This report presents the results of August 1993 and March 1994 sediment studies in the West Basin of Great Slave Lake. In August 1993, a series of 10 surficial sediment samples were collected in the vicinity of the Slave River mouth. PCB was the predominant organochlorine (OC) compound detected followed by chlorobenzene, total DDT, HCH, and dieldrin. Concentrations were low and comparable to values reported for other subarctic and arctic lakes. There was no apparent pattern in the distribution of these compounds relative to the Slave River outflow. PAHs were very abundant and were dominated by benzo(g,h,i)perylene, benzo(e)pyrene, and phenathrene: concentrations were slightly higher offshore the river mouth than elsewhere. PCDD and PCDF concentrations were exceedingly low. PCDDs were dominated by DiCDD and OCDD while PCDFs were dominated by DiCDF and TriCDF. The presence of the lower chlorinated forms may be suggestive of a pulp and paper mill influence. Similarly the presence of pentachloroanisole, trichloroveratrole, and tetrachloroveratrole may be suggestive of a pulp and paper mill influence.
Two cores were collected in August 1993 on the shelf region immediately west of the Slave River. Station depths were less than 30 m. Both cores were in areas of high erosion and could not be assigned meaningful dates.
In March 1994, a series of sediment cores was collected at five sites (Sites 12, 13, 16, 19, and 23) in the West Basin, to the west of the August 1993 surficial sediment and coring studies. A single core from each site was dated at the Freshwater Institute and two additional cores (one each from Sites 13 and 19) were dated at the National Water Research Institute. Cores from four of the sites were in depositional areas while the core collected offshore of the Slave River mouth (Site 23) was in an erosional area. However, sufficient sediment deposition had occurred at this site for the core to be dated. The two cores examined from Site 13, in the central region of the West Basin, gave similar dating estimates. However, the two cores examined from Site 19, further to the east, gave somewhat different estimates from each other. These two cores were collected in a less physically-stable region of the lake with some evidence of postdepositional erosion of older material on top of newer
sediments. The core from Site 23 was in the least stable region o f the five sites examined.
Sedimentation rate estimates were similar to those for Lake Athabasca and Lakes Ontario and Erie. They were higher than estimates for subarctic and arctic lakes and for Lake Superior. Based on estimates of the suspended sediment loading to Great Slave Lake, we conclude that our cores were not collected in the high-sedimentation regions of Great Slave Lake. The greatest sedimentation may occur offshore of the Slave River mouth. Analysis of cores collected in March 1995 should allow us to confirm this hypothesis.
Two cores (Cores 12B and 19B) were analyzed for organochlorine compounds. Concentrations of OCs in Core 12B, collected offshore of Hay River, were relatively high and require verification. Thus, these data are not presented in this report. For Core 19B, there was some evidence of increasing PCB, chlorobenzene, and HCH concentrations over the 1949 - 1994 period investigated. Dieldrin showed a weaker time trend. Based on the analysis of sediment trap material collected in
August 1994, we conclude that the Slave River is a significant source of organochlorine compounds to Great Slave Lake.
Cores from Sites 12 and 19 were analyzed for PAHs. Although the same number of slices were examined for both cores, slices from Core 12B did not extend as far back in time: thus, the PAH record has not been determined for Core 12B prior to the mid 1960s. Both cores were dominated by naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene suggesting a petrogenic source. Concentrations were higher at Site 19, closer to the Slave River, than Site 12. There was strong evidence that concentrations of these compounds increased since the 1960s suggesting an additional anthropogenic source of these PAHs: temporal patterns of increase differed for Core 12B and Core 19B. Fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo(a)anthracene, and chrysene all occurred in higher concentrations in Core 12B than Core 19B. Concentrations varied little over time for Core 19B but showed some evidence of higher concentrations in Core 12B for two periods - the late 1970s and the late 1980s. This is suggestive of a localized input, possibly from Hay River. Higher molecular weight PAHs occurred in similar concentrations in Cores 12B and 19B. There was a suggestion of slightly higher concentrations of these compounds in Core 12B during the late 1970s than earlier and later times.
PCCDs and PCDFs concentrations were determined in Cores 19D and 23A. Concentrations of PCDDs were substantially higher during the 1950s through the 1970s than in more recent times. Temporal patterns of increase differed for Core 19D and Core 23A. PCDDs were dominated by HpCDDs and OCDDs with only low concentrations of the lower chlorinated forms being detected. Total PCDFs were less abundant than PCDDs: this is in notable contrast to the surficial samples where PCDDs and PCDFs occurred in similar concentrations to one another. PCDFs (primarily TriCDF and TCDF) showed some evidence of increasing concentrations since the 1950s for Core 23A while this trend was less apparent for Core 19D. These increases in PCDD and PCDF concentrations may be related to increased atmospheric sources and/or paper and pulp mill activities. There was some evidence of a pulp and paper mill signature in Core 19B with pentachloroanisole increasing in concentration from 1949 to the early 1980s and then declining somewhat thereafter: trichloroveratrole and tetrachloroveratrole occurred in low concentrations in the 1950s and in increasing concentrations thereafter.
Total organic carbon (TOC) and total organic nitrogen (TON) concentrations were determined in Cores 13C and 19D. Concentrations of both compounds have increased since the early 1900s with the greatest increase occurring since the 1950s. Moreover, the increase was more pronounced in Core 13C than Core 19D. This suggests that the West Basin of Great Slave Lake has undergone a slight increase in productivity, possibly due to land clearing and increased anthropogenic development in the Peace and Athabasca River watersheds. Localized activities, occurring at the towns of Hay River and Yellowknife, may also have been important.
While Great Slave Lake is essentially a pristine system, it does show signs of recent anthropogenic contamination. A significant fraction of OCs, PAHs, PCCDs, and PCDFs probably entered the West Basin of Great Slave Lake with Slave River inflow. However, the primaiy source of these compounds is less certain, e.g., localized inputs from industries along the Peace and Athabasca Rivers and/or atmospheric deposition (wet and dry) over the broader watershed with the eventual transport of these compounds into the Peace, Athabasca, and Slave rivers and then into Great Slave Lake.