PCBs and OCPs in Sediment Cores from the Lower St. Lawrence

Jan 29, 2005 - Three sediment cores were collected along the longitudinal axis of the Laurentian Trough in the Lower St. Lawrence Estuary (LSLE) and a...
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Environ. Sci. Technol. 2005, 39, 1470-1478

PCBs and OCPs in Sediment Cores from the Lower St. Lawrence Estuary, Canada: Evidence of Fluvial Inputs and Time Lag in Delivery to Coring Sites M I C H E L L E B E U F * ,† A N D T E R E S A N U N E S ‡ Department of Fisheries and Oceans, Maurice Lamontagne Institute, P.O. Box 1000, Mont-Joli, Quebec, Canada G5H 3Z4, and Instituto Espan ˜ ol de Oceanografı´a, Centro Oceanogra´fı´co de Vigo, Apartado 1552, 36200 Vigo, Spain

Three sediment cores were collected along the longitudinal axis of the Laurentian Trough in the Lower St. Lawrence Estuary (LSLE) and an additional one at the junction of the Estuary and the Gulf of St. Lawrence. After core-slicing, each sediment layer was analyzed for polychlorinated biphenyls (PCBs) and some organochlorine pesticides (OCPs) including p,p′-dichlorodiphenyltrichloroethane (DDT) and its metabolites, hexachlorobenzene (HCB) and Mirex. 210Pb activity was also measured in these sediments, which allowed us to confirm that these cores were too much affected by the overall impact of surface mixing to be dated. Nevertheless, POP sedimentary profiles in cores from the LSLE upstream stations showed well-defined subsurface peak concentrations. Apparently, the peak inputs of POPs to these sediment cores had occurred after the years of maximum sales and production of these chemicals in North America, suggesting a time lag in the delivery of POPs to the LSLE sediments. Concentrations of POPs in the LSLE surface sediments as well as POP inventories in sediment cores decreased in the seaward direction, confirming that the head of the LSLE acts as a sink for sediments and associated constituents. Surface concentrations of ΣPCBs, ΣDDTs, and HCB in the most upstream core were on average similar to those reported in two fluvial lakes of the St. Lawrence River but were between 12 and 39 times lower than those from Lake Ontario. For Mirex, the surface concentration in that core was 5 and 130 times lower than the average values found in the fluvial lakes and Lake Ontario, respectively. Differences between Lake Ontario sediment cores and the most upstream core from the LSLE were much smaller on the basis of POP inventories than surface concentrations of POPs, but were still important. The total burdens of POPs in LSLE sediments below the 200 m isobath were 8704 kg for ΣPCBs, 1825 kg for ΣDDTs, 319 kg for HCB, and 27.5 kg for Mirex. These values represent between 1% and 10% of the total burdens of these POPs in Lake Ontario sediments. The estimated contribution of POPs by direct atmospheric deposition into the water column area of the LSLE represented at most 30% of the total sediment burdens of * Corresponding author phone: (418)775-0690; fax: (418)775-0718; e-mail: [email protected]. † Maurice Lamontagne Institute. ‡ Instituto Espan ˜ ol de Oceanografı´a. 1470

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POPs in the LSLE, illustrating the importance of fluvial inputs.

Introduction As a part of the St. Lawrence system, the Lower St. Lawrence Estuary (LSLE) begins at the confluence of the Saguenay Fjord and extends to some 200 km to the east (Figure 1). It is located downstream of the zone of the salt intrusion and maximum turbidity, the Upper St. Lawrence Estuary, and upstream of the Gulf of St. Lawrence. The LSLE receives approximately 90% of its waters from the St. Lawrence River, which drains the Great-Lakes basin, while most of the remaining waters come from the Saguenay Fjord. The St. Lawrence River delivers an average annual freshwater discharge of 12 000 m3 s-1, whereas 5-7 million tons of suspended solids are carried out annually to the Estuary (1). The LSLE is characterized by one important feature as it comprises the starting point of the Laurentian Trough, a 350 m deep channel that crosses the Gulf of St. Lawrence and opens into the Atlantic Ocean. The Laurentian Trough has been recognized as the first significant zone of permanent accumulation of sediments downstream of Lake Ontario (2, 3), and the main deposition zone of fine-grained sediment, and associated contaminants, within the whole Estuary and Gulf area (4, 5). Sediment cores have been used to study temporal trends, burdens, and fate of several metallic contaminants in the LSLE, including Pb, Hg, and Ag (2, 6-8). The same methodology has been applied to reconstruct the input history of lignin-derived materials, considered as marker of terrigenous organic matter released from pulp and paper industries and sawmill outfalls, to the LSLE (9, 10). Except for one study on Mirex (11), only preliminary works have reported data on persistent organic pollutants (POPs) in sediments of the LSLE. These were limited to PCBs and DDTs in cores collected in the 1980s (12, 13). The present work aimed to report levels, accumulation trends, inventories, and burdens of polychlorinated biphenyls (PCBs) and some organochlorine pesticides (OCPs) such as p,p′-dichlorodiphenyltrichloroethane (DDT) and its metabolites, hexachlorobenzene (HCB), and Mirex, in sediment cores from the LSLE. It also intended to assess the importance of the fluvial inputs of these compounds to the LSLE relative to those from other sources. This work is complementary to earlier studies on PCBs, DDTs, HCB and Mirex using dated sediment cores from areas located upstream the LSLE including fluvial lakes of the St. Lawrence River (14) and Lake Ontario (15-18).

Experimental Methods Sampling. Four sediment cores were collected between 1993 and 1994 along the longitudinal axis of the Laurentian Trough at water depths greater than 300 m (Figure 1, Table 1). Stations 24A, E3, and C2 were located in the Lower St. Lawrence Estuary, and station C1 was located at the junction of the Estuary and the Gulf of St. Lawrence (Figure 1). At each station, an Ocean Instruments Mark II box-corer with a 600 cm2 by 60 cm stainless steel box was used to recover the cores. The sediment cores were immediately placed on a specially designed slicing table (19) and then sliced in 0.5 cm increments at the top, and progressively increasing from 1-2 cm increments in the middle of the core to up to 3 cm increments at the bottom. Aliquots of each sediment layer were transferred into solvent-cleaned glass jars for POP 10.1021/es049051c CCC: $30.25

 2005 American Chemical Society Published on Web 01/29/2005

FIGURE 1. Map of the Lower St. Lawrence Estuary (LSLE) showing the location of the sediment coring sites. The depositional area of the Laurentian Trough within the LSLE is delineated by the 200 m isobath and divided into three zones, each one represented by one sediment core.

TABLE 1. Characteristics of the LSLE Sediment Cores Examined and Results of Optimized Parameters Obtained from the Two-Zone 210Pb Model

station

location

24A

48°28.64′ N 69°03.32′ W 48°50.63′ N 68°20.58′ W 49°06.95′ N 67°22.47′ W 49°14.95′ N 67°00.95′ W

E3 C2 C1 a

collection year

water depth (m)

porosity of surface sediment

sediment mass accumulation rate (g cm-2 yr-1)

sedimentation rate (cm yr-1)a

mixing depth (cm)

mixing rate (cm2 yr-1)

1993

326

0.87

0.42

14

1.0

1993

357

0.90

0.18

11

0.6

1994

315

0.88

0.09

14

0.6

1994

321

0.87

0.085

0.71 (0.55-1.26) 0.36 (0.32-0.67) 0.19 (0.15-0.29) 0.17 (0.15-0.28)

9

0.25

Sedimentation rates reported as mean values with range from bottom to top layers reflecting sediment compaction.

analysis, and into polypropylene vials for water content and 210Pb analysis, and stored at -20 °C until analysis. Analysis of POPs. Sediment samples were thawed at room temperature, transferred into solvent-rinsed foil dishes, and placed in a 40 °C convection oven. Approximately 10 g of semi-dried sediment was spiked with a surrogate mixture of 10 [13C12] labeled PCBs (IUPAC 28, 52, 118, 153, 180, and 209) and OCPs (HCB, Mirex, p,p′DDE, and p,p′DDT), mixed with 50-100 g of sodium sulfate and then Soxhlet extracted overnight with 250 mL of acetone/hexane (1:1). After the addition of a few drops of nonane, the extraction solution was concentrated by rotary evaporation to a volume of 2 mL. The extract was cleaned up using a 10% deactivated neutral silica gel column, followed by treatment with copper powder to remove sulfur. The cleaned-up extract containing the PCBs and OCPs was evaporated to 2 mL with solvent exchange

with nonane, and a mixture of two [13C12] labeled injection standards (1234-TCDD, 123789-HxCDD) was added. The GC-analyses of PCBs (IUPAC 28, 37, 44, 49, 52, 70, 74, 77, 81, 87, 99, 101, 105, 110, 118, 128/167, 138, 149, 151, 153, 156, 157, 158, 168, 170, 171/180, 177, 178, 183/187, 194, 199/ 201, 206, 208, 209), and OCPs (HCB, Mirex, and o,p′- and p,p′-DDE, DDD, and DDT), were performed on a HewlettPackard 5890 Series II gas chromatograph equipped with CTC 200 autosampler and a programmable split/splitless injector operated in splitless mode. PCBs and OCPs were determined separately by injecting each time a 2.0 µL sample extract. For both analyses, the injector temperature was held at 250 °C and the chromatographic separation was achieved on DB-5 type capillary column (60 m, 0.25 mm-i.d. and 0.25 µm stationary phase) with helium as the carrier gas. For the PCB analysis, the column temperature was held at 90 °C for VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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2 min, increased to 180 °C at 6 °C min-1 and then to 240 °C at 3 °C min-1, and, finally, to 300 °C at 8 °C min-1 with a 8 min hold. For the OCP analysis, the column temperature was held at 140 °C for 1.5 min, increased to 200 °C at 20 °C min-1 and then increased to 270 °C at 1.5 °C min-1 with a 1 min hold, and, finally, to 300 °C at 10 °C min-1. The GC was coupled to a VG 70SE mass spectrometer operated at a resolution of 10 000 in low-energy electron impact mode using selected ion monitoring. Two ions were monitored for each PCB congener or OCP. It has been verified that fragments of PCB congeners of high molecular weight were not interfering with fragments of lower molecular weight. The concentration of the compounds was reported when: (1) the isotopic ratio was within 15% of its theoretical value, (2) the retention time was within 3 s of the reference standard, and (3) the signal-to-noise ratio was above the threshold value of 10. Results were corrected on the basis of the 13C12 surrogate recoveries. The mean recovery of surrogate compounds in samples was 94 ( 16% for PCB congeners and 89 ( 20% for OCPs. A procedural blank sample was run for every series of 10 sediment samples, revealing no systematic contamination. Occasionally, however, positive values of some PCB congeners and OCPs were measured in blanks and were subtracted from the results in sediment samples from the same series. Blank corrections were only significant in deeper sediment layers but did not affect any of the sedimentary profiles of individual POPs. The precision of the PCB congeners and OCPs analyses was determined for two sediment samples (one from core 24A and one from core C1) analyzed in duplicate. The coefficients of variation of these analyses were on average 6.1 ( 5.8% and 7.6 ( 7.9% for PCB congeners and OCPs, respectively. The analytical accuracy of the method was assessed by measuring the recoveries of known quantities of native PCB congeners and OCPs added to four different sediment samples after subtracting the contribution of the compounds from the original samples. The recovery of these added native compounds was on average 94 ( 16% for PCB congeners and 98 ( 22% for OCPs. PCB results were reported as the sum of the 38 individual congeners measured (ΣPCBs) and as the sum of the individual congeners measured in each homologue group. DDT group pesticides (ΣDDTs) were calculated as the sum of the concentrations of o,p′- and p,p′DDE, DDD, and DDT. Concentrations of POPs are reported in dry weight (d wt) of sediments. 210Pb Activities. Radionuclide 210Pb analysis was conducted on a freeze-dried sample of each sediment layer by the measurement of the activity of its granddaughter 210Po (20). To ensure that a secular equilibrium was reached between the two elements, a minimum of 6 months elapsed before samples were analyzed. Radionuclides 210Po and 209Po, the latter used as a yield tracer, were measured using a 300-mm silicon surface barrier detector interfaced to a Canberra 8180 MCA. Precision in duplicate analysis of sediment samples was within 10% error for activities above 75 Bq kg-1 and within 20% error below that threshold.

Results and Discussion Sedimentation and Mixing. Unsupported 210Pb activity was determined in each sediment layer by subtracting from the total 210Pb activity the value of 21 Bq kg-1 obtained from the constant 210Pb activity at depth in these cores and representing the 210Pb activity supported by 226Ra decay in the sediment. The effect of sediment compaction was removed by calculating the mass of sediment in each sediment layer from the measured water content and the sediment density of 2.65 g cm-3 (8). For the four sediment cores examined, natural logarithms of the unsupported 210Pb activities are plotted as a function of sediment cumulative mass (g cm-2, d wt) (Figure 2). 1472

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FIGURE 2. Distribution profiles of unsupported 210Pb as a function of cumulative mass in sediment cores 24A, E3, and C2 from the Lower St. Lawrence Estuary and C1 from the Gulf of St. Lawrence. Solid lines are best fits obtained with the two-zone 210Pb model. Depth distribution profiles of unsupported 210Pb were interpreted by a two-zone model, which takes into account mixing in the top layers of the sediment cores. This model is commonly used to describe 210Pb profiles and has been recently applied with success to LSLE sediments (2). The two-zone model assumes constant flux of 210Pb and sedimentation rate, as well as constant mixing rate in the upper zone of the sediment core and no mixing below that zone (21). Several parameters, such as sedimentation rate, depth of the mixing zone, mixing rate, as well as the 210Pb activity at the sediment-water interface have to be optimized to best fit the 210Pb data. Although it is possible to obtain a valid estimate of the sedimentation rate from the two-zone 210Pb model, in most cases surface sediment mixing makes the assignment of a unique date to each individual sediment layer no longer possible. 210Pb sedimentary profiles simulated by the two-zone model are shown as solid lines in Figure 2, and optimized parameters are reported in Table 1. The two-zone model accurately described the 210Pb data and gave estimated sediment mass accumulation rates (g cm-2 yr-1) falling in the range of previous measurements from the same area (2, 8). The sedimentation rate (cm yr-1), calculated from the sediment mass accumulation rate, density, and porosity of the sediment decreases as a function of sediment depth as a result of compaction (Table 1). Estimated average sedimentation rates (cm yr-1) were also similar to those previously reported for the LSLE when surface mixing is taken into account (22). In addition, results of sedimentation rates in the examined cores confirm the existence of a strong decreasing gradient going downstream the LSLE (2, 22). The thickness of the mixing layers, expressed as sediment mass depths, were 7, 5, 7, and 4.5 g cm-2 in cores 24A, E3, C2, and C1, respectively, corresponding to a range from 9 to 14 cm (Table 1). These values are slightly higher than the ones reported in earlier studies (2, 22), which ranged from 3 to 10 cm, likely because these authors have only considered the upper part of the mixing layer where mixing is intense and highest and 210Pb activity is approximately constant. In the present study, the model fitting results of the 210Pb data suggest deeper but less intense mixing activity in these sediment cores (Figure 2). Surface sediment mixing can significantly alter sedimentary profiles of contaminants by creating, after deposition,

FIGURE 3. ΣPCBs, ΣDDTs, HCB, and Mirex concentration profiles in sediment cores from the Lower St. Lawrence Estuary (24A, E3, and C2) and the Gulf of St. Lawrence (C1). 0 represent concentrations below detection limits. The history of sales and production of these POPs in North America is also shown (16). a downward movement and the dispersion of contaminants within the cores (23, 24). As a result, surface sediment mixing may prevent the accurate reconstruction of pollution history from sediment cores. On the other hand, sediment cores affected by surface mixing can be useful in validating the movement of contaminants, or tracers, within sediments, assuming contaminant input functions are reasonably well known. Recently, Smith and Schafer (2) assessed the impact of surface mixing and sedimentation rate on Hg sedimentary profiles in several cores from the LSLE. The authors were able to accurately simulate depth distribution profiles of Hg from an estimated input function. Their results indicated that the history of contamination (input function) was relatively well conserved in the LSLE cores with the highest sedimentation rates, located at the head of the Laurentian Trough. On the other hand, the overall impact of mixing and low sedimentation rate in cores located further east, toward the Gulf of St. Lawrence, was too important to consider the POP sedimentary profiles at these sites as an accurate reflection of the local pollution history. Concentration Profiles of POPs. Concentration profiles of ΣPCBs, ΣDDTs, HCB, and Mirex in sediment cores are presented in Figure 3. Different depth scales were selected to take into account the different sedimentation rates among the examined cores and to represent, whenever possible, about 100 years of sediment accumulation. The sedimentary profile of ΣPCBs in the core collected at the most western station (24A) showed steadily increasing concentrations from the bottom of the core to a well-defined subsurface maximum, followed by a sharp decrease to the sediment-water interface. The other contaminants (i.e.,

ΣDDTs, HCB, and Mirex) show profiles similar to ΣPCBs in core 24A. A clear subsurface maximum for both ΣPCBs and ΣDDTs was also detected in the core collected at station E3. In that core, however, a less apparent decrease of levels was seen for both HCB and Mirex. A dampening of the peak and a broadening of the POP sedimentary profiles were observed at station E3 with respect to those of 24A. The sedimentary profiles of ΣPCBs, ΣDDTs, HCB, and Mirex in cores 24A and E3 seem to reflect the historical records of sales and production of these POPs in North America (Figure 3) (16). All of these compounds have been increasingly produced, sold, and found in the environment until their bans or use limitations, which had as a consequence to reduce their environmental concentrations. In cores C2 and C1, contaminant profiles were very similar for all of the POPs investigated, showing a continuous increase in levels from the bottom of the core to the sediment-water interface. In contrast to 24A and E3, it was not possible to identify any well-defined subsurface maximum of POP concentrations in cores C2 and C1. A gradual change of the POPs profiles in the seaward direction was observed, which results in the disappearance of the subsurface peak in the sedimentary profiles of cores C2 and C1. This gradual change of POP profiles was concomitant with the decrease of sedimentation rates in these cores (Table 1). These observations are in good agreement with the gradual disappearance of the subsurface contaminant peak in sedimentary profiles of the LSLE cores in seaward direction reported in earlier works (2, 6, 8, 11). Late Occurrence of Subsurface Contaminant Peaks. Although sediment dating is not possible in cores affected by surface mixing, it is still possible to get relevant information from sedimentary profiles of POPs exhibiting a well-defined VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Deposition Year of the Peak Input of Contaminant Calculated from the Depth of the Subsurface Peak and the Sedimentation Rate in Cores 24A and E3, and Year of Maximum Sales or Production of these Contaminants in North America station

ΣPCBs

ΣDDTs

HCB

Mirex

24A E3 sales and productionb

g1970 g1971 1968

g1964 g1969 1960

g1975 g1969 1962

g1975 g1974a 1965

b

a Calculated from the first appearance of the maximum concentration. According to ref 16.

and sharp subsurface peak. For instance, it is possible to estimate the maximum time period since the peak deposition of a contaminant onto the surface of the sediment. Evidence supporting this approach results from considering that the depth of the subsurface contaminant peak in the sedimentary profile is derived from two factors: (1) sedimentation, which adds up new sediment on the top of the sediment layer containing the contaminant (the higher is the sedimentation rate, the deeper will be the contaminant in the sediment core), and (2) surface sediment mixing, which moves downward the sediment and its associated contaminants to a depth which varies according to the intensity of mixing (or mixing rate) and the thickness of the mixing zone (23). Both sedimentation and surface sediment mixing move the contaminant toward the same direction, deeper in the sediment core, and their effect is combined in the resulting sedimentary profile. Therefore, the maximum possible time period after the peak deposition of contaminant onto the sediment can be calculated from the observed depth of the subsurface contaminant peak in the sediment core and the rate of sedimentation. As an example, sediment core 24A exhibits a clear subsurface Mirex peak at 14 cm depth (Figure 3), which corresponds to a cumulative mass of 7.5 g cm-2. From the sediment mass accumulation rate in that core (0.42 g cm-2 yr-1; Table 1), the calculated maximum possible time period after the deposition of the peak input of Mirex onto the sediment corresponds to 18 years. Knowing that the 24A sediment core was collected in 1993, the peak input of Mirex to that coring site occurred in 1975 or later (i.e., g1975). This result suggests a time lag in the delivery of Mirex to site 24A of at least 10 years when compared to the occurrence, in 1965, of the maximum input of Mirex in the environment (16). Table 2 shows for each of the different POPs the earliest deposition year of the peak input of contaminant in cores 24A and E3. According to these calculations, the maximum input of POPs observed in these cores generally occurred later in time (i.e., deposited more recently) in LSLE sediment cores than the years of North American maximum sales and production of these compounds (Table 2). There are a few possible explanations for the late occurrence of peak concentrations of the examined POPs in the LSLE sediment cores. For instance, remobilization or degradation of contaminants can significantly alter their sedimentary profiles. This possibility was examined by comparing the relative pattern of PCBs in every sediment layer throughout the investigated cores. To avoid the bias of several nondetected congeners in the deeper sediment layers, only layers with ΣPCB concentration higher than 3 ng g-1 were considered in defining the PCB patterns. The results indicate minimal changes in patterns between the upper and lower sediment layers, which suggest that negligible degradation or remobilization of PCBs occurred within these cores (Figure 4). Thus, it can be concluded that these processes did not significantly alter the overall profiles of ΣPCBs, including the depth of the subsurface peak of contaminant concentrations in these cores. 1474

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FIGURE 4. Average PCB patterns (% composition of PCB homologue groups ( std dev) in the upper and lower sediment layers of the cores. Tri, tetra, penta, hexa, hepta, and octa-deca represent the sums of the individual congeners measured within each homologue group. It would also be possible that the maximum use of these chemicals happened some years after the years of maximum sales and production, assuming that large stocks were stored. Thus, these chemicals could have been intensively used in the 1970s, between their peak production and their subsequent ban or restriction (16, 25). Several studies, however, have reported good agreement between the historical records found in Lake Ontario dated sediments for ΣPCBs, ΣDDTs, HCB, and Mirex, and the history of production and sales of these chemicals (15-17, 26). Another hypothesis is that all of these chemicals could have been significantly used at later time in the vicinity of the LSLE. Local uses and releases of PCBs (Baie des Anglais) and DDTs (Quebec and New Brunswick) have been reported in the surrounding areas of the LSLE (27, 28). However, Mirex, which has never been used locally, clearly shows a delay in the occurrence of subsurface peaks in both 24A and E3 sediment cores relative to its peak accumulation recorded in Lake Ontario sediments. The example of Mirex suggests that POPs from nonlocal sources, especially Lake Ontario, are delivered to the LSLE sediments, resulting in a local contribution of both PCBs and DDTs either negligible or in phase with the contribution of other potential sources. The late peak deposition of the examined POPs in the LSLE sediments could possibly result from a delay in their

transition between Lake Ontario and the LSLE. This is likely the case for Mirex, a chemical exclusively exported from Lake Ontario and transported to LSLE primary by outflowing water (11). Considering that it takes several years to totally renew the water mass in Lake Ontario, this phenomenon should favor some additional retention of POPs within the lake, creating a time lag between the peak inputs of POPs into the lake and the peak flux of POPs outflowing Lake Ontario (29). Additionally, a delay could also occur during the transport of POPs along the St. Lawrence River. For instance, it has been shown that suspended particles, and likely their associated POPs, can be retained for months, according to their size, in the Upper St. Lawrence Estuary before being flushed out to the LSLE (30). A part of POPs reaching the LSLE is accumulated and temporarily retained by biota, especially in species with long life span such as marine mammals, which would slow their delivery to sediments. Late occurrence of subsurface peak concentrations of POPs in sediments can be explained, in a more conceptual way, as a consequence of system time averaging in which particleassociated constituents accumulate and mix in a “reservoir” before transferring to permanent sediments (31). According to Robbins et al. (31), such a “reservoir” does not refer to any physical part of a given system but acts by retaining elements attached to particles, such as radionuclides or contaminants, creating a time lag before their permanent accumulation in sediments. Contrary to the observed time lag in delivery of POPs to LSLE sediments, Hg profiles in sediments located at the head of the Laurentian Trough show subsurface peak concentrations at depths that correspond to a maximum possible deposition year that occurred, as expected, before the year of the peak input (i.e., 1971), in agreement with the downward transport of Hg due to surface mixing (2). In that particular case, there was no late occurrence of subsurface peak of Hg, likely because the main source of Hg, the Saguenay River, is located downstream the Upper St. Lawrence Estuary and much closer to the LSLE. Sediment Concentrations of POPs. Surface sediment mixing may significantly alter the vertical distribution of POPs in sediment cores. For instance, high inputs of POPs are dampened by surface sediment mixing, whereas low inputs are enriched by contaminated sediments already in place. As a result, surface concentrations of POPs in sediment cores affected by intensive mixing and low sedimentation do not accurately reflect the local and most recent inputs of POPs and therefore must be compared with caution with other sites or areas. On the other hand, concentrations of POPs in surface sediments give relevant information on the exposure of benthic organisms to these contaminants. The highest concentrations of POPs in LSLE sediments were found in core 24A, with subsurface peak reaching 40 ng g-1 for ΣPCBs, 7.6 ng g-1 for ΣDDTs, 1.3 ng g-1 for HCB, and 0.16 ng g-1 for Mirex (Figure 3). Decreasing concentrations toward surface sediments were systematically seen in that core, with the peak-to-surface ratio ranging from 1.6 for HCB to 2.5 for ΣDDTs. In core E3, peak concentrations were lower, and the depletion of POP concentrations from peak to surface was less pronounced as compared to core 24A. Surface concentrations in both 24A and E3 sediment cores were very similar, in the range of 20 ng g-1 for ΣPCBs, 3 ng g-1 for ΣDDTs, 0.8 ng g-1 for HCB, and 0.1 ng g-1 for Mirex (Figure 3). Surface concentrations of POPs in cores C2 and C1 were at most 2 times lower than those found in cores 24A and E3. The most noticeable difference among these four cores is that cores C2 and C1 do not exhibit any well-defined subsurface peaks, and, as such, the highest POP concentrations were found at or near the surface. Only a few studies have reported POPs in LSLE sediment cores. For instance, Cossa (12) measured PCBs and DDT

metabolites (p,p′DDE and p,p′DDD) in a core collected in the Laurentian Trough, near Rimouski, in 1985. They reported sedimentary profiles very similar to those of ΣPCBs and ΣDDTs in cores 24A and E3. Surface and peak concentrations were, however, slightly lower than those found in core E3. PCB concentrations were also reported in three cores from the LSLE collected in 1987 (13). Concentrations of ΣPCBs were in good agreement with those in nearby stations reported in the present study, but profiles were more erratic than those shown in Figure 3. Mirex was reported in five sediment cores collected between 1987 and 1989 in the LSLE (11). These authors reported large variability in Mirex concentrations from one sediment layer to another, leading to irregular profiles. In addition, Mirex levels were about 3-4 times higher than those reported in the present work. Observed differences in levels and sedimentary profiles of POPs with previous works are likely due to the different analytical techniques used and, in the case of the data reported by Cossa (12), to the smaller number of congeners or metabolites analyzed. POP concentrations were reported in sediment cores for two sites in fluvial lakes (St. Francis and St. Louis) of the St. Lawrence River, located at some 400 km upstream the LSLE (14). The authors reported multipeak shape of the POP profiles in these two cores collected in 1992 and suggested that historic trends in pollution by POPs have been influenced not only by changing anthropogenic inputs, but also by river discharge. Peak concentrations of ΣPCBs (sum of 13 congeners), ΣDDTs (sum of p,p′DDD and p,p′DDE), and HCB in both cores were on average between 2 and 3 times higher than those observed in LSLE sediment core 24A, whereas surface concentrations were very similar. This indicates that the overall depletion of POP concentrations from peak to surface was more pronounced in the fluvial lakes than in the LSLE sediments. For Mirex, however, surface and peak concentrations were on average 5 times higher in cores from the fluvial lakes than in core 24A, indicating a similar depletion of Mirex toward the surface sediments in both areas. Located upstream of the St. Lawrence system, at a distance of about 750 km from the LSLE, Lake Ontario has been the site of several studies on chronology accumulation of organic contaminants during the last 25 years. Sedimentary profiles of ΣPCBs, ΣDDTs, HCB, and Mirex for at least 10 sediment cores from deep basins, collected between 1981 and 1991, have been reported in the scientific literature (15-18). Surface and peak concentrations for ΣPCBs and ΣDDTs reported in Lake Ontario sediment cores were on average from 14 to 22 times higher than those found in LSLE core 24A. For HCB, average surface and peak concentrations in Lake Ontario sediments were 39 and 63 times higher than those in core 24A, respectively. Among the POPs examined, Mirex showed the highest difference of concentrations between Lake Ontario and LSLE sediments. For this specific contaminant, surface concentration and subsurface peak concentration in core 24A were on average 130 and 220 times lower than in Lake Ontario sediment cores, respectively. For all of these POPs accumulated in Lake Ontario sediments, the average peak-to-surface ratios were relatively similar, ranging from 3 to 5. In sediment core 24A, the peak-to-surface ratios were between 1.6 and 2.1 times lower than in Lake Ontario cores. Concentrations of POPs in sediment cores depend on several factors including the history of POP inputs and the sediment mixing but also on the organic carbon content and the grain size of particles, and the sediment accumulation rates. In sediment cores from Lake Ontario, the St. Lawrence River, and the LSLE, the range of organic carbon content was rather narrow, from 4-5% (d wt) at the surface to about 1-2% in depth (9, 14, 15). Similarly, the size of sediment particles accumulating at these sites was mainly constituted of silt and clay-sized particles (2, 14). Sedimentation rates, VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Inventories (ng cm-2) and Burdens (kg) of ΣPCBs, ΣDDTs, HCB, and Mirex in the LSLE Sediments ΣPCBs

HCB

ΣDDTs

Mirex

station

surface > 200 m (km2)

inventory (ng cm-2)

burden (kg)

inventory (ng cm-2)

burden (kg)

inventory (ng cm-2)

burden (kg)

inventory (ng cm-2)

burden (kg)

24A E3 C2 sum

721 1804 2255 4780

488 197 72

3523 3557 1624 8704

90 41 19

647 743 435 1825

18 7.4 2.4

131 134 54 319

1.45 0.66 0.23

10.4 12.0 5.1 27.5

however, differed drastically among the examined sediment cores, making the interpretation of POP concentrations among sites or areas more difficult. For example, sedimentation rates at the head of the LSLE were more than 5 times higher than in Lake Ontario, whereas POP concentrations were at least 15 times lower (2, 15). Therefore, any given contaminant input would result in lower sediment concentration at the site where sedimentation rate is higher. One way to compare the sedimentary accumulation of POPs among different sites is to calculate their inventories. This approach is independent of the influence of surface mixing (vertical redistribution of POPs after deposition) and sedimentation rates (dilution of the input of POPs by settling particles). These considerations, together with the fact of the observed decreasing gradient of sedimentation rates downstream, in the seaward direction (Table 1), suggest that inventory data may provide a suitable insight for further discussion of transport of contaminants and accumulation in the sediments along the LSLE. Inventories. The inventories of ΣPCBs, ΣDDTs, HCB, and Mirex (ng cm-2) were calculated for each core according to the following equation: n

I)

∑(1 - φ )F[ ] Z i

i i

i)1

where øi, [ ]i, and Zi represent the measured porosity, the concentration of the contaminant (ng g-1), and the thickness (cm) of the layer i in the core, respectively. A single density value, F, of 2.65 g cm-3 was used for all the n sediment layers (8). The POP inventories within the LSLE varied considerably between locations (Table 3). For instance, the inventories of the examined POPs at station 24A were on average 2.3 and 6.4 times higher than those at stations E3 and C2, respectively. This contrasts with the very similar concentrations of POPs measured in surface sediments at both stations 24A and E3, and with the relative small concentrations gradient of POPs, on average less than a factor of 2, between 24A and C2 (Figure 3). The POP inventories in the investigated sediment cores decreased radically in the seaward direction, which confirms that the head of the LSLE, where the Laurentian Trough begins, acts as a sink for sediments and associated constituents (5, 10). Strong inventory gradients have been reported for Pb, Hg, and Ag within the LSLE (2, 6, 8). The POP inventories reported in Lake Ontario cores were in the range of 650-1550 ng cm-2 for ΣPCBs, 240-420 ng cm-2 for ΣDDTs, 60-100 ng cm-2 for HCB, and 22-54 ng cm-2 for Mirex (15). The mean inventory values in Lake Ontario cores as compared to those in core 24A, the LSLE site with the highest POP inventories, were between 2.5 and 4.3 times higher for ΣPCBs, ΣDDTs, and HCB. The highest difference of POP inventories was seen for Mirex, which was about 25 times higher in Lake Ontario cores as compared to core 24A from the LSLE. These results indicate that, although differences of POP inventories were still observed between Lake Ontario cores and core 24A from the LSLE, these were 1476

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between 1.8 and 3.8 times smaller than the differences of POP concentrations in surface sediments. Burdens. The calculated inventories of POPs in the LSLE sediment cores were used to estimate the total burdens in the Laurentian Trough sediments between the head of the Trough and Pointe-des-Monts (Figure 1). The depositional area of the Trough, delineated by the 200 m isobath, covers a surface area of approximately 4800 km2 within the LSLE. This area was divided into three zones, each one being represented by one sediment core and having axial boundaries equidistant from the adjacent sediment cores (Figure 1). This approach has been used to evaluate the total burdens of Pb, Hg, and Ag in the LSLE sediments (2, 6, 8). In these studies, seven cores were used to characterize the main zone of sediment deposition and contaminant burdens in the LSLE. Although it is always preferable to consider as many cores as possible to estimate contaminant burdens in sediments from a given area, it is possible to obtain very similar results, within 20% error for trace metal burdens, by considering only three representative cores instead of seven. The total sediment burdens of ΣPCBs, ΣDDTs, and HCB in the LSLE were 8704, 1825, and 319 kg, respectively (Table 3). For Mirex, the estimated total burden in the LSLE sediments was 27.5 kg, a much lower value as compared to the 184 kg reported by Comba et al. (11). As mentioned earlier, these authors reported much higher concentrations of Mirex in the LSLE sediments, which explains the discrepancy between the two burden estimates. Total burdens of POPs in Lake Ontario sediments from Oliver et al. (16) were 50 000 kg for ΣPCBs, 30 000 kg for ΣDDTs, 10 000 kg for HCB, and 2000 kg for Mirex, whereas Wong et al. (15) reported 130 000 ( 18 000 kg for ΣPCBs, 35 000 ( 5000 kg for ΣDDTs, 8500 ( 1500 kg for HCB, and 3800 ( 570 kg for Mirex. These values are much higher than the estimated total burdens of POPs in the LSLE sediments. In comparison, the total burdens of POPs in LSLE sediments represent between 3% and 10% of the burdens in Lake Ontario sediments for ΣPCBs, ΣDDTs, and HCB, and only 1% of the Mirex burden. Sources of POPs to the LSLE Sediments. Despite its modest total burdens of POPs in sediments as compared to Lake Ontario, the LSLE still shows significant sedimentary accumulation and relatively high sediment inventories of POPs, especially in cores collected at the head of the Laurentian Trough. Although POPs are reaching the LSLE and accumulating in its sediments, their main input pathways have not been well established. POPs are potentially delivered to the LSLE sediments by fluvial and local inputs as well as by direct atmospheric deposition. The fluvial input integrates POPs coming from the Saguenay Fjord, the St. Lawrence River, and Lake Ontario, deposited on both their water surface area and drainage basin, and delivered to the LSLE through the flowing waters. The local source represents the POPs discharged to the LSLE from local point-sources, small rivers, and streams. The third source is from the direct atmospheric deposition of POPs into the water column of the LSLE.

TABLE 4. Estimated Contributions of ΣPCBs, ΣDDTs, HCB, and Mirex to the LSLE Sediments, from Different Sources inventory at station C1 (ng cm-2) ΣPCBs ΣDDTs HCB Mirex

31.8 11.5 1.13 0.13

direct atmospheric deposition kg % 1520 548 54 6.1

17 30 17 22

fluvial and local inputs kg % 7185 1277 265 21.4

82 70 83 78

The contribution of POPs from the direct atmospheric deposition input was estimated from the inventories of POPs calculated in core C1 (Table 4). It was first assumed that core C1, collected at the junction of the Estuary and the Gulf, has received POPs exclusively via direct atmospheric deposition into the water column of the specific area of the LSLE and from erosional input from its local drainage basin. This implies that no significant contribution of POPs from local point-sources has reached C1. If these assumptions were proved to be false, then the estimated direct atmospheric input of POPs to C1 would be overestimated, as further discussed below. In support of these assumptions, however, ΣDDTs and HCB inventories calculated at C1 were similar to those reported from peat cores located in the northeastern part of Canada resulting from direct atmospheric deposition (32). In that study, the inventory for PCBs was about 3 times lower than the value calculated in core C1, whereas Mirex inventory from atmospheric source was not reported. Furthermore, unsupported 210Pb inventory in core C1, estimated at 0.67 Bq cm-2, was also similar to the direct atmospheric input of 0.57 Bq cm-2 calculated for this area (33) or the value of 0.54 Bq cm-2 reported by Smith et al. (34). The estimated input of POPs from direct atmospheric deposition to the LSLE sediments was calculated from the POP inventories in core C1 extrapolated to the whole area (4780 km2) of permanent sediment deposition below the 200 m isobath (Table 4). The contribution of ΣPCBs to the LSLE from direct atmospheric deposition was estimated to be approximately 1500 kg, which represents 17% of the total sediment burden for ΣPCBs. Similarly, 17% of HCB accumulated in the LSLE sediments came directly from the atmosphere. The atmospheric contribution of Mirex and ΣDDTs to the LSLE sediments, resulting from direct deposition, represents 22% and 30% of the total burdens, respectively. These results indicate that ΣPCBs, ΣDDTs, HCB, and Mirex contributions to the LSLE sediments via direct atmospheric deposition appear to be relatively minor as compared to those from upstream fluvial waters and from local point-sources, which represent at least 70% of the total burdens of POPs in the LSLE sediments (Table 4). As mentioned above, these calculated contributions of POPs to the LSLE resulting from direct atmospheric deposition could be overestimated because it is very possible that a portion of the burdens of POPs in core C1 originated from local or fluvial inputs. One way to assess the local inputs of POPs to the LSLE is to compare their estimated contributions with Mirex, a compound considered to exclusively originate from Lake Ontario (11, 35). Results indicate that direct atmospheric contributions of both ΣPCBs and HCB were similar to that of Mirex, suggesting that the main contributor of these compounds to the LSLE sediments is Lake Ontario and its drainage basin. For DDTs, however, the estimated direct atmospheric contribution is slightly higher than for Mirex, suggesting a contribution from local sources, which is in agreement with the known local uses of these chemicals (28, 36).

Acknowledgments We thank Dr. C. Gobeil for his help and advice in sampling and Dr. C. Marvin for his comments and suggestions on a previous version of the manuscript. POP analyses were done by Wellington Laboratories, and 210Pb analyses were done by Flett Research. Funding was provided by the Department of Fisheries and Oceans under the Green Plan Program. We also thank the Instituto Espan ˜ ol de Oceanografı´a and the Fundacio´n Alfonso Martı´n Escudero, Spain, for financial support during the stay of T.N. in Canada.

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Received for review June 22, 2004. Revised manuscript received November 5, 2004. Accepted December 6, 2004. ES049051C