Frank Wania

Peer reviewed: tracking the distribution of persistent organic pollutants.

Control strategies for these contaminants will require a better understanding of how they move around the globe.

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.

A global distribution model for persistent organic chemicals

Abstract A nonsteady-state, multi-compartmental mass balance model of organic contaminant fate is presented in which the global environment is represented by nine sequentially arranged climatic zones. Each zone has an air, ocean water, fresh water, fresh water sediment, and two soil compartments, connected by advective and intermedia transport processes. Degradation can take place in every compartment and zone. The time, magnitude, and medium of chemical discharge is specified for each climatic zone. The seasonal variability of some key parameters such as air and ocean water temperature is taken into account. The mass balances for each of the compartments result in a system of 54 differential equations, solved numerically to yield estimates of concentrations, masses, transport fluxes, and reaction rates as a function of time. A series of illustrative calculations studies the major factors governing the global dispersion of persistent organic chemicals: (1) temperature, (2) chemical degradability, (3) environmental descriptors such as transport rates and organic carbon contents, (4) location and amount of chemical discharge and (5) physico-chemical properties. The calculations confirm that condensation at low temperatures can result in elevated contaminant concentrations in the polar regions and that chemicals show distinct global distribution patterns based on their physico-chemical properties. Location and medium of discharge into the environment and several environmental parameters strongly influence the calculated global fate of contaminants. The factors that primarily influence pollutant enrichment and behavior in polar regions should be studied more thoroughly with a view to obtaining more accurate parameter values and improved equilibrium and rate expressions.

A Comprehensive and Critical Compilation, Evaluation, and Selection of Physical–Chemical Property Data for Selected Polychlorinated Biphenyls

Accurate physical–chemical properties (aqueous solubility SW, octanol–water partition coefficient KOW, vapor pressure P, Henry’s law constant H, octanol–air partition coefficient KOA, octanol solubility SO) are of fundamental importance for modeling the transport and fate of organic pollutants in the environment. Energies of phase transfer are used to describe the temperature dependence of these properties. When trying to quantify the behavior of contaminant mixtures such as the polychlorinated biphenyls, consistent physical–chemical properties are required for each individual congener. A complete set of temperature dependent property data for sixteen polychlorinated biphenyls (PCB-3, 8, 15, 28, 29, 31, 52, 61, 101, 105, 118, 138, 153, 155, 180, 194) was derived, based on all experimentally obtained values reported for these congeners in the literature. Log mean values derived from the experimental data were adjusted to yield an internally consistent set of data for each congener. These adjusted data also...

Estimation of vapor pressures, solubilities and Henry's law constants of selected persistent organic pollutants as functions of temperature

Abstract An internally consistent set of temperature dependent physical-chemical property data was derived for 73 persistent organic pollutants, including polychlorinated biphenyls, diphenylethers, dibenzo-p-dioxins, and dibenzofurans, organochlorinated pesticides and polycyclic aromatic hydrocarbons. Measured data reported in the literature were expanded by different estimation and regression methods. Temperature dependent vapor pressures were estimated from relative gas chromatographic retention times and by a modified Watson equation. Temperature dependent water solubilities were estimated by equations based on mobile order theory modified from Ruelle and Kesselring. The temperature dependence of Henrys law constants was estimated from the temperature dependence of both the vapor pressure and water solubility. The results are presented as intercepts A and slopes B of log-linear relationships of the type log property = A - B / T. Octanol-water partition coefficients of the same compounds at 25°C were also compiled and estimated. A comparison of the estimated data with literature values showed fair agreement. The estimated property values may serve as interim input values to models of the fate of persistent organic pollutants under variable climatic conditions.

The effects of snow and ice on the environmental behaviour of hydrophobic organic chemicals

A review is presented of the roles of snow and ice as they influence the environmental fate of hydrophobic organic chemicals (HOCs). Measurements of HOC concentrations in snow are reviewed and present information on the partitioning and depositional and post-depositional behaviour of HOCs in snow is described and implications for environmental monitoring and assessment of fate are discussed. It is concluded that snow is an efficient scavenger of HOCs from the atmosphere both by adsorption of gaseous HOCs to the ice interface, and by particle scavenging. The post-depositional fate of HOCs in ageing snow packs is poorly understood. Suggested structures of quantitative models describing HOC interactions with ice and snow are presented. Key parameters in these models include the interfacial area of snow and the extent of HOC sorption to the ice surface. Recent laboratory determinations of these parameters are reviewed. Finally, research needs and gaps are identified with a view to compiling more accurate estimates of net atmospheric wet deposition of HOCs, establishing their fate in snow packs, developing reliable sampling protocols and assessing the usefulness of the glacial record as an indicator of past atmospheric compositions.

Estimating the contribution of degradation in air and deposition to the deep sea to the global loss of PCBs

A series of theoretical and semi-empirical approaches, including the zonally averaged global fate and transport model Globo-POP, were used to estimate the rate of loss of selected polychlorinated biphenyl (PCB) congeners from the global environment, and the relative contribution of atmospheric degradation and transfer to the deep sea to this loss. The atmospheric fate of PCBs is highly dependent on the OH radical concentration, temperature and the latters influence on the gas/particle partitioning equilibrium. The PCB loss was found to be very congener specific. Atmospheric reaction with the OH radical is the major loss process for the lighter and intermediate PCB congeners, whereas particle-bound transfer to the deep sea dominates the loss of the congeners with a large number of chlorine substituents. As concentrations in air and seawater decline more rapidly than in the terrestrial environment, other loss processes such as degradation in soils are gaining in relative importance with time. Estimated lifetimes of the PCBs in the global environment are on the order of decades, with significant differences between congeners. Research needs are identified that would allow improvements of these estimates.

Transport of contaminants to the Arctic: partitioning, processes and models

Abstract This paper presents a review of the processes by which organic contaminants are transported to the Arctic and become subject to deposition and absorption into terrestrial, aquatic and marine ecosystems, with subsequent transport and transformation within the abiotic and biotic media that comprise these ecosystems. The rates of these processes are controlled by the physico-chemical properties of the contaminants, notably vapor pressure, solubility in water and various partition coefficients such as those to organic carbon, lipids, plant matter and snow and ice surfaces. These properties are, in turn, profoundly affected by temperature, especially those involving air to condensed phase partitioning. Calculations of contaminant behavior in generic environments suggest that the unexpectedly high concentrations of certain organochlorine chemicals in polar regions can be explained, at least in part, by the temperature dependence of these partitioning phenomena. At low temperatures, partition coefficients from the atmosphere to condensed phases such as water, soil and biota increase significantly, thus causing higher ecosystem concentrations. Further, the rates of transport from the atmosphere may increase because of partitioning to acrosols and reduced water side mass transfer resistances. The authors propose a simple model to explain the migration and condensation of a contaminant from warm to cold regions, which we hope will represent a hypothesis that can be tested experimentally. We conclude that (1) there is a need to measure and correlate these fundamental properties and partition coefficients over the relevant range of temperatures, (2) the full implications of the effects of low temperatures on chemical fate can become clear only through construction of models describing the multimedia partitioning, transport and transformation in representative ecosystems and validating these models with experimental data, and (3) there is a need to identify the characteristics of the class of chemicals susceptible to what has been termed ‘cold condensation’ in polar regions.

Organic contaminant amplification during snowmelt.

The release of organic contaminants from melting snow poses risks to aquatic and terrestrial organisms and to humans who rely on drinking water and food production from regions that are seasonally snow-covered. Measured and model-predicted spring peak concentrations in waters receiving snowmelt motivate a thorough investigation of organic contaminant behaviour during melting. On the basis of the current understanding of snow metamorphosis, snowmelt hydrology and chemical partitioning in snow, this critical review aims to provide a qualitative picture of the processes involved in the release of organic contaminants from a melting snowpack. The elution sequence of organic substances during snowmelt is strongly dependent on their environmental partitioning properties and the physical properties of the snowpack. Water-soluble organic contaminants can be discharged in greatly elevated concentrations at an early stage of melting, while the bulk of the hydrophobic chemicals attached to particles is often released at the end of the melt period. Melting of a highly metamorphosed and deep snowpack promotes such shock load releases, whereas a shallow snow cover over a relatively warm ground experiencing irregular melting over the winter season is unlikely to generate notable peak releases of organic substances. Meltwater runoff over frozen ground directly transfers contaminant shock loads into receiving water bodies, while permeable soils buffer and dilute the contaminants. A more quantitative understanding of the behaviour of organic contaminants in varying snowmelt scenarios will depend on controlled laboratory studies combined with field investigations. Reliable numerical process descriptions will need to be developed to integrate water quality and contaminant fate models.

Empirical and modeling evidence of regional atmospheric transport of current-use pesticides.

Water samples from 30 lakes in Canada and the northeastern United States were analyzed for the occurrence of 27 current-use pesticides (CUPs). Eleven CUPs were frequently detected in lakes receiving agricultural inputs as well as in remote lakes hundreds of kilometers from known application areas. These included the triazine herbicide atrazine and its desethylated degradation product; the herbicides alachlor, metolachlor, and dacthal; the organophosphate insecticides chlorpyrifos, diazinon, and disulfoton; the organochlorine insecticides alpha-endosulfan and lindane; and the fungicides chlorothalonil and flutriafol. For six of the pesticides, empirical half-distances on the order of 560 to 1,820 km were estimated from the water-concentration gradient with latitude. For most of the pesticides, a suite of assessment models failed to predict such atmospheric long-range transport behavior, unless the effect of periods of lower hydroxyl radical concentrations and dry weather were taken into account. Observations and model results suggest that under the conditions prevailing in south-central Canada (relatively high latitude, low precipitation rates), many CUPs will be able to undergo regional-scale atmospheric transport and reach lakes outside areas of agricultural application. When assessing the potential of fairly reactive and water-soluble substances to undergo long-range transport, it is imperative to account for periods of no precipitation, to assure that degradation rate constants are correct, and to apply oxidant concentrations that are valid for the region and time period of interest.