries and Oceans through its Toxic Chemicals Program. The laboratory work was carried out with the collaboration of G. Paquette, L. Beaudin, Y. Clermont, and M. Mongrain. Some of the sediment samples were provided by G. H. Tremblay. This paper benefited from the review of B. Sundby.
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(21) Carignan, R.; Lorrain, S.; Lum, K. Can J. Fish. Aquat. Sci. 1994, 51, 1088-1100. (22) Carignan, R.; Nriagu. J. O. Geochim. Cosmochim. Acta 1985, 49, 1753-1764. (23) Carignan, R.; Tessier, A. Science 1985, 228, 1524-1526. (24) Rosenthal, Y.; Lam, P.; Boyle, E. A.; Thomson, J. Earth Planet. Sci. Lett. 1995, 132, 99-111. (25) Gobeil, C.; Macdonald, R. W.; Sundby, B. Geochim. Cosmoschim. Acta 1997, 61, 4647-4654. (26) Lasorsa, B.; Casas, A. Mar. Chem. 1996, 52, 211-220. (27) Stumm, W.; Morgan, J. J. Aquatic Chemistry; Wiley: New York, 1981. (28) Di Toro, D. M.; Mahony, J. D.; Hansen, D. J.; Scott, K. J.; Carlson, A. R.; Ankley, G. T. Environ. Sci. Technol. 1992, 26, 96-101. (29) Rivera-Duarte, I.; Flegal, A. R. Mar. Chem. 1997, 56, 15-26.
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Received for review December 21, 1998. Revised manuscript received May 25, 1999. Accepted June 15, 1999. ES981322U
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