Anthropogenic organic compounds

Long-range atmospheric transport of PCBs and organochlorine pesticides from industrialized areas is probably the main process which explains the presence of serni-volatile compounds in lake sediments in the Mackenzie Delta. In winter, air flows mainly from the Eurasian continent into the Arctic and then out over North America (BARRIE et al., 1992), facilitating direct transfer to the Arctic of contarninants from industrialized areas of eastern and northern Europe and Asia (BARRIE, 1986; PACYNA, 1991; PATION et al., 1991; WELCH et al., 1991). However, it should be noted that the contaminant input from the arctic haze during late winter is probably smaller in the Mackenzie Delta than in the High Arctic (for instance, Ellesmere Island and Baffin Island), as the delta area and the Canadian Beaufort Sea tend to be outside of the main zone of airflow (fig. 5.28) (RAATZ. 1991, YUNKER et al., 1996). In summer, the Eurasian flow reverses and airflow into the Arctic is frequently from the North Pacifie and North Atlantic (BARRIE et al., 1992). Episodic transport of contarninants may occur from North America, southern Europe, northern Africa and Asia (PACYNA, 1991; RAATZ, 1991, WELCH et al., 1991).

The presence of organochlorine compounds in sediments in the Mackenzie Delta is probably explained by the following processes. During the winter, atmospheric organochlorines scavenged by snow are deposited in the Mackenzie River watershed. While the most volatile compounds are volatilized during the snowmelt, the compounds with lower vapor pressures are adsorbed to organic matter and clay particles. Because of their relatively low vapor pressures and low aqueous solubilities, organochlorines are preferentially adsorbed to particles and removed from the water column by sedimentation (EISENREICH et al., 1989). During the spring break-up, organochlorines adsorbed to particles eroded from soils and from the tributary banks of the Mackenzie River are transported in the river system and then deposited in the subaerial delta or in the Beaufort Sea. Organochlorines deposited in the watershed during the summer are directly scavenged by particles.

The contribution of local sources of PCBs and other organochlorine compounds is probably rninor, as lake sediments contain very low concentrations of high chlorinated PCBs and organochlorine pesticides.

The historical deposition of anthropogenic organic compounds and their potential sources are discussed in the following sections.

~ primary sources of pollution

~ major rivers flowing into Arctic Ocean . - / surface ocean currents

Winter air circulation

@

(b)

" " Prevailing winds

Figure 5.28: Transport of contaminants into the Arctic during late winter (after RAATZ et al., 1991;

McLAUGHLIN et al., 1995; RUDELS et al., 1995; MACDONALD and BEWERS, 1996).

Polychlorobiphenyl (PCB) sources

The accumulation of PCBs in lake sediments in the Mackenzie Delta may be explained by three different processes:

• direct deposition of PCBs coming from industrialized areas by long-range atmospheric transport,

• transport in the river system of PCBs adsorbed to material eroded from the tributary banks of the Mackenzie River (indirect deposition),

• releases of PCBs from local sources such as closed military radar sites belonging to the Distant Early Waming Line.

The relative contribution of PCBs coming from direct and indirect atmospheric deposition and from local sources are discussed in the following sections.

Atmospheric input of PCBs

PCBs have been manufactured and used extensively since 1929 and production in U.S peaked between 1968 and 1970. Manufacturing stopped in 1977 and the importation of PCBs in Canada was prohibited in 1980 (CCREM, 1987). In mid-latitudes lakes, such as in Lake Ontario, the historical record of PCBs in sediments shows the decrease of PCB input after 1970 (EISENREICH et al., 1989). The profile of PCB

-

151-Chapter 5: Recent Historical Deposition of Trace Organic Compounds

fluxes over time in lake sediments reflects the production/sales curve which accurately describes the PCB atmospheric input function with the exception that recent fluxes, although reduced in quantity, continue

(RAPAPORT and EISENREICH, 1988; EISENREICH et al., 1989). ln the Mackenzie Delta, profiles of PCBs are different from those observed in mid-latitude lakes. PCB fluxes in lake 7 increased slightly from 1962 to 1970, decreased after 1970 and increased again until 1993 (fig. 5.24). Since 1985, the PCB flux increased more quickly and the mean fluxes between 1985 and 1993 (8.3 ng/cm²*y) is higher than the mean flux between 1968 and 1970 (5.2 ng/cm²*y). The comparison between the PCB fluxes over time in lakes 6 and 7 and the U.S. production and sales curve of PCBs (fig. 5.24) indicates that sediments collected by the Mackenzie River and deposited in the Mackenzie Delta have not been affected significantly by the high input of PCBs observed in industrialized regions between 1950 and 1970. The PCB signature in sediments deposited in the southern part of the Mackenzie River watershed appears to be sirnilar to the one in the northern part, as the distribution and concentrations of PCBs in Great Slave Lake (MUDROCH et al., 1992) are similar to those determined in lake sediments in the Mackenzie Delta.

The predominance of lower chlorinated PCBs (91 ± 7% of PCBs are mono- through penta-CBs, n

=

51) over higher chlorinated PCBs (hexa- to deca-CBs) in lake sediments (fig. 5.20) suggests that long-range atmospheric transport of PCBs from industrialized areas is the main process explaining the presence of PCBs in sediments. The comparison of PCB distribution between lake sediments and snowpack collected from lake ice surfaces in the Canadian Arctic (GREGOR, 1995) indicates that lower chlorinated PCBs predominate in snow and lake samples and that their homologue patterns are sirnilar.

Lake sediments contain higher proportions of tetrachlorobiphenyls, while snow samples contain higher proportions of mono- and dichlorobiphenyls (GREGOR, 1995). Both contain a very law portion of higher chlorinated PCBs (from hexa- to octachlorobiphenyls). These observations strongly suggest that PCBs scavenged by snow from the atmosphere are deposited in the Canadian Arctic, including the Mackenzie River watershed. While mono- and dichlorobiphenyls are evaporated during snowmelt because of their higher vapor pressure (increasing with temperature), higher chlorinated PCBs are adsorbed to sail or clay particles. During the spring break-up, PCBs adsorbed to particles eroded from soils and from the tributary banks of the Mackenzie River are transported in the river system and then deposited in the subaerial delta or in the Beaufort Sea. The preferential adsorption of penta- and tetra-chlorobiphenyls to clay particles is illustrated by the positive correlation between the concentrations and the fraction of particles smaller than 4 J.Lm contained in lake sediments (table 5.14). The correlation coefficient between particle size and tri-chlorobiphenyls is lower (table 5.13), but still suggests a preferential accumulation in clay particles.

Table 5.14: Correlation coefficients between PCB homologues 2 through 7 with grain size distribution in lake sediments (n = 47). Coefficients with *are significant at p<O.OSOO.

Di-CBs Tri-CBs Tetra-CBs Penta-CBs Hexa-CBs Hepta-CBs :EPCBs

<4Jlm 0.21 0.47* 0.53* 0.59* 0.40* 0.19 0.53*

4-16 J.Lm 0.17 0.38* 0.48* 0.53* 0.28 0.07 0.48*

16-32 Jlm -0.27 -0.52* -0.57* -0.65* -0.41 * -0.19 -0.57*

32-63 Jlm -0.12 -0.33 -0.44* -0.48* -0.26 -0.07 -0.46*

The relative contribution of riverine PCBs (indirect deposition) and of direct deposition of atmospheric PCBs in the delta cannot be estimated as the atmospheric flux of PCBs in the Mackenzie Delta is unknown. However, the calculated fluxes of PCBs are much higher (between 0.94 and 13

ng/cm²*y) in connected lakes than in arctic lakes receiving PCBs only via the atmosphere (between 0.01 and 0.15 ng/cm²*y, MUIR et al., 1996), which indicates that the fraction of riverine PCBs is much more important than the direct deposition of PCBs.

The PCB pattern in lake sediments has not changed significantly since 1955 (fig. 5.20 combined with 5.24) and has been similar to the pattern of one marketed mixed product, Aroclor 1248 (fig. 5.29). The persistence of the PCB signature over time probably indicates that the indirect deposition of PCBs (long-range atmospheric transport+ riverine transport) has been the main source of PCBs during the last forty years and that PCB degradation has not been effective. However, the increase of PCB concentrations since 1985 observed in lake 7 and in (sub)surface sediments in lakes 3, 5 and 6 does not follow the general trend of the PCB atmospheric input which has decreased sin ce 1971 (ADDISON et al., 1986; MUIR et al., 1988; WANIA and MACKAY, 1993; GREGOR, 1995). In contrast, it appears that the indirect atmospheric input to the Mackenzie River basin has increased since 1985. The continuous volatilization of law chlorinated PCBs from contaminated soils in warm and temperate regions (JONES et al., 1992;

ALCOK et al., 1993) and the atmospheric transport of these PCBs to cold regions could explain the regular input of PCBs in the arctic environment. Moreover, the use of PCBs in many Eurasian countries after 1970 (BARRIE et al., 1992) probably maintain a continuous PCB input to the arctic. A recent increase of higher chlorinated PCBs coming from local sources such as closed oil exploration sites or military radar sites could explain the larger fraction of these compounds in (sub)surface sediments. Even if sample sites are located far from these potential sources, contaminated partic~es may have been transported by wind. However, no data on potential PCB contamination about closed oil exploration sites in the delta or about the radar site in the Tuktoyaktuk Peninsula can support this assumption.

60

-lakes

50 IZZJ Aroclor 1242

cf?-en 40

en

c=J Aroclor 1248

~ Aroclor 1254

c::::J Aroclor 1260 0 0...

~ 30

0 c:

.Q ü 20

.... C1l lL.

10 '

0 n 1 h.

~ L ^.J

2 3 4 5 6 7 8 9 10

PCB homologues

Figure 5.29: PCB homologue distribution in lake sediments (average) and the main Aroclors 1242, 1248, 1254 and 1260 (analysis of Aroclors given in BACKUS, 1996).

The higher concentrations of PCBs in surface sediments could be explained by the deposition of very fine particles with high concentrations of PCBs during the winter. MUIR et al. (1996) also observed higher concentrations in surface slices of arctic lakes and they suggested that higher concentrations "may be due to particulates deposited during the long ice covered season, which would be resuspended after ice melt and not permanently buried". This assumption could be true for lakes collected in the Mackenzie Delta as they had been ice covered for four or five months prior to sampling. High chlorinated PCBs

-153-Chapter 5: Recent Historical Deposition of Trace Organic Compounds

(hepta- and octa-chlorobiphenyls) would be adsorbed to the low-density material deposited during the winter period as they constitute a large fraction (about 10%) in surface samples relative to deeper samples.

Local sources of PCBs

The Iow proportion of higher homologues in lake sediments indicates that the Mackenzie Delta is far from local sources of PCBs, as higher chlorinated PCBs are usually confined to the area surrounding contaminated sites because of their very low vapor pressure and their strong affinity for soi! particles (SA WHNEY, 1986; ERICKSON, 1997). This observation suggests also that releases of PCBs from disposai about closed military radar sites (Distant Early Waming Line) do not contribute significantly to the PCB input to Iakes. The PCB mixture observed in two contaminated DEW Line sites, Cambridge Bay and Iqaluit, contains 86 and 98% in homologues 5 through 8 (GREGOR, 1995) while in lake sediments, homologues 5 through 8 represent 31% on average (n =51). Moreover, the soil surface contaminated by PCBs about DEW Line sites is usually 300 km² (GREGOR, 1995), which implies that collected sites in the Mackenzie Delta are probably not affected by local contamination, as the closest DEW Line site, located in the Tuktoyaktuk Peninsula, is distant from 70 km.

Closed oil exploration stations in the Mackenzie Delta could be a potential source of PCBs. However, no spills of PCB fluids or disposai of material containing PCBs have been mentioned in the literature.

Organochlorine pesticide sources

The low concentrations of semi-volatile organochlorine compounds (OCs) measured in lake sediments indicate that the Mackenzie Delta is far from industrial and agricultural sources of OCs. For the purpose of illustration, concentrations of the DDT family (DDT +DDE+DDD) in lake sediments deposited in the early 1980s is about 500 times lower than in Lake Ontario surface sediments dated to the early 1980s (EISENREICH et al., 1989). The accumulation of OCs in lake sediments is probably explained by the direct and indirect (riverine inputs) deposition from the atmosphere of semi-volatile compounds coming from industrialized areas via long-range atmospheric transport. The interpretation of isomer distribution for each family of compounds in terms of potential sources is tricky, as uncertainties attached to low concentrations, close to the detection limit, are quite important.

The relative contribution of riverine organochlorine pesticides (indirect deposition) and of direct deposition from the atmosphere in the delta cannot be estimated as the atmospheric flux of these compounds has not been determined in the area.

Atmospheric deposition of chlorobenzenes (CBZs)

The distribution of chlorobenzenes in lake sediments (appendix 4.5a) indicates that the relative proportions of di- through hexachlorobenzenes are variable with depth in two lake cores in lakes 7 and 3 and constant in lakes 5 and 6. As described in chapter 5.3.2, the Jack of dichlorobenzenes in samples containing very low concentrations of total CBZs ( <3 nglg) suggests that light chlorobenzenes have been volatilized because of their relatively high vapor pressure. The predominance of dichlorobenzenes in surface sediments suggests that they have been deposited on the lake bottom during the winter season preceding the sampling. As di-, tri and tetrachlorobenzenes are characterized by a relatively high vapor pressure, it was chosen to focus on Jess volatile compounds such as penta- (PECB) and hexachlorobenzenes (HCB). The proportion of HCBs represents on average 63% of the sum of PECB and HCB. The input of the sum of PECB and HCB in lakes 6 and 7 has regularly increased since 1955 until the sampling date (fig. 5.30). Two periods of higher fluxes can be observed, the first one between 1972 and 1976 in lake 7 (1966-1971 in lake 6) and the second one between 1985 and 1990 ( 1979-1985 in

lake 6). The lag of 5 years between peaks in lakes 6 and 7 is probably explained by sediment mixing processes occurring in lake 6 (see chapter 3.3.4). Individual concentrations of PECB and HCB have also increased since 1955. profiles of di- though hexachlorobenzenes in Lake Ontario (DURHAM and OLIVER, 1983) should provide the general hape of the sum of PECB and HCB input function in the U.S. (EISENREICH et al., 1989). The highest input of CBZs in the U.S. environment occurred between 1960 and 1970 (fig. 5.30) and has been recorded probably 10 years later in the Mackenzie Delta (peak between 1971 and 1976 in lake 7).

-155-Chapter 5: Recent Historical Deposition of Trace Organic Compounds

Concentrations of HCB decreased from 1970 to 1980 in Lake Ontario sediments (EISENREICH et al., 1989) while they increased in the Mackenzie Delta. However, the level of contamination is much less important in the Mackenzie Delta, as concentrations of HCB in lake sediments deposited in the early I980s is about 300 times lower than in Lake Ontario surface sediments dated to the early l980s

(EISENREICH et al., 1989). The regular increase of PECB and HCB input observed in the Mackenzie Delta lakes can be explained by the regular usage of chlorobenzenes in sorne countries (BARRIE et al., 1992). Moreover, the analysis of semi-volatile OCs in lake sediments along a North American mid-continental transect from 49° N to 82°N has suggested that more volatile OCs are preferentially accumulated in polar regions and that temporal trends in deposition of these contaminants is delayed and prolonged relative to temperate regions (MUIR et al., 1996).

Atmospheric deposition of hexachlorocyclohexanes ( HCHs)

The historical deposition of HCHs ( a.-HCH + ~-HCH) in lake sediments is similar to the deposition of CBZs (PECB + HCB) described previously (fig. 5.30). The input of HCHs has increased since 1955 and two periods of higher flux are observed in lake 7. The first one between 1973 and 1976 and the second one between 1985 and 1990. The flux profile in lake 6 is similar, but a lag of 5 years with the flux profile in lake 7 can be observed. The lag is probably explained by sediment mixing processes which have affected surface sediments in lake 6 (see chapter 3.3.4). In lake 6, only a.-HCH has been detected, while in lake 7 a.-HCH and ~-HCH have been detected. The proportion of a.-HCHs decreases from the surface to the bottom of the core while the ~-HCH proportion increases from the surface to the bottom. The most pesticidally active isomer of HCHs, lindane, has not been detected in lake sediments, except in two samples in lake 6. The dominance of a.-HCH over ~-HCH and y-HCH (lindane) in lake sediments reflects the composition of the technical product which usually contains 55-70% a.-HCH, 5-14% ~-HCH and

10-18% y-HCH (BARRIE et al., 1992).

The production of HCH began in 1945 and lindane has been used as insecticide over the last 50 years.

While the mixed isomer was banned in Canada in 1971 and in 1978 in U.S., the importation of lindane is still allowed under restrictions (CCREM, 1987). Technical HCH is still widely used in China and India where 20,000 tons was produced in 1991 (BARRIE et al., 1992; ALLOWA Y and A YRES, 1997). The regular increase of HCH input in lake sediments in the Mackenzie Delta reflects probably the continuous worldwide input of HCH in the atmosphere. The very small proportions of lindane (below detection li mit in most samples) in lake sediments suggests that the use of this insecticide in Canada has not affected the sediment composition in the Mackenzie Delta. The analysis of HCH in Great Slave Lake sediments also indicated that a.-HCH is predominant over lindane (MUDROCH et al, 1992).

Atmospheric deposition of DDT

Concentrations of the DDT family (DDT +DDE+DDD) in subsurface sediments collected in lakes 6 and 7 are very low, between non-detected and 0.52 ng/g. Except for the higher deposition of DDT (1.3 ng/g) in surface sediments in lake 6, the historical profile of DDT flux does not indicate any significant trend over time (fig. 5.30). Considering only subsurface samples, DDT concentrations tend to decrease from the bottom of the core to the top in lakes 6, 3 and 5 but samples with high concentrations altemate with non-detected levels (appendix 4.5b). P,p' -DDT is dominant in lake 6 while o,p-DDD is the main compound in lake 7. The metabolites o,p-DDE and p,p'-DDE have been detected only occasionally. The predominance of p,p' -DDT over metabolites in lakes 3, 5 and 6 suggests that DDT has not been weathered and that the source of this DDT is probably fresh (RAPAPORT et al., 1985; GREGOR, 1989).

Methoxychlor was not detected in any lake sample.

The comparison between the DDT production curve and the DDT flux profile in lake sediments (fig.

5.30) indicates that the deposition of DDT in the Mackenzie Delta lakes is different from the production curve which reflects the atmospheric input fonction in the U.S (RAPAPORT and EISENREICH, 1988;

EISENREICH et al., 1989). DDT was first produced in the early 1940s, reaching peak production and use in North America in 1958-1960 (fig. 5.30). Use of DDT was banned in 1972 in U.S. and in Canada.

Although DDT was banned in North America and in Europe, it continues to be manufactured and used in southem Asia, Africa, Central America and Southem America (VOLDNER and ELLENTON, 1987). The flux profile in lake 6 before 1992 reflects the decrease of DDT input since the 1970s (fig. 5.30). The sudden increase of DDT in surface samples could reflect a recent increase of atmospheric deposition of DDT in the Mackenzie Delta. Higher concentrations of DDT in surface sediments may also be explained by the deposition of low density material ( organic matter) during the win ter season which would be resuspended after ice melt and not permanently buried.

Atmospheric deposition of other organochlorine compounds.

The atmospheric deposition of cyclodienes in lake sediments in the Mackenzie Delta is very weak as most of these compounds are only occasionally detected. Mirex, heptachor and heptachlor-epoxyde, ~­

endosulfan and y-chlordane were below detection limit in ali samples (appendix 4.1).

Endrin and dieldrin were occasionally detected in lakes 7, 3 and 5. Dieldrin was more frequently detected than endrin. a-chlordane was detected in sorne samples in lakes 3 and 5 and a-endosulfan in lake 7 in the two upper slices (appendix 4.1). As concentrations of these compounds are very low ( < 0.37 ng/g and < 1.8 ng/g for a-endosulfan), it is not possible to observe a significant trend of concentrations over time.

Cyclodiene concentrations in lake sediments are similar to the concentrations measured in the Great Slave Lake (MUDROCH et al., 1992), which indicates that deposition of cyclodienes in the Mackenzie River watershed is quite homogeneous.

5.5 Conclusions

Hydrocarbons, pesticides and organochlorine pesticides have been analyzed in four lake cores and in

Hydrocarbons, pesticides and organochlorine pesticides have been analyzed in four lake cores and in