4 RECENT HISTORICAL DEPOSITION OF TRACE METALS AND THEIR SPATIAL
5.2 Spatial distribution of trace organic compounds within the delta
5.3.2 Concentration profiles of anthropogenic organic compounds
Profiles of PCB and organochlorine pesticide concentrations in lake sediments are presented in the following sections. Profiles in overbank sediments are not available as most results are below the detection limit. lt should be reminded that detection limits determined during the analysis of overbank sediments are ten times greater that those determined during the analysis of lake sediments (see chapter 2.7.2). Two samples in each overbank core were analyzed. The first one is the surface sample (from 0 to a depth of 2 cm) and the second one was collected between 6 and 8 cm of depth.
Polychlorobiphenyl (PCB) concentration profiles
Profiles of the sum of PCBs (97 congeners from 1 to 209) in lakes 6 and 7 are illustrated in figure 5.20.
The concentrations of the sum of PCBs in lake 7 decrease from the top of the core until the bottom. One sample, between 16 and 18 g/cm2 of cumulative dry weight, con tains very high concentrations of PCBs (144 ng/g). This value is more than 10 times higher than the one measured in the sample at the top of the core. Such high concentrations have not been determined in any other lake or overbank cores, suggesting that this sample has been contaminated during the sampling or the analytical procedure. However, the distribution of homologues in this sample is similar to the distribution observed in the other sample, which should indicate that the source of PCBs is similar. As this sample raises doubts, it has been excluded from the data set. In lake 6, the profile of the sum of PCBs does not indicate the same decrease of concentrations observed in lake 7. The sample with the highest PCB concentration (3.3 ng/g) is located below the surface sample (between 0.8 and 2.5 g/cm2), but the sum of PCBs in this sample is much lower than the one measured in surface sediments in lake 7 (10.8 ng/g). Concentrations of the sum of PCBs range from 2.8 to 10.8 in lake 7 and from 1.6 to 3.3 in lake 6. The median concentration is 4.9 ng/g in lake 7 and 2.5 in lake 6.
The distribution of PCB homologues is usually constant over depth in lakes 6 and 7 (fig. 5.20). lt is dominated by tetrachlorobiphenyls, followed by tri- and pentachlorobiphenyls (fig. 5.20). Light (mono-and dichlorobiphenyls) (mono-and heavy (hepta, oeta-, nona- (mono-and decachlorobiphenyls) PCBs are not detected or detected at very low concentrations (fig. 5.20). However, heptachlorobiphenyls represent almost 10%
of the sum of PCBs in the surface samples of lakes 6 and 7 but concentrations decrease below the surface and represent only 0 to 4% of the sum of PCBs. The most abundant congener is 52 and the heaviest and most abundant congener is 149/118 (appendix 4.1).
Lake 7 Lake 6
Figure 5.20: PCB profiles in lakes 6 and 7 with distribution of homologues.
PCB profiles in lake cores collected in the outer delta (lakes 3 and 5) are sirnilar to the ones measured in lakes 6 and 7 in the middle delta (appendix 4.4). The range of concentrations in lake 3 is comparable to the one deterrnined in lake 6 (from 0.9 to 3.7 ng/g). Like in lake 7, PCB concentrations are higher at the top of the core (from the top until 12 g/cm2) and decrease toward the bottom. The same trend of concentrations is observed in lake 5 where the maximal PCB concentration is found in the first upper sample (6.9 ng/g). On average, the median, maximal and minimal PCB concentrations are sirnilar in the middle and outer delta (table 5.9).
Table 5.9: Median, maximal and minimal concentrations of the sum of PCBs in lake sediments in the outer delta (lakes 3 and 5) and the middle delta (lakes 6 and 7).
Outer delta (lakes 3 and 5, n
=
19) Middle delta (lakes 6 and 7, n=
28)Min. Median Max. Min. Median Max.
I
PCBs [nglg] 0.91 2.1 6.9 1.6 3.6 10.8In overbank sediments, only two samples (usually from 0 to 2 cm and from 6 to 8 cm) were analyzed in each core, as PCBs concentrations were very low, close to the detection lirnit. As described in chapter 2.7.2, the method detection lirnit is 10 times higher (0.1 ng/g) in overbank sediment analysis than in lake sediment analysis (0.01 ng/g), which explains the small number of detected PCBs in overbank sediments.
As well several congeners with responses between the two capillary colurnns differing by more than 30%
were excluded from the data set, as this difference exceeded the confidence test used to confirm PCB
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concentrations.
On levee 8, the sum of PCBs in the surface sample (from 0 to 2 cm) is 0.84 ng/g and below the detection limit in the sample between 6 and 8 cm. In the surface sample, the most abundant congener is 52, followed by 44, 49 and 101, which are all tetrachlorobiphenyls except 101.
PCB concentrations in the other overbank samples in levees 1 to 7 range from non detected to 3.9 ng/g. Congener 52 is the most abundant congener, followed by 101, 44, 87, 49, which are all tetra- or pentachlorobiphenyls (appendix 4.1). These congeners are also dominant in lake sediments.
The median, minimal and maximal concentrations in overbank cores collected in the middle delta are similar to the ones measured in samples collected in the outer delta (table 5.10).
Table 5.10: Median, maximal and minimal concentrations of the sum of PCBs in overbank sediments in the outer delta (levees 1 to 5) and in the middle delta (levees 6 to 8).
Outer delta (levees 1 to 5, n
=
10) Middle delta (levees 6 to 8, n=
6)Min. Median Max. Min. Median Max.
I
PCBs [ng/g] ND' 2.8 3.7 ND' 2.4 3.91 ND
=
not detected.The comparison between PCB concentrations in lake and overbank samples indicates that the median and minimal concentrations of the sum of PCBs are similar in lake and overbank sediments, while high concentrations (between 7 and 11 ng/g) are only measured in lake sediments. This is partly explained by the different detection limits.
Concentration profiles of organochlorine pesticides Chlorobenzenes (CBZs), the DDT family, hexachlorocyclohexanes (HCHs) and various cyclodiene concentrations in lake cores are not available as most of these compounds are below the detection limit.
Among the organochlorine pesticides detected in lake and overbank sediments, the dominant compounds are chlorobenzenes (table 5.11). located at the bottom of the cores ( 1.4 and 0.76 ng/g respectively).
By considering the distribution of the CBZ homologues, dichlorobenzenes are dominant when the sum of CBZs is higher than 3 or 4 ng/g (appendix 4.5a). When the sum of CBZs is very low (<3 ng/g), heavier chlorobenzenes such as tri- and tetrachlorobenzenes are more abundant.
The range of the sum of CBZs in the outer delta (lakes 3 and 5) is similar to the one in the middle delta (lakes 6 and 7) (table 5.11). Like in lakes 6 and 7, surface sediments in lakes 3 and 5 contain high concentrations of CBZs and below the surface, the concentrations decrease dramatically (appendix 4.5a).
The sum of CBZs in lake 3 is particularly low ( <0.5 ng/g) below 10 g/cm2 of cumulative dry weight and heavier chlorobenzenes such as tri-, tetra-, penta- and even hexachlorobenzenes are dominant (appendix 4.5a). CBZ concentrations and homologue distributions below the surface samples in lakes 6 and 5 are different from th ose in lakes 3 and 7. CBZ concentrations are lower in lakes 3 and 7 than tho se in lakes 6 and 5 and the profile of the homologue distribution is irregular in lakes 3 and 5 while it is uniform in lakes 6 and 5 (appendix 4.5a). This difference cannot be explained at this time. It may result from problems during the analytical procedure (for instance, volatilization of dichlorobenzenes) or from natural processes in delta lakes.
In surface sediments, the sum of chlorobenzenes in levees 1, 3, 4 and 7 is higher than in lakes, but the median concentration in samples below the surface (between 6 and 8 cm) is lower in levees than in lakes (see maximal and median concentrations in table 5.11). Tetra-, penta- and hexachlorobenzenes are dominant in surface sediments. In levees 2, 5, 6 and 8, the concentrations of CBZs in surface sediments and in samples from 6 to 8 cm are low (<1.7 ng/g) (appendix 4.1).
Table 5.11: Concentrations of organochlorine pesticides (nglg). A. in lake sediments in the middle delta (lakes 6 and 7, n
=
29) and in the outer delta (lakes 3 and 5, n=
20). B. ln overbank sediments in the middle delta (levees 6 to 8, n=
6) and in the outer delta (levees 1 to 5, n=
10).:E CBZs (ng/g) :E DDT (nglg) :E HCHs (nglg) :E Cyclodiene (nglg)
A. Lakes Min Med Max Min Med Max Min Med Max Min Med Max
Outer 0.04 1.8 42 ND1 0.33 3.1 ND 0.10 0.88 ND ND 0.32
delta
Middle 0.76 2.4 50 ND 0.12 1.3 0.04 0.72 1.4 ND ND 1.8
delta B. Levees
Outer ND 0.8 166 ND ND ND ND 0.05 0.23 ND ND ND
delta
Middle ND 0.05 160 ND ND ND ND 0.05 0.14 ND ND ND
delta
1ND = not detected
Seven compounds belonging to the DDT farnily (table 2.10) were measured in lake and overbank sediments. In overbank sediments, none of these compounds was detected.
The profiles of the sum of DDT in lakes 6 and 7 are illustrated in figure 5.22. The first upper sample in lake 6 contains relatively high concentrations of DDT (1.3 ng/g) compared to concentrations below the surface (median= 0.21 ng/g). In lake 7, the median concentration is 0.08 ng/g and the sum of DDT in surface sediments is below the detection limit.
In the outer delta, the profiles of the sum of DDT in lakes 3 and 5 are sirnilar to the profiles
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determined in lake 6. They are characterized by relatively high concentrations in surface sediments (about 1 ng/g) and by a decrease of concentrations below the surface (appendix 4.5b). However, in lake 3, the sample at the bottom of the core contains 3.1 ng/g (appendix 4.5b).
In lakes 6, 3 and 5, the dominant DDT is p,p' -DDT white only o,p-DDD was detected in lake 7. O,p-DDE and p,p' -DDE were detected occasionally.