Seasonal and Decadal Variations in Lead Sources to Eastern North

Jan 20, 2010 - ... and 2005 at three sites along the French Atlantic coast, one in the La Fresnaye Bay and the others in the Loire and Seine River est...
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Environ. Sci. Technol. 2010, 44, 1211–1216

Seasonal and Decadal Variations in Lead Sources to Eastern North Atlantic Mussels R A O U L - M A R I E C O U T U R E , * ,† JEAN-FRANC ¸ OIS CHIFFOLEAU,‡ DOMINIQUE AUGER,‡ DIDIER CLAISSE,‡ CHARLES GOBEIL,† AND DANIEL COSSA§ Universite´ du Que´bec, INRS-ETE, 490 Rue de la Couronne, Que´bec, Canada, G1K 9A9, IFREMER, Centre de Nantes, Rue de l’Ile d’Yeu, Nantes, France, 44311, and IFREMER, Centre de Me´diterrane´e, Zone Portuaire de Bre´gaillon, La Seyne-sur-mer, France, 83507

Received August 3, 2009. Revised manuscript received December 14, 2009. Accepted December 16, 2009.

The concentration of Pb and its stable isotope composition were measured in 216 composite samples of 50 blue mussels (Mytilus edulis (M. edulis)) collected quarterly between 1985 and 2005 at three sites along the French Atlantic coast, one in the La Fresnaye Bay and the others in the Loire and Seine River estuaries. Depending on the sites and time periods, Pb concentrations were 5-66 times higher than the natural background value for the North Atlantic. Even for the samples with the lowest Pb concentrations, the isotopic signature of Pb is very different than that of the regional natural Pb, suggesting that most of the bioaccumulated Pb is anthropogenic in origin. Stable Pb isotope ratios measured in the mussels differ markedly from that of Pb emitted in Western Europe as a result of leaded gasoline combustion, which was still a dominant source of contaminant Pb to the atmosphere during most of our study period. The isotope composition of Pb in the mussels was instead more typical of that of the Pb released to the environment by wastewater treatment plants, municipal waste incinerators, and industries such as metal refineries and smelters. Continental runoff, rather than atmospheric deposition, is therefore identified as the leading transport pathway of Pb alongtheFrenchAtlanticcoast.Fromthestrongseasonalvariations in 206Pb/208Pb ratios in the mussels from the Seine Estuary site we also conclude that the resuspension of contaminated sediments, triggered by high river runoff events, is a chief factor affecting the bioaccumulation of Pb in M. edulis. The value of this organism as a biomonitor of coastal contamination is thus further demonstrated.

organism soft tissues have consequently been carried out worldwide to identify hotspots of contamination and determine temporal trends in trace metals, particularly in the United States (5) and in France (6). The interpretation of results from mussel watch programs is, however, not always straightforward because a variety of processes can affect metal concentrations in mussel soft tissues. For example, metal concentrations can vary naturally during an annual cycle due to metabolic changes initiated by spawning, a process that causes the soft tissue weight of the mussel to fluctuate while the metal content remains constant (7-9). Besides this biological dilution effect, variations in metal concentrations in mussel soft tissues at a given site can also be sporadically induced as a result of sediment resuspension events, spills, or discharges from point sources (e.g., refs 10 and 11). To resolve this complexity and improve the benefit of mussel watch programs, sample collection has to be repeated preferably several times per year and new approaches need to be developed to better understand all of the factors influencing metal inputs in coastal systems. In this respect, measurements of the isotopic composition of Pb in mussel soft tissues stand as a promising tracer as revealed by experiments of mussels transplanted into contaminated coastal systems (2, 11, 12). There are four stable Pb isotopes (204Pb, 206Pb, 207Pb, and 208Pb), the three heaviest and most abundant of which result respectively from the radioactive decay of isotopes 238U, 235U, and 232Th. Stable Pb isotope ratios are thus a function of the characteristics of the geological system from which the metal derives, and, because there is no measurable biological, chemical, or physical fractionation of Pb isotopes as they migrate in the environment, these ratios can be insightful indicators of pathways impacted by human activities at regional and global scales (13). Despite convincing evidence, provided by experiments of mussel transplantation, the potential use of Pb isotope ratios in marine mussel as tracers of metal sources and pathways to the coastal zone remains insufficiently exploited in biomonitoring programs. We measured the concentration of Pb and its stable isotope composition in mussel (Mytilus edulis (M. edulis)) soft tissue samples collected four times a year between 1985 and 2005 at three sites along the French Atlantic coast. While the sources and chronologies of the past century Pb emissions into the Western Europe atmosphere are relatively well understood (e.g., ref 14), this knowledge does not likely apply to the coastal zone because a significant part of Pb in such systems comes, through river runoff, from drainage basins where the residence time of anthropogenic Pb is longer than in the atmosphere (15, 16). The aim of this study is to demonstrate that the spatial patterns and intricate temporal trends in coastal Pb contamination can be better deciphered using measurements of stable Pb isotope ratios in marine mussels.

Introduction Marine mussels are among the few organisms that can be used as biomonitors of coastal contamination by trace metals (1). The interest in mussels as biomonitors arises from their wide geographical distribution, from their resistance to environmental stress, and because they accumulate trace metals in proportion to their local bioavailabilities (e.g., refs 1-4). Mussel watch programs based on the analysis of whole * Corresponding author email: [email protected]. † Universite´ du Que´bec. ‡ IFREMER Centre de Nantes. § IFREMER Centre de Me´diterrane´e. 10.1021/es902352z

 2010 American Chemical Society

Published on Web 01/20/2010

Materials and Methods Environmental Setting. Three sites spanning a range of environmental conditions were chosen for this study (Figure 1). Site 1 is located in the La Fresnaye Bay within the NormanBreton Gulf (48°38′60′′ N, 2°17′40′′ W), whose surrounding areas are not industrialized and not heavily populated. The waters of the Norman-Breton Gulf are typical of those of the English Channel since they receive no important freshwater inputs from rivers. Site 2 is located in the estuary of the Loire River at Pointe Chemoulin (47°14′00′′ N, 02°17′90′′ W). The Loire River, which is the longest in France, has a mean annual discharge of 850 m3 s-1 and drains an area of 1.2 × 105 km2 VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Sampling site locations along the French Atlantic shoreline. inhabited by a population of 11.5 million people, mainly located in rural regions and midsize cities (17). Our sampling site in the Loire Estuary was positioned 15 km downstream from the town of Paimboeuf, where a chemical plant, operated by Octel-Kulhman, produced alkyl-lead added to gasoline in France and other European countries from 1938 to 1996 (17). Site 3 is located in the estuary of the Seine River at Villerville (49°24′30′′ N, 00°07′50′′ E). The mussel bank at this site lies in estuarine waters and receives Pb inputs from the Seine Bay and the plume escaping the high-turbidity zone of the Seine Estuary. This zone is formed as a result of the tidal-induced resuspension of sediments from local mudflats and salt marshes (18). The Seine River drainage basin has been shown to be highly contaminated by Pb and other metals (19, 20) owing to the presence of 17 million inhabitants and 40% of French chemical industries. Sampling and Analysis. Fifty individuals (35-65 mm) of the mussel M. edulis were collected four times a year between 1985 and 2005 within the framework of the French mussel watch program (6). Following a depuration step, during which all individuals were kept for 24 h in a polyethylene aquarium containing decanted water from the sampling site, the soft tissue of the 50 mussels was removed from the shell, homogenized with a stainless steel blender, and freeze-dried. Aliquots of 250 mg were then digested at 90 °C for 3 h with 8 mL of ultrapure concentrated nitric acid. After appropriate dilutions, Pb concentrations ([Pb]) were measured using a graphite furnace atomic absorption spectrometer equipped with a Zeeman background corrector (Perkin-Elmer 3030, Varian Spectra AA 800) for the samples collected between 1985 and 2002 and a quadrupole inductively coupled plasmamass spectrometer (ICP-MS Thermo Elemental, X7 series) for those collected after 2003. Replicate measurements (n ) 18) of [Pb] in the mussel certified standard reference material CRM-278R from the European Community Bureau of Reference (BCR) was within 7% of its certified value; the precision, expressed as the coefficient of variation of replicate analyses, was 3%. Stable Pb isotope ratios were determined by ICPMS on the same solutions. Signal intensity was measured 100 times over 1.2 × 10-2 s for 206Pb and 207Pb and 1 × 10-2 s for 208Pb. This procedure was repeated five times for each sample. Mass bias and instrumental drift was corrected with a standard bracketing method by analyzing the certified reference material SRM-981 from NIST after every second sample (21); the internal precision of the instrument, expressed as relative standard deviation (n ) 46), was 0.11 and 0.16% for the 206Pb/207Pb and 206Pb/208Pb ratios, respectively. The external precision for the digestion of replicate (n ) 18) aliquots of the mussel CRM BCR-278R was 0.13 and 0.19% for the 206Pb/207Pb and 206Pb/208Pb ratios, respectively.

Results and Discussion Temporal Trends in Pb Concentration and Isotopic Composition. Variations in the mussel [Pb] over the sampling period at each of the sites, as well as those of the 206Pb/207Pb 1212

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and 206Pb/208Pb ratios, are illustrated in Figure 2. The results of the 216 composite samples analyzed are also listed in the Supporting Information. The contrasting Pb levels and isotope ratios measured among sites, as well as their different time trends, suggest that the relative strength of the Pb sources impacting each site varied over time (Figure 2). The overall average mussel [Pb] at site 1 (1.1 ( 0.5 µg g-1, n ) 70) was five times higher than what is considered as background concentrations (∼0.2 µg g-1) for the same species in the Eastern North Atlantic (22). At site 1, [Pb] increased slightly from 1985 to the mid-1990s and then progressively decreased until 2005 (Figure 2a). Furthermore, the mussel [Pb] varied over an annual cycle, with the highest concentrations in the winter and spring and the lowest in the summer and fall (Figure 2a). Similar short-term variations in trace metal concentrations in mussel soft tissues were also observed in previous studies and attributed to biodilution, referring to seasonal fluctuations in the soft tissue weight (6–9). The 206Pb/207Pb and 206Pb/208Pb ratios in mussels collected at site 1 ranged from 1.158 to 1.173 and from 0.476 to 0.482, respectively. Exhibiting no systematic seasonal variations, the ratios both progressively increased between 1985 and the early 1990s (Figure 2d,g) and then remained relatively constant around mean values of 1.167 ( 0.002 and 0.479 ( 0.001, respectively. The effect of biodilution on the concentrations of Pb in mussel tissues can be taken into account using the following empirical relationship (8): PbCOR ) PbMES + m(CI - CIREF) + 2b

(1)

where PbCOR is the Pb concentration corrected for biodilution, PbMES is the measured concentration, CI is a condition index defined as the measured soft tissue weight to shell weight ratio, CIREF is the average CI over the entire time series, and m and b are respectively the slope and the intercept of the linear regression of PbMES as a function of CI. As part of the French mussel watch program, CI has been routinely determined since 2001. Assuming that these results can be extrapolated to the whole 1985-2005 time period, sitespecific values of CIREF, m, and b were calculated (see Table S1 of the Supporting Information) and subsequently used to approximate PbCOR. It follows that biodilution at site 1 induces a 60 ( 19% (n ) 36) increase of the Pb concentration in mussel soft tissues during winter time, hence explaining most of the observed seasonal variations at this site. The average [Pb] over the whole sampling period at site 2 (2.8 ( 1.1 µg g-1, n ) 76) was more than twice that at site 1. The concentrations followed a strong seasonal pattern prior to 1992, again with the highest concentrations in winter and spring and the lowest concentrations in summer and fall; values as high as 6.4 µg g-1 and as low as 2.1 µg g-1 were reached in spring 1992 and fall 1988, respectively (Figure 2b). After 1992, the [Pb] generally decreased over time with nonsystematic seasonal variations. In contrast to site 1, much less (11 ( 5%, n ) 38) of the observed seasonal increase in Pb concentration during winter time can be attributed to biodilution at site 2. Values of the 206Pb/207Pb and 206Pb/208Pb ratios measured in samples collected at site 2 were lower than those collected at site 1 during most of the sampling period (Figure 2e,h). Alongside a steady decrease in [Pb], the 206 Pb/207Pb and 206Pb/208Pb ratios generally increased between 1985 and 2005, from 1.130 to 1.163 and from 0.470 to 0.480, respectively (Figure 2b), reaching values similar to those found at site 1. It is noteworthy that both ratios measured in mussels from site 2 temporarily decreased in 1997-2001, closely mirroring an increase in [Pb] during this time interval (Figure 2b,e,h). With an average of 3.7 ( 2.0 µg g-1 (n ) 70), the mussel [Pb] at site 3 was significantly higher than at the other sites

FIGURE 2. Distribution of Pb concentrations on a dry weight basis (panels a-c) and of the 206Pb/208Pb (panels d-f) and 206Pb/207Pb (panels g-i) ratios in mussel samples collected from site 1 (left panels), site 2 (center panels), and site 3 (right panels) between 1985 and 2005. In each panel the mean value is indicated by a dashed line. In panels a-c the dotted line indicates the background Pb concentration in mussels in the North Atlantic (22). In panels a-i, open symbols represent samples collected during summer and fall while solid symbols represent samples taken during winter and spring. The insets in panels b, e, and h are magnifications of the time period during which the former alkyl-lead plant site was decontaminated (1997-2001). In the insets, the thick solid lines are the smoothed average of the data. (p < 0.001). Moreover, the [Pb] record revealed strong seasonal variations, again with high values during winter and spring and low values during summer and fall (Figure 2c); the highest concentration (13.3 µg g-1) was reached in April of 1990 (Figure 2c). Calculations of PbCOR using eq 1 indicate that only 14 ( 4% (n ) 35) of the Pb concentration increase in mussel soft tissues during winter and spring is due to biodilution. The 206Pb/207Pb ratio at site 3 did not vary much over the whole sampling period, and its average value (1.161 ( 0.002) was close to that at site 1 (Figure 2i). In contrast, the 206Pb/208Pb ratio strongly varied over time (Figure 2f). The general trend was a decrease from 1985 to 2002, followed by an increase between 2002 and 2005. Moreover, the chronological record of the 206Pb/208Pb ratio at site 3 exhibited short-term fluctuations; the lowest values occurred in winter and spring, along with the highest [Pb] (>5.4 µg g-1). Constraints on Pb Sources. The presence of anthropogenic Pb in the coastal zone is due to atmospheric inputs and continental runoff, but the relative importance of these pathways is not well-defined. Although the sources of anthropogenic Pb into the atmosphere have been numerous, the combustion of leaded gasoline was by far the dominant human activity responsible for Pb emissions into the atmospheric reservoir during the 20th century (14, 23-25).

This was inferred from both global scale emission inventories (e.g., ref 26) and measurements of [Pb] and its isotope composition in various environmental archives, such as sediment and peat bog cores, tree rings, and archival vegetal samples (27). Even though the origin of anthropogenic Pb in the continental runoff remains poorly characterized, it is also likely affected by multiple sources of contaminant Pb, including leaded gasoline. However, there are undoubtedly important differences in the relative proportion of Pb inputs from various anthropogenic sources and in the chronology of such inputs between continental runoff and atmospheric deposition. First, Pb from leaded gasoline in water runoff should not be as dominant as in the atmosphere due to its dilution by the Pb released directly into streams with industrial and municipal waste waters (20). Second, because the migration of anthropogenic Pb in the soil reservoir is a relatively long process with respect to its atmospheric counterpart, the latter responds faster to changes in Pb input (15). At our three sampling sites, most of the bioaccumulated Pb in the mussels is from anthropogenic sources given that [Pb] in the mussel tissues were significantly higher than the background value for the North Atlantic (22). This interpretation is also supported by the Pb isotope ratio results. For VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. 206Pb/208Pb ratios as a function of the 206Pb/207Pb ratios for mussels from site 1 (open diamonds), site 2 (open squares), and site 3 (open circles) along with rectangular zones representing the isotopic signature of Pb in French leaded gasoline (23, 24), industrial emissions (32-35, 40), coal (31), and natural sources (23, 28-30). example, the 206Pb/207Pb ratio at the three sites (∼1.13-1.17) was consistently much lower than that of the natural Pb reported in the literature for preindustrial sediments in the North Atlantic Ocean (206Pb/207Pb ∼ 1.20-1.23; refs 28 and 29) and for preindustrial sediments of the Loire River (206Pb/ 207 Pb ∼ 1.197-1.206; ref 23) and the Seine River (206Pb/207Pb ∼ 1.201 ( 0.01; ref 30). The 206Pb/207Pb ratio measured in the mussel tissues at site 1 (∼1.16-1.17) was consistently higher than that of leaded gasoline produced in European countries during the 20th century mainly using Pb from Broken Hill, Australia (206Pb/ 207 Pb ∼1.04; ref 24). The 206Pb/207Pb ratio of alkyl-lead gasoline ranges from 1.06 to 1.10 in France (23, 24) and from 1.06 to 1.13 in Europe in general (27). Our observations therefore point out that Pb from leaded gasoline released in the coastal zone through atmospheric deposition and continental runoff, at least since 1985, was not the major source of anthropogenic Pb at site 1. Similarly, it seems unlikely that Pb emitted into the environment by coal burning was a major source of Pb to the mussels, since Diaz-Somoano et al. (31) reported an average 206Pb/207Pb ratio of 1.19 ( 0.04 (n ) 32) for European coal, which is higher than the mean value of this ratio (∼1.17) found at site 1. The isotopic composition of Pb in mussels from site 1 (206Pb/207Pb ∼1.16-1.17; Figure 3) is in the middle of the range reported for Pb from industries such as metal refineries and smelters in France (206Pb/207Pb ∼ 1.10-1.20 in 2006; ref 32) and close to values for Pb emitted by wastewater treatment plants (206Pb/207Pb ∼ 1.15-1.16 in 1994; ref 33) and found in European urban aerosols (206Pb/207Pb ∼ 1.14-1.17 in 1995-1996; ref 34 and 35). Assuming that Pb in the atmosphere of France in the 1980s and 1990s was still mainly deriving from the alkyl-lead added to gasoline (23, 24, 35), our measurements show that Pb found in mussel tissues at site 1 is predominantly transported to the coastal zone by continental runoff. The relative importance of the various sources of Pb accumulated in mussel tissues at site 2 is different than that at site 1. Between 1985 and 1990, the 206Pb/207Pb ratio was as low as 1.12-1.14 (Figure 2), potentially due to a greater proportion of Broken Hill Pb used to produce leaded gasoline in Europe. Some of this Pb might have been deposited from the atmosphere, but most likely originated from the alkyl-lead plant located 15 km upstream of our sampling site. During the 1980s this chemical plant released up to 26 × 103 kg year-1 of Pb to the Loire Estuary, hence overwhelming all other Pb sources (17). It is notable that the temporary decrease of the Pb isotopes ratios as the [Pb] increased during 1214

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FIGURE 4. Water flow of the Seine River (dashed lines) as a function of time along with biodilution-corrected Pb concentration (thick line, panel a) and 206Pb/208Pb ratio (thick line, panel b) in mussel soft tissues from site 3. the 1997-2001 period (see insets in Figure 2b,e,h), can likely be linked to the decontamination of the industrial site where the alkyl-lead plant was located. This plant was shut down in 1996 and subsequently demolished. The decontamination operation started in 1997 and was completed in the 2001; during that period ∼103 t of contaminated waste and groundwater were removed from the site. Recent measurements of dissolved [Pb] in the Loire Estuary revealed that the closure of the plant brought a 10-fold decrease in [Pb] in the past decade, from 0.4-1.7 nM in 1990 to 0.04-0.26 nM in 2002 (36). Consistent with this observation, the 206Pb/207Pb ratio at site 2 progressively increased after 1996 and reached values close to those found at site 1, thus demonstrating that after this date the anthropogenic sources of Pb at sites 1 and 2 became progressively similar. At site 3, the elevated Pb levels detected in mussels (up to 13.3 µg g-1) are consistent with the high Pb concentrations measured in waters and sediments of the Seine River and Estuary (19, 20, 37), which are thought to be mainly derived from industrial sources (20). At this site, the average 206Pb/ 207 Pb ratio (206Pb/207Pb ∼ 1.161 ( 0.002) in the mussels was similar to that found at site 1 (206Pb/207Pb ∼ 1.16-1.17), which shows again that most of the Pb does not come predominantly from atmospheric deposition of anthropogenic Pb due to leaded gasoline combustion (Figure 3). The 206Pb/207Pb ratio measured at site 3 is comparable to the value of 1.169 measured in Seine River sediment sampled at its mouth (38). Furthermore, both the 206Pb/207Pb and the 206Pb/208Pb ratios in site 3 mussels are very close to those found in surface mudflat sediments from the Seine Estuary (1.160-1.163 and 0.475-0.477, respectively; ref 39). There is limited information in the literature regarding the geological origin of Pb used by the industries emitting Pb to the Seine River and Estuary. The isotopic composition of ore bodies from several countries, known to provide Pb to the European market, spans a wide range of 206Pb/207Pb values (1.01-1.20; refs 32 and 40). Among these potential Pb sources, ore from Northern Europe (206Pb/208Pb < 0.475 and 206Pb/207Pb ∼ 1.160; ref 40) is characterized by signatures similar to those found in site 3 mussels. Seasonal Riverine Imprint on the Pb Isotope Composition at Site 3. The isotopic composition of Pb in the mussel soft tissues at site 3 exhibits pronounced changes over short periods of time, and, as illustrated in Figure 4, these changes are matched by strong shifts in both the mussel’s biodilutioncorrected Pb concentration ([PbCOR]) and the water discharge of the Seine River (ref 41). Indeed, for 16 of the 20 years

studied, the lowest annual 206Pb/208Pb ratio appeared at river discharge peaks, concurrent with the highest Pb concentrations (Figure 4). Cross-spectrum analysis with XLSTAT confirms that the primary period at which all the time series (i.e., 206Pb/208Pb, 206Pb/207Pb, [PbCOR], and river flow) oscillate synchronously is 1 year (p < 0.04). Our observations thus strongly suggest that both the isotopic composition of Pb and [PbCOR] are tightly coupled to a key variable which fluctuates with water discharge, i.e. the sediment load. This is consistent with the reported 2 orders of magnitude variation over time in the concentration of suspended particulate matter in the Seine River (42). The increase in turbidity is duetosedimentresuspensioneventswithintheriver-estuarine system and to the intensification of erosion, both processes being triggered by the high river flow lasting up to 7 weeks in winter time (18). It should also be noted that hydrodynamic modeling has shown that the extent to which the river’s plume reached our study site is a function of the river flow (43). Linking the changes in the isotopic composition of Pb in the mussels with the sediment discharge of the Seine River is rational because the kinetics of the accumulation of Pb in the mussels is relatively rapid. The prompt response of the mussels to their surrounding environment has been demonstrated in transplantation experiments, during which the concentrations of Pb in the soft tissues of transplanted organisms progressively change to reach a pseudoequilibrium with their new environment after ∼1 month (12, 44). Likewise, according to Casas et al. (9), Pb concentrations in transplanted mussels (M. edulis) at contaminated sites reach a plateau within a similar time period. Therefore, M. edulis is a biomonitor able to register the influence of sediment resuspension events due to high river discharge, whose time scales are of the order of a few weeks (18). Data showing that the isotopic composition of Pb in suspended particulate matter of the Seine River varies on a yearly basis are currently not available. However, such periodic variations are highly probable considering that sediments susceptible to resuspension have been contaminated by trace metals over a long period of time (37) and that the isotopic signature of anthropogenic Pb in these sediments is likely different than that of today’s contaminant Pb (37, 39). Along the same line, it should be emphasized that, due to the long residence time of Pb in soils (15, 16), the isotopic composition of Pb deriving from the watershed during the raining season must be different than that of the Pb currently discharged into the environment. We can therefore conclude that, even at our least contaminated site, [Pb] in the soft tissues of the mussels collected in 1985-2005 was five times higher than in remote coastal areas of the North Atlantic. Most of the anthropogenic Pb accumulating in the mussels is transported to the coastal zone via continental runoff rather than atmospheric deposition. The isotopic composition of Pb in the mussels is not typical of that of Pb used to produce leaded gasoline in Western Europe but instead is close to that of Pb emitted by wastewater treatment plants and found in European urban aerosols. Furthermore, simultaneous variations in the Pb isotope ratios and Pb concentrations over most of the annual cycles indicate that the latter cannot only be attributed to a biological dilution effect. Showing an almost perfect match between the occurrences of the lowest annual values of the 206 Pb/208Pb ratio in the mussels collected in the Seine Estuary and the river discharge peaks during the winter and spring, our results reveal the high potential of M. edulis to monitor the impact of sediment resuspension events on coastal contamination.

Acknowledgments This research has been partially supported by Ifremer and ´ cologie, de l’Energie, du De´veloppement the “Ministe`re de l’E

durable et de la Mer”, within the framework of the Re´seau National d’Observation de la Qualite´ du Milieu Marin, and the Que´bec-France Cooperation Program (Grant No. 60-107). Thanks are due to K. Mueller for editing the manuscript.

Supporting Information Available Tables of results and of parameter values needed to calculate biodilution-corrected Pb concentration with Equation 1.This information is available free of charge via the Internet at http://pubs.acs.org.

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