R. G. Semkin

Contaminants in the Canadian Arctic: 5 years of progress in understanding sources, occurrence and pathways

Recent studies of contaminants under the Canadian Northern Contaminants Program (NCP) have substantially enhanced our understanding of the pathways by which contaminants enter Canadas Arctic and move through terrestrial and marine ecosystems there. Building on a previous review (Barrie et al., Arctic contaminants: sources, occurrence and pathways. Sci Total Environ 1992:1-74), we highlight new knowledge developed under the NCP on the sources, occurrence and pathways of contaminants (organochlorines, Hg, Pb and Cd, PAHs, artificial radionuclides). Starting from the global scale, we examine emission histories and sources for selected contaminants focussing especially on the organochlorines. Physical and chemical properties, transport processes in the environment (e.g. winds, currents, partitioning), and models are then used to identify, understand and illustrate the connection between the contaminant sources in industrial and agricultural regions to the south and the eventual arrival of contaminants in remote regions of the Arctic. Within the Arctic, we examine how contaminants impinge on marine and terrestrial pathways and how they are subsequently either removed to sinks or remain where they can enter the biosphere. As a way to focus this synthesis on key concerns of northern residents, a number of special topics are examined including: a mass balance for HCH and toxaphene (CHBs) in the Arctic Ocean; a comparison of PCB sources within Canadas Arctic (Dew Line Sites) with PCBs imported through long-range transport; an evaluation of concerns posed by three priority metals--Hg, Pb and Cd; an evaluation of the risks from artificial radionuclides in the ocean; a review of what is known about new-generation pesticides that are replacing the organochlorines; and a comparison of natural vs. anthropogenic sources of PAH in the Arctic. The research and syntheses provide compelling evidence for close connectivity between the global emission of contaminants from industrial and agricultural activities and the Arctic. For semi-volatile compounds that partition strongly into cold water (e.g. HCH) we have seen an inevitable loading of Arctic aquatic reservoirs. Drastic HCH emission reductions have been rapidly followed by reduced atmospheric burdens with the result that the major reservoir and transport agent has become the ocean. In the Arctic, it will take decades for the upper ocean to clear itself of HCH. For compounds that partition strongly onto particles, and for which the soil reservoir is most important (e.g. PCBs), we have seen a delay in their arrival in the Arctic and some fractionation toward more volatile compounds (e.g. lower-chlorinated PCBs). Despite banning the production of PCB in the 1970s, and despite decreases of PCBs in environmental compartments in temperate regions, the Arctic presently shows little evidence of reduced PCB loadings. We anticipate a delay in PCB reductions in the Arctic and environmental lifetimes measured in decades. Although artificial radionuclides have caused great concern due to their direct disposal on Russian Shelves, they are found to pose little threat to Canadian waters and, indeed, much of the radionuclide inventory can be explained as remnant global fallout, which was sharply curtailed in the 1960s, and waste emissions released under license by the European reprocessing plants. Although Cd poses a human dietary concern both for terrestrial and marine mammals, we find little evidence that Cd in marine systems has been impacted by human activities. There is evidence of contaminant Pb in the Arctic, but loadings appear presently to be decreasing due to source controls (e.g. removal of Pb from gasoline) in Europe and North America. Of the metals, Hg provokes the greatest concern; loadings appear to be increasing in the Arctic due to global human activities, but such loadings are not evenly distributed nor are the pathways by which they enter and move within the Arctic well understood.

Tracing the Sources of Exported Nitrate in the Turkey Lakes Watershed Using 15N/14N and 18O/16O isotopic ratios

Nitrate produced by bacterially mediated nitrification in soils is isotopically distinct from atmospheric nitrate in precipitation. 15N/14N and 18O/16O isotopic ratios of nitrate can therefore be used to distinguish between these two sources of nitrate in surface waters and groundwaters. Two forested catchments in the Turkey Lakes Watershed (TLW) near Sault Ste. Marie, Ontario, Canada were studied to determine the relative contributions of atmospheric and microbial nitrate to nitrate export. The TLW is reasonably undisturbed and receives a moderate amount of inorganic nitrogen bulk deposition (8.7 kg N · ha−1· yr−1) yet it exhibits unusually low inorganic nitrogen retention (average = 65% of deposition). The measured isotopic ratios for nitrate in precipitation ranged from +35 to +59‰ (VSMOW) for δ18O and −4 to +0.8‰ (AIR) for δ15N. Nitrate produced from nitrification at the TLW is expected to have an average isotope value of approximately −1.0‰ for δ18O and a value of about 0 to +6‰ for δ15N, thus, the isotopic separation between atmospheric and soil sources of nitrate is substantial. Nitrate produced by nitrification of ammonium appears to be the dominant source of the nitrate exported in both catchments, even during the snowmelt period. These whole catchment results are consistent with the results of small but intensive plot scale studies that have shown that the majority of the nitrate leached from these catchments is microbial in origin. The isotopic composition of stream nitrate provides information about N-cycling in the forested upland and riparian zones on a whole catchment basis.

Enantioselective Breakdown of .alpha.-Hexachlorocyclohexane in a Small Arctic Lake and its Watershed.

Water and snow samples were collected at Amituk Lake on Cornwallis Island to investigate the enantioselective degradation of α-hexachlorocyclohexane (α-HCH)in the Arctic. The two enantiomers were separated by gas chromatography on permethylated cyclodextrin capillary columns. The enantiomeric ratio (ER=(+)α-HCH)(-)α-HCH) for an α-HCH standard was 1.00±0.005, which is in excellent agreement with a theoretical ER of 1.00 for unmetabolized α-HCH. ERs of snow samples were racemic (0.9810.03). Degradation was found in Amituk Lake at 15-21 m where ERs were 0.7710.004; however, stream runoff and lake outflow ERs varied considerably during the study. ERs of the outflow traced the meltwater running over the surface of the lake, being close to streamwater values during peak runoff and returning to deep lake water values during low flow. Streamwater ERs decreased within a few weeks of snowmelt and showed a large variability (0.970.62), which may be due to the differences in temperature and amount of suspended sediments. The rapid enantioselective breakdown of a-HCH suggests that the ability of arctic microbial systems to degrade organic contaminants is greater than commonly thought

Trends in the Output of First-order Basins at Turkey Lakes Watershed, 1982–96

In 1981, we began to monitor the stream flow and chemistry of 13 first-order basins at the Turkey Lakes Watershed, with the objective of measuring the response of an undisturbed forested ecosystem to acid deposition. There was no trend in total annual precipitation received by the watershed, but the average annual water yield (percentage of annual precipitation) declined over the observation period. The proportion of runoff occurring in different seasons also changed, decreasing in the winter and increasing in the spring. In most streams the concentration of SO42− has decreased coincident with the decline in precipitation inputs. Recovery of the basins from acid deposition (as evidenced by increased pH, increased alkalinity, and decreasing base cation fluxes) has not been uniform. Basins that do not show signs of recovery are characterized by deeper flowpaths and greater potential neutralizing capacity, which minimizes the impacts of acid deposition. Basins that are dominated by shallow flowpaths and lower levels of potential neutralizing capacity are showing some signs of recovery, but their recovery is not complete and it is possible that cation depletion may prevent or retard it.

Modelling the fate of non-polar organic chemicals during the melting of an Arctic snowpack

Recent advances in the understanding of the interactions between non-polar organic chemicals and frozen water have revealed the immense importance of the ice-air interface and the possibility of quantitatively treating the adsorption process on the ice surface in terms of chemical-specific interfacial partition coefficients and the snow specific surface area. A simple fugacity-based chemical fate model is used to describe quantitatively the behaviour of selected non-polar organic contaminants during snowpack melting, namely atmosphere-snow exchange, redistribution within the snowpack and draining with meltwater. The simulations are evaluated using field data on snowpack and meltwater concentrations of these chemicals measured in a small watershed in the Canadian Arctic during the melting period of 1993. The model reproduced the observed preferential elution of hexachlorocyclohexanes in the first meltwater fractions, and the relative retention of polychlorinated biphenyls within the snowpack. The model calculations indicate that the specific surface area of the ice crystals and the air-ice and air-water partition coefficients of the chemical are key parameters controlling the extent and timing of chemical loss with the meltwater. For particle-sorptive substances, the fate of particlcs during snowmelt must also be considered. Uncertainty in describing interfacial partitioning and the behaviour of particles during melting presently limits the capabilities of models to simulate and ultimately predict meltwater concentrations of non-polar organic contaminants.

Development of Stream Water Chemistry during Spring Melt in a Northern Hardwood Forest

The role of snowmelt and subsurface hydrology in determiningthe chemistry of a small headwater stream in the TurkeyLakes Watershed (TLW) was evaluated for the spring meltperiods 1992 to 1996. Spring runoff is the dominanthydrological event at the TLW each year. Processesoccurring within the snowpack during snowmelt wereprincipally responsible for the above-ground changes inchemical fluxes relative to bulk deposition (the effect ofwinter throughfall was minimal). Large changes in chemicalfluxes occurred below ground. Organic matter decomposition,weathering, nitrification, and element cycling are some ofthe more important below-ground processes that operateduring the snow accumulation and ablation season and controlthe composition of the water ultimately appearing in thestream. Maximum stream discharge was accompanied byelevated concentrations of H+, NO3-, K+,NH4+, DOC, Al and Mn, but reduced levels ofCa2+, Mg2+, SO42- and SiO2. Theconcentration-discharge relationships were consistent withwater movement through and above the forest floor duringpeak discharge, a flowpath facilitated by rapid infiltrationof meltwater and the existence of a relatively impermeablelayer in the mineral soil creating a perched water table. Averaged over the five periods of snow accumulation andablation, it was estimated that pre-melt stream flow, andwater routed through the forest floor and through the uppermineral soil contributed 9, 28 and 63%, respectively, ofthe discharge measured at the outlet of the catchment. Theforest floor contribution would be greater at peak dischargeand at higher elevations. An end-member mixing modelestimated concentrations of SO42-, NO3-,Cl-, Ca2+, Mg2+, Na+ and Al that werecomparable to average values measured in the stream. Othervariables (NH4+, H+, K+ and DOC) wereover-estimated implying retention mechanisms operatingoutside the model assumptions.

Hydrologic Pathways during Snowmelt in First-order Stream Basins at the Turkey Lakes Watershed

The chemical composition of stream and soil water collected from two first-order stream basins in the Turkey Lakes Watershed (TLW) during the spring melt periods of 1992–1996 was examined to determine the flowpaths of snowmelt to the stream channel. Soil water was intensively sampled from within the soil organic layers as well as above (shallow soil water) and within (deep soil water) a compact basal till. Stream SiO2 concentrations of the high-elevation basin 47 were the same as the levels found in shallow soil water, and forest-floor percolate SiO2 concentrations were elevated to these levels during intense melting periods. The SiO2 concentrations from the stream and the shallow and deep soil water were similar at the low-elevation basin 31. With the exception of deep soil water, water collected from the soil and stream at basin 47 had higher H+ and Al and lower base cation concentrations than basin 31. Stream Al concentrations were significantly correlated with forest-floor percolate Al concentrations at the high-elevation basin, whereas stream Al concentrations were correlated with mineral soil water Al concentrations at the low-elevation site. There were significant positive correlations between stream and shallow soil water H+ at both basins. Shallow soil water pathways, therefore, were an important contributor to streamflow, and influenced stream chemical response during the spring snowmelt at TLW.

Contaminant fate in high arctic lakes: development and application of a mass balance model

Abstract Steady- and unsteady-state models based on the QWASI fugacity/aquivalence approach and describing chemical fate in high Arctic lakes were developed and applied to Amituk and Char Lakes on Cornwallis Island, NWT, Canada. The model considered characteristics of Arctic lakes, such as water and chemical throughflow, development and depletion of ice cover, and temperature dependence of physical-chemical properties. The model of Char Lake was parameterized and calibrated with literature data for phosphorus, and for Amituk Lake, data were obtained from the Amituk Lake project, focusing on ΣDDT. Model results indicate that Arctic lakes act as conduits, not sinks for chemicals. Most loadings are from snowmelt that enter via stream inflow and most is exported from the lake; minimal amounts of chemicals volatilize or are retained in sediments. Burial is restricted by low-suspended particle concentrations that convey chemicals to the sediment. An attendant implication of the low-suspended particle concentrations is that nearly all chemicals remain in the dissolved phase in the water column. Consequently, chemical persistence is mainly controlled by water retention time which, for these small lakes, is several years. The illustrative unsteady-state model shows seasonal effects on chemical processes such as cryoconcentration that may increase water column concentrations by up to 15% in early May.

Temporal Trends in Water Chemistry in the Turkey Lakes Watershed Ontario, Canada, 1982-1999

The Turkey Lakes Watershed (TLW) was established in 1980 as asite for study of the ecosystem effects of acidic deposition, andsince then there has been ∼40% reduction in North AmericanSO2 emissions. Monitoring records for bulk deposition,shallow and deep ground water, two headwater streams and two lakeoutflows have been tested to identify statistically significantmonotonic trends. The TLW appears to be responding to decliningacidifying emissions because the most prevalent chemical trendacross sample types/stations was decreasing SO42-. Increasing pH was detected in four of the seven data sets, butonly the H+ decrease in bulk deposition was of a magnitudeto be an important ionic compensation for the SO42-decline. There is little evidence of acidification recovery inTLW waters however. Increasing alkalinity was found only in theoutflow of the penultimate lake of the basin, and in fact, deepground water and the other lake outflow had decreasing alkalinitytrends (i.e., continuing acidification). For the surface waterstations, the greater part of the ionic compensation fordeclining SO42- was decreasing base cations, and as aresult, these waters are probably becoming more dilute with time,although only the headwater streams exhibited decliningconductivity. Five of seven data sets had increasing dissolvedorganic carbon concentrations. Increasing NO3- wasimportant in ground waters. Drought has strongly influencedtrends and delayed recovery by mobilizing S stored in catchmentwetlands and/or soils.

A premilinary assessment of the chemical and hydrological interaction of acidic snowmelt water with the terrestrial portion of a Canadian Shield Catchment

This study examines certain aspects of the hydrological and chemical interaction of springmelt runoff with the terrestrial portion of a Canadian Shield catchment. Though NO3− is the dominant anion in the snowpack for most of the winter, SO4−2 deposition increases at the end of the winter season (March) such that it is the dominant anion in snowpack meltwater runoff. Pronounced formation office lenses in the snowpack are responsible for notable movement of H+, NO3− and SO4 ions downslope prior to contact with the ground. Subsurface meltwater flow through the lower portion of the slope is much more acidic (pH 5.18 ) than that flowing within the subsurface in the upper slope ( pH 6.44 ).