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Don Mackay

Publications and source records attributed to Don Mackay.

14 recordsLinked to original sources
PubMed2006

Screening level risk assessment model for chemical fate and effects in the environment.

A screening level risk assessment model is developed and described to assess and prioritize chemicals by estimating environmental fate and transport, bioaccumulation, and exposure to humans and wildlife for a unit emission rate. The most sensitive risk endpoint is identified and a critical emission rate is then calculated as a result of that endpoint being reached. Finally, this estimated critical emission rate is compared with the estimated actual emission rate as a risk assessment factor. This "back-tracking" process avoids the use of highly uncertain emission rate data as model input. The application of the model is demonstrated in detail for three diverse chemicals and in less detail for a group of 70 chemicals drawn from the Canadian Domestic Substances List. The simple Level II and the more complex Level III fate calculations are used to "bin" substances into categories of similar probable risk. The essential role of the model is to synthesize information on chemical and environmental properties within a consistent mass balance framework to yield an overall estimate of screening level risk with respect to the defined endpoint. The approach may be useful to identify and prioritize those chemicals of commerce that are of greatest potential concern and require more comprehensive modeling and monitoring evaluations in actual regional environments and food webs.

Animals↗
PubMed2005

Mass balance modelling of contaminants in river basins: a flexible matrix approach.

A novel and flexible approach is described for simulating the behaviour of chemicals in river basins. A number (n) of river reaches are defined and their connectivity is described by entries in an n x n matrix. Changes in segmentation can be readily accommodated by altering the matrix entries, without the need for model revision. Two models are described. The simpler QMX-R model only considers advection and an overall loss due to the combined processes of volatilization, net transfer to sediment and degradation. The rate constant for the overall loss is derived from fugacity calculations for a single segment system. The more rigorous QMX-F model performs fugacity calculations for each segment and explicitly includes the processes of advection, evaporation, water-sediment exchange and degradation in both water and sediment. In this way chemical exposure in all compartments (including equilibrium concentrations in biota) can be estimated. Both models are designed to serve as intermediate-complexity exposure assessment tools for river basins with relatively low data requirements. By considering the spatially explicit nature of emission sources and the changes in concentration which occur with transport in the channel system, the approach offers significant advantages over simple one-segment simulations while being more readily applicable than more sophisticated, highly segmented, GIS-based models.

Environmental Monitoring↗
PubMed2005

Five-stage environmental exposure assessment strategy for mixtures: gasoline as a case study.

A five-stage strategy is suggested for conducting an exposure assessment of mixtures that may contain numerous chemical components. The stages are: (1) determination of mixture composition and variability, (2) selection of component groups within the mixture and documentation of criteria used for this selection, (3) compilation of relevant property data for each group, (4) assessment of environmental fate of each group, and (5) assessment of environmental and human exposure to each group and to the mixture as a whole. A subsequent step is the assessment of environmental and/or human risk associated with the individual and aggregate exposure to each group. The approach is illustrated by application to gasoline, which is treated as 24 component groups or hydrocarbon blocks. Focusing on stages 2-4, the illustration shows that the groups display widely different environmental fates as a result of their different physicochemical properties, degradation half-lives, and mode-of-entry into the environment. As a result, the relative proportions of groups in each environmental medium (such as air and water) differ greatly from that of the original mixture. It is thus important to treat gasoline and similar mixtures as a number of component groups instead of as a single substance. A generic procedure is suggested in which the model is run for unit emissions of each component group to air, water, and soil. These results are compiled into matrices that can then be conveniently scaled to actual emission rates without rerunning the model. Methods for determining subsequent exposure and risk are also briefly outlined.

Air↗
PubMed2004

Applications of contaminant fate and bioaccumulation models in assessing ecological risks of chemicals: a case study for gasoline hydrocarbons.

Mass balance models of chemical fate and transport can be applied in ecological risk assessments for quantitative estimation of concentrations in air, water, soil, and sediment. These concentrations can, in turn, be used to estimate organism exposures and ultimately internal tissue concentrations that can be compared to mode-of-action-based critical body residues that induce toxic effects. From this comparison, risks to the exposed organism can be evaluated. To demonstrate the use of fate models in ecological risk assessment, we combine the EQuilibrium Criterion (EQC) environmental fate model with a simple screening level biouptake model for three representative organisms: a bird, a mammal, and a fish. This effort yields estimates of internal body concentrations that can be compared with levels known to elicit toxic effects. As an illustration, we present an analysis of 24 hydrocarbon components of gasoline that differ in properties but are assumed to elicit toxicity by a common narcotic mode of action. Results demonstrate that differences in chemical properties and mode of entry into the environment lead to profound differences in the efficiency of transport from emission to target biota. We discussthe implications of these results and draw attention to the insights gained about regional fate and ecological risks associated with gasoline. This approach is suitable for assessing single chemicals or mixtures that have similar modes of action. We conclude that the model-based methodologies presented are widely applicable for screening level ecological risk assessments that support effective chemicals management.

Animals↗
PubMed2004

Regional differences in chemical fate model outcome.

The fate of anthropogenic substances in the environment is increasingly determined using multimedia mass balance models. It is, therefore, critical to fully understand how such models work and what their limitations are. The effects of uncertainty and variation in the chemical properties, discharges, and landscape parameters on model outcome have been examined by other researchers. Here, the role of landscape properties in controlling region-to-region differences in chemical fate is examined. Specifically, regions of Canada and the ChemCAN model are used to explore the region-to-region difference in fate for benzo[a]pyrene, hexachlorobenzene, tetrachloroethylene, alpha-hexachlorocyclohexane, 2,2',5,5'-tetrachlorobiphenyl (PCB 52), and atrazine emitted individually to air, water, and soil. To facilitate the same analysis in other places a description of the model and the methods for obtaining the landscape parameters used here are given. Differences in fate are the unique result of combining the input parameters of chemical properties, emission data, and landscape parameters. While region-to-region differences are small compared to the chemical-to-chemical differences that may span many orders of magnitude for physical-chemical or degradation properties, chemical fate is not the same for regions of differing landscape parameters. It is therefore concluded that the quality of results obtained from regional environmental fate models can be improved by the use of region-specific landscape parameters.

Atrazine↗
PubMed2004

Comparison of two methods for obtaining degradation half-lives.

Given the paucity of experimental degradation half-life data for most organic chemicals, there is a compelling incentive to use available estimation software when undertaking assessments of chemical persistence and mass balance modeling studies. In this study, half-life data obtained from estimation software for a set of 233 organic chemicals in air, water, soil and sediments were shown to differ significantly from half-life data listed in handbooks. It is suggested that the widely available and used estimation software, EPIWIN (Estimations Program's Interface for Windows), overestimates the reactivity of persistent organic pollutants (POPs). Reasons for this overestimation are explored. It is concluded that the maximum "default half-life values" used by the EPIWIN software are too short for estimating half-lives of highly persistent chemicals such as PCBs. There is a need for estimation software such as EPIWIN to be more thoroughly calibrated against experimental derived half-life data for a wide range of chemicals, including potential POPs, thus improving their reliability.

Chemistry Techniques, Analytical↗
PubMed2004

Modeling transport and deposition of contaminants to ecosystems of concern: a case study for the Laurentian Great Lakes.

Transfer efficiency (TE) is introduced as a model output that can be used to characterize the relative ability of chemicals to be transported in the environment and deposited to specific target ecosystems. We illustrate this concept by applying the Berkeley-Trent North American contaminant fate model (BETR North America) to identify organic chemicals with properties that result in efficient atmospheric transport and deposition to the Laurentian Great Lakes. By systematically applying the model to hypothetical organic chemicals that span a wide range of environmental partitioning properties, we identify combinations of properties that favor efficient transport and deposition to the Lakes. Five classes of chemicals are identified based on dominant transport and deposition pathways, and specific examples of chemicals in each class are identified and discussed. The role of vegetation in scavenging chemicals from the atmosphere is assessed, and found to have a negligible influence on transfer efficiency to the Great Lakes. Results indicate chemicals with octanol-water (K(ow)) and air-water (K(aw)) partition coefficients in the range of 10(5)-10(7) and 10(-4)-10(-1) combine efficient transport and deposition to the Great Lakes with potential for biaccumulation in the aquatic food web once they are deposited. A method of estimating the time scale for atmospheric transport and deposition process is suggested, and the effects of degrading reactions in the atmosphere and meteorological conditions on transport efficiency of different classes of chemicals are discussed. In total, this approach provides a method of identifying chemicals that are subject to long-range transport and deposition to specific target ecosystems as a result of their partitioning and persistence characteristics. Supported by an appropriate contaminant fate model, the approach can be applied to any target ecosystem of concern.

Air Pollutants↗
PubMed2004

Quantifying the fate of mercury in the Great Lakes Basin: toward an ecosystem approach.

We argue that the need to reduce human exposure to mercury in the Great Lakes Basin and thus reduce the risk of adverse effects can be accomplished only by reducing the quantity of all mercury species cycling in the ecosystem. It is pointed out that much can be learned from experiences with PCBs. PCB concentrations and exposures have been reduced, in part, because a clear picture has been established of relative sources and PCB' s environmental fate in the form of mass balance models, which document the "big picture" of PCB behavior at a range of scales. It is suggested that a similar strategy is needed for mercury as part of the effort to protect human health throughout the entire Great Lakes ecosystem.

Ecosystem↗
PubMed2004

Finding fugacity feasible, fruitful, and fun.

A review is presented concerning the evolution of the fugacity concept as applied to environmental science. The series of serendipitous events that ultimately resulted in publication of the paper "Finding Fugacity Feasible" in 1979 is described. The use of fugacity as a surrogate for concentration is shown to facilitate the compilation and solution of mass-balance equations. It has proved to be valuable in a number of contexts, notably the description of chemical fate in unit worlds at various levels of complexity. More complex systems can be simulated as sets of connected unit worlds. The fugacity approach enables the multimedia character of organic chemicals to be deduced, thus contributing to the evaluation of chemical properties that impact persistence and long-range transport. It has proved to be especially insightful for describing bioconcentration, bioaccumulation, and pharmacokinetic phenomena. Applications to the sensing and monitoring of chemical presence in the environment are described. Suggestions are made for subject areas in which the fugacity concept may prove to be particularly valuable in the future. Finally, the many colleagues who have contributed to the use of fugacity when quantifying chemical behavior in the environment are acknowledged.

Environmental Monitoring↗
PubMed2002

A regionally segmented national scale multimedia contaminant fate model for Canada with GIS data input and display.

Regional scale mass balance models are valuable tools for describing the fate of chemicals in areas with defined and fairly homogeneous environmental characteristics and chemical use patterns. These models often show that contaminant inflows from outside the region of interest are significant compared with local emissions. This is most likely for persistent chemicals and those that are efficiently transported in air or water. As a result regional levels of environmental contamination are controlled by external factors and meaningful evaluation requires assessment of contaminant fate in neighboring regions. A linked set of regional models thus has the potential to describe quantitatively the impact of chemical emissions over a wider geographic scale with significant spatial differences in environmental characteristics and chemical use patterns. We describe here a national scale contaiminant fate model for Canada based on the existing 24-region ChemCAN model. The ecological regions, which were previously treated individually, are linked with flows of air and water deduced from GIS analysis to provide a comprehensive description of contaminant fate over the entire country, including long-range transport between regions. The model is applied to describe the national-scale fate of three chemicals in Canada, benzene, trichloroethene, and diethylhexyl phthalate, exploiting GIS analysis for interpretation and presentation of model results. Agreement between predicted multimedia environmental concentrations and measured values is satisfactory for all three chemicals. In total this work represents an initial attempt to address the different processes of both linking a regional model and using GIS as a tool for data analysis and management.

Air Movements↗
PubMed2002

Modeling biomagnification and metabolism of contaminants in harp seals of the Barents Sea.

A simple fugacity-based bioaccumulation model is presented for harp seals (Phoca groenlandica), which feed primarily on polar cod (Boreogadus saida) and a pelagic crustacean (Themisto libellula). Using concentration data reported for 15 polychlorinated biphenyl (PCB) congeners and 27 pesticides in the food and blubber of harp seals from the Barents Sea, the model was used to determine biomagnification factors and metabolic half-lives as well as rates of contaminant uptake and loss processes in seals, including a discussion of uncertainty in biomagnification factors and half-lives. Examination of the model output shows considerable, but highly variable, biomagnification attributable to differences in metabolic rates. It is suggested that two biomagnification factors can be defined and should be used in such assessments, one based on concentration ratios and the other on fugacity ratios or lipid-normalized concentrations. A maximum biomagnification factor specific to the seal is deduced using biomagnification data for the most persistent substances, and from this, metabolic half-lives are estimated for all substances. The approach can be applied to other biomagnification situations, thus quantifying metabolic half-lives as a function of the properties of the bioaccumulating substance and ultimately its molecular structure and the physiological characteristics of the consuming animal.

Animals↗
PubMed2002

Evaluating and expressing the propagation of uncertainty in chemical fate and bioaccumulation models.

First-order analytical sensitivity and uncertainty analysis for environmental chemical fate models is described and applied to a regional contaminant fate model and a food web bioaccumulation model. By assuming linear relationships between inputs and outputs, independence, and log-normal distributions of input variables, a relationship between uncertainty in input parameters and uncertainty in output parameters can be derived, yielding results that are consistent with a Monte Carlo analysis with similar input assumptions. A graphical technique is devised for interpreting and communicating uncertainty propagation as a function of variance in input parameters and model sensitivity. The suggested approach is less calculationally intensive than Monte Carlo analysis and is appropriate for preliminary assessment of uncertainty when models are applied to generic environments or to large geographic areas or when detailed parameterization of input uncertainties is unwarranted or impossible. This approach is particularly useful as a starting point for identification of sensitive model inputs at the early stages of applying a generic contaminant fate model to a specific environmental scenario, as a tool to support refinements of the model and the uncertainty analysis for site-specific scenarios, or for examining defined end points. The analysis identifies those input parameters that contribute significantly to uncertainty in outputs, enabling attention to be focused on defining median values and more appropriate distributions to describe these variables.

Environmental Pollutants↗
PubMed2002

A dynamic mass budget for toxaphene in North America.

A continental-scale dynamic mass budget for toxaphene in North America is presented, based on available information on physicochemical properties, usage patterns, and reported environmental concentrations and using the Berkeley-Trent North American mass balance contaminant fate model (BETR North America). The model describes contaminant fate in 24 ecological regions of North America, including advective transport between regions in the atmosphere, freshwater, and near-shore coastal water. The dynamic mass budget accounts for environmental partitioning, transport, and degradation of the estimated 534 million kg of toxaphene that were used in North America as an insecticide and piscicide between 1945 and 2000. Satisfactory agreement exists between model results and current and historically reported concentrations of toxaphene in air, water, soil, and sediments throughout North America. An estimated 15 million kg of toxaphene are believed to remain in active circulation in the North American environment in the year 2000, with the majority in soils in the southern United States and Mexico, where historic usage was highest. Approximately 70% of total toxaphene deposition from the atmosphere to the Great Lakes is attributed to sources outside the Great Lakes Basin, and an estimated total of 3.9 million kg of toxaphene have been transported to this region from other parts of the continent. The toxaphene mass budget presented here is believed to be the first reported continental-scale multimedia mass budget for any contaminant.

Agriculture↗