Contaminant Fate Model Research Articles

Accumulation of Current-Use Pesticides in Neotropical Montane Forests

In Central America, chemical-intensive tropical agriculture takes place in close proximity to highly valued and biologically diverse ecosystems, yet the potential for atmospheric transport of pesticides from plantations to national parks and other reserves is poorly characterized. The specific meteorological conditions of mountain ranges can lead to contaminant convergence at high altitudes, raising particular concern for montane forest ecosystems downwind from pesticide use areas. Here we show, based on a wide-ranging air and soil sampling campaign across Costa Rica, that soils in some neotropical montane forests indeed display much higher concentrations of currently used pesticides than soils elsewhere in the country. Specifically, elevated concentrations of the fungicide chlorothalonil, the herbicide dacthal, and the insecticide metabolite endosulfan sulfate on volcanoes Barva and Poas, lying directly downwind of the extensive banana plantations of the Caribbean lowland, indicate the occurrence of atmospheric transport and wet deposition of pesticides at high altitudes. Calculations with a contaminant fate model, designed for mountain regions and parametrized to the Costa Rican environment, show that chemicals with a log K(AW) between -3 and -5 have a greater potential for accumulation at high altitudes. This enrichment behavior is quantified by the Mountain Contamination Potential and is sensitive to contaminant degradability. The modeling work supports the hypothesis suggested by the field results that it is enhanced precipitation scavenging at high elevations (caused by lower temperatures and governed by K(AW)) that causes pesticides to accumulate in tropical montane areas. By providing for the first time evidence of significant transfer of currently used pesticides to Central American montane cloud forests, this study highlights the need to evaluate the risk that tropical agricultural practices place on the region's ecological reserves.

The slow recovery of San Francisco Bay from the legacy of organochlorine pesticides

The use of organochlorine pesticides, including DDTs, chlordanes, and dieldrin, peaked in San Francisco Bay's watershed 30–40 years ago, yet residues of the pesticides remain high. Known as legacy pesticides for their persistence in the Bay decades after their uses ended, the compounds and their breakdown products occur at concentrations high enough to contribute to advisories against the consumption of sport fish from the Bay. Combined with other data sets, the long-term monitoring data collected by the San Francisco Estuary Regional Monitoring Program (RMP) for trace substances allow us to track recovery of the Bay from these inputs and predict its future improvement. Legacy pesticides enter the water and sediment of San Francisco Bay from a variety of sources, including runoff from California's Central Valley and local watersheds, municipal and industrial wastewater, atmospheric deposition, erosion of historically contaminated sediment deposits, and dredging and disposal of dredged material. Runoff from small-urbanized tributaries may contribute as much or more to the loads than runoff from the agricultural Central Valley, even though 90 percent of the freshwater flow comes from the Central Valley via the Sacramento and San Joaquin rivers. The fates of legacy pesticides in San Francisco Bay are controlled by their chemical properties, including their solubilities and partition coefficients. Degradation in the sediments, outflow through the Golden Gate, and volatilization—in that relative order—result in removal of pesticides from the Bay. A contaminant fate model was used to estimate recovery times of the Bay under various scenarios. For example, under a scenario in which no new legacy pesticides entered the Bay, model predictions suggested that concentrations of pesticides in the water and the active sediment layer would reach risk-reduction goals within one to three decades. Under scenarios of continued inputs to the Bay, recovery time would be considerably longer or not reached at all. Long-term tissue monitoring corroborates model predictions of slow declines in DDT and chlordane concentrations. Field-transplanted bivalve samples indicate declines since 1980, and lipid-weight concentrations of pesticides have declined in fishes, but the declines are slow. The critical management question for the Bay is whether there are feasible management actions that would decrease concentrations in sport fish significantly faster than the existing slow progress that has been observed.

Using Mathematical Models to Assess Sediment Stability

The application of mathematical models to a sediment stability study is presented, with an emphasis on using models as a component of an effort to develop, refine, and potentially validate a conceptual site model (CSM) for sediment transport at a study site. The utility of mathematical models is discussed, in which the modeling framework consists of linked hydrodynamic, sediment transport, and contaminant fate and transport models. Benefits and drawbacks of empirically based mechanistic models are presented. An approach for integrating modeling analyses into the development, refinement and validation of a CSM is provided. This approach focuses on a phased study that combines modeling and data-based analyses to test hypotheses related to the CSM and sediment stability. Uncertainty in modeling results is a primary concern in sediment stability studies, and issues related to model uncertainty are discussed. Finally, communication of modeling results to stakeholders is addressed.

Assessing the Influence of Climate Variability on Atmospheric Concentrations of Polychlorinated Biphenyls Using a Global-Scale Mass Balance Model (BETR-Global)

We introduce a new global-scale multimedia contaminant fate model (the Berkeley-Trent Global Model; BETR-Global) that integrates global climate data from the National Centers for Environmental Prediction (NCEP). BETR-Global represents the global environment as a connected set of 288 multimedia regions on a 15 degrees grid. We evaluate the model by simulating the global fate and transport of seven PCB congeners over a 70 year period and find satisfactory agreement between model output and observations of atmospheric PCB concentrations at 11 long-term monitoring stations in the Northern Hemisphere. We demonstrate the use of the model as a tool for understanding global pollutant dynamics by examining the hypothesis that variability in global-scale climate conditions, as reflected bythe North Atlantic Oscillation (NAO), influences atmospheric PCB concentrations in the Northern Hemisphere. We estimate that the maximum variability in atmospheric PCB concentrations attributable to NAO variability is approximately a factor of 2. The influence of variability in the NAO on PCB concentrations in air is most likely to be observed in the winter and spring at monitoring sites in Northern Europe and the Arctic. Analysis of long-term monitoring data from 11 sites shows some statistically significant relationships between NAO indices and atmospheric PCB concentrations during the winter and spring. Giving consideration to competing factors that influence atmospheric PCB concentrations, longer time series of monitoring data are required to fully evaluate the modeling results and to improve our understanding of the role of climate variability on the long-term fate of persistent semivolatile pollutants.

Probabilistic Modeling of One-Dimensional Water Movement and Leaching from Highway Embankments Containing Secondary Materials

Predictive methods for contaminant release from virgin and secondary road construction materials are important for evaluating potential long-term soil and groundwater contamination from highways. The objective of this research was to describe the field hydrology in a highway embankment and to investigate leaching under unsaturated conditions by use of a contaminant fate and transport model. The HYDRUS2D code was used to solve the Richards equation and the advection–dispersion equation with retardation. Water flow in a Minnesota highway embankment was successfully modeled in one dimension for several rain events after Bayesian calibration of the hydraulic parameters against water content data at a point 0.32 m from the surface of the embankment. The hypothetical leaching of Cadmium from coal fly ash was probabilistically simulated in a scenario where the top 0.50 m of the embankment was replaced by coal fly ash. Simulation results were compared to the percolation equation method where the solubility is multiplied by the liquid-to-solid ratio to estimate total release. If a low solubility value is used for Cadmium, the release estimates obtained using the percolation/equilibrium model are close to those predicted from HYDRUS2D simulations (∼10–4–10–2 mg Cd/kg ash). If high solubility is used, the percolation equation over predicts the actual release (0.1–1.0 mg Cd/kg ash). At the 90th percentile of uncertainty, the 10-year liquid-to-solid ratio for the coal fly ash embankment was 9.48 L/kg, and the fraction of precipitation that infiltrated the coal fly ash embankment was 92%. Probabilistic modeling with HYDRUS2D appears to be a promising realistic approach to predicting field hydrology and subsequent leaching in embankments.

Model for In Situ Perchloroethene Dechlorination via Membrane-Delivered Hydrogen

A one-dimensional contaminant fate and transport model was developed to simulate reductive dechlorination of perchloroethene (PCE) in an anaerobic aquifer supplied with hydrogen via a gas-permeable membrane curtain. The model predicted that providing hydrogen at transfer rates equal to the reducing-equivalent demand associated with the groundwater PCE flux would mineralize 75% of the PCE-bound chlorine to chloride and, furthermore, that 0.55 moles of chloride would be released per mole of hydrogen transferred. Supplying higher hydrogen transfer rates was predicted to result in slightly lower dechlorination efficiencies and significantly lower dechlorination yields due to greater methanogenic growth and concomitant displacement of dehalorespirers away from the hydrogen-supply membranes. The model also predicted that high hydrogen-utilizing biomass concentrations would develop near the membranes, resulting in minimal hydrogen dispersal. Model predictions were qualitatively similar to results attained in experimental soil column studies; however, incorporation of homoacetogenesis and acetate utilization by dehalorespirers, as well as hydrogen production via fermentation of biomass decay products, would have improved agreement between model simulations and experimentally observed dechlorination performance.

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.

Dependence of Intake Fraction on Release Location in a Multimedia Framework

SummaryA defining feature of industrial ecology is the design of processes to minimize any disruption of the functioning of the natural ecosystem that supports life, including human beings. The extent of human exposure to anthropogenic contaminants in the environment is a complex function of the amount of chemical emitted, its physicochemical properties and reactivity, the nature of the environment, and the characteristics of the pathways for human exposure, such as inhalation, dermal contacts, and intake of food and water. For some chemicals, the location of emissions relative to areas of high population density or intense food production may also be an important factor. We explore the relative importance of these variables using the regionally segmented Berkeleyf‐Trent (BETR) North America contaminant fate model and data for food production patterns and population density for North America. The model is applied to fourfff contaminants emitted to air: benzene, carbon tetrachloride, benzo‐a‐pyrene, and 2,3,7,8 tetrachlorodibenzo‐pff‐dioxin. The total continental intake fraction (iF), relating exposure quantity to emission quantity, is employed as a metric for assessing population exposure to these contaminants. Results show that the use of continentally averaged parameters for population density and food production provides an accurate estimate of the median of iF calculated for emissions in individual regions; however, iF can range from this median by up to 3 orders of magnitude, especially for chemicals transferred to humans through foods. The locations of population and food production relative to sources of chemicals are important variables that should be considered when assessing the possible human health impacts of chemical emissions as in life‐cycle assessment.

Modelling the atmospheric fate and seasonality of polycyclic aromatic hydrocarbons in the UK

This paper presents the results from an exercise in atmospheric contaminant fate modelling, which had three main objectives: (1) to investigate the balance between estimated national atmospheric emissions of six selected PAHs and observed ambient measurements for the UK, as a means of testing the current emission estimates; (2) to investigate the potential influence of seasonally dependent environmental fate processes on the observed seasonality of air concentrations; and (3) after undertaking the first two objectives, to make inferences about the likely magnitude of seasonal differences in sources. When addressing objective 1 with annually averaged emissions data, it appeared that the UK PAH atmospheric emissions inventory was reasonably reliable for fluorene, fluoranthene, pyrene, benzo[ a]pyrene and benzo[ ghi]perylene––but not so for phenanthrene. However, more detailed analysis of the seasonality in environmental processes which may influence ambient levels, showed that the directions and/or magnitudes of the predicted seasonality did not coincide with field observations. This indicates either that our understanding of the environmental fate and behaviour of PAHs is still limited, and/or that there are uncertainties in the emissions inventories. It is suggested that better quantification of PAH sources is needed. For 3- and 4-ringed compounds, this should focus on those sources which increase with temperature, such as volatilisation from soil, water, vegetation and urban surfaces, and possible microbially-mediated formation mechanisms. The study also suggests that the contributions of inefficient, diffusive combustion processes (e.g. domestic coal/wood burning) may be underestimated as a source of the toxicologically significant higher molecular weight species in the winter. It is concluded that many signatory countries to the UNECE POPs protocol (which requires them to reduce national PAH emissions to 1990 levels) will experience difficulties in demonstrating compliance, because source inventories for 1990 and contemporary situations are clearly subject to major uncertainties.

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.

A quantitative methodology for indexing environmental sensitivity and pollution potential.

A methodology for rating the suitability of sites for the location of industrial facilities is formulated and applied to the case of a coal-fired power plant location. The methodology comprises two major interlinked components: the environmental plant location indexing component, which involves the identification, scaling and weighting of environmental sensitivity factors; and the impact analysis component, which involves the superimposition of the pollution generation impacts of an industrial facility on spatially gridded zones of various environmental sensitivities. For each rectangular areal unit defined by a square grid, the Unit Pollution Potential Index is determined by the severity and distribution of key environmental sensitivity factors and the coverage of superimposed pollutant effects as determined by contaminant fate and transport models. For any alternative site of a planned facility, the summation of the unit indices over the area of influence of the facility provides the quantitative Pollution Index, which can be used as a basis for comparison of alternative sites for planned facilities. For this paper, this methodology is applied to the hypothetical case of the siting of a coal-fired power plant in the northeastern region of the United States, in which three alternative sites are considered. The three sites: A, B and C yielded indices of 47.83, 47.91 and 47.6, respectively, indicating that site C is the most suitable for siting the power plant.

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.

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.

Sediment and Metals Modeling in Shallow River

It is a challenge to apply coupled hydrodynamic, sediment process, and contaminant fate and transport models to the studies of surface water systems. So far, there are few published modeling studies on sediment and metal transport in rivers that simulate storm events on an hourly basis and use comprehensive data sets for model input and model calibration. The United States Environmental Protection Agency (USEPA) in 1997 emphasized the need for credible modeling tools that can be used to quantitatively evaluate the impacts of point sources, nonpoint sources, and internal transport processes in 1D/2D/3D environments. A 1D and time-dependent hydrodynamic, sediment, and toxic model, within the framework of the 3D Environmental Fluid Dynamics Code (EFDC), has been developed and applied to Blackstone River, Mass. The Blackstone River Initiative (USEPA) in 1996, a multiyear and multimillion-dollar project, provided the most comprehensive surveys on water quality, sediment, and heavy metals in the river, and served as the primary data set for this study. The model simulates three storm events successfully. The river flow rates are well calculated both in amplitude and in phase. The sediment transport and resuspension processes are depicted satisfactorily. The concentrations of sediment and five metals (cadmium, chromium, copper, nickel, and lead) during the three storm events are also simulated very well. Numerical analyses are conducted to clarify the impacts of contaminant sources and sediment resuspension processes on the river. While point sources are important to sediment contamination in the river, other sources, including nonpoint sources from watershed and bed resuspension, were found to contribute significantly to the sediment and metals in the river. Point sources alone cannot account for the total metals in the river. The model presented in this paper can be a useful tool for studying sediment and metals transport in shallow rivers and for water resource management.

A mass balance model describing multiyear fate of organochlorine compounds in a high Arctic lake.

Data collected over a 3-year study of a high arctic watershed and lake are used to understand the fate of organochlorine compounds (OCs) and form the basis of a mass balance contaminant fate model. The model uses the fugacity/aquivalence approach to describe OC dynamics between air, stream inflows and outflow, the water column, and surficial sediments. The steady-state model results indicate that stream inflows contributed from 96 to >99% of total chemical loadings, but 57-98% of total loadings were lost from the lake via the outlet, the percentage of which is controlled by the hydrologic regime of the high arctic lake. Conversely, only 0.4-3.4% of loadings were retained within the sediments due to the high export rate, minimal scavenging from the water column and low organic carbon fraction of the sediments. Using the unsteady-state model, which includes year-round processes, degradation was estimated to account for losses of 7-32% for the more persistent OCs and 42-50% for the less persistent OCs (alpha-HCH, gamma-HCH, and endosulfan I). If loadings were eliminated, water column concentrations would decline with half-lives <1 year for less persistent OCs and 1-2 years for the more persistent OCs, whereas the half-lives for OCs in sediment are 8-25 years.

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 contaminant 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.

Identification of Contaminant Source Location and Release History in Aquifers

In this study, we formulate a contaminant source characterization problem as a nonlinear optimization model, in which contaminant source locations and release histories are defined as explicit unknown variables. The optimization model selected is the standard model, in which the residuals between the simulated and measured contaminant concentrations at observation sites are minimized. In the proposed formulation, simulated concentrations at the observation locations are implicitly embedded into the optimization model through the solution of ground-water flow and contaminant fate and transport simulation models. It is well known that repeated solutions of these models, which is a necessary component of the optimization process, dominate the computational cost and adversely affect the efficiency of this approach. To simplify this computationally intensive process, a new combinatorial approach, identified as the progressive genetic algorithm, is proposed for the solution of the nonlinear optimization model. ...

BETR North America: a regionally segmented multimedia contaminant fate model for North America.

We present the Berkeley-Trent North American contaminant fate model (BETR North America), a regionally segmented multimedia contaminant fate model based on the fugacity concept. The model is built on a framework that links contaminant fate models of individual regions, and is generally applicable to large, spatially heterogeneous areas. The North American environment is modeled as 24 ecological regions, within each region contaminant fate is described using a 7 compartment multimedia fugacity model including a vertically segmented atmosphere, freshwater, freshwater sediment, soil, coastal water and vegetation compartments. Inter-regional transport of contaminants in the atmosphere, freshwater and coastal water is described using a database of hydrological and meteorological data compiled with Geographical Information Systems (GIS) techniques. Steady-state and dynamic solutions to the 168 mass balance equations that make up the linked model for North America are discussed, and an illustrative case study of toxaphene transport from the southern United States to the Great Lakes Basin is presented. Regionally segmented models such as BETR North America can provide a critical link between evaluative models of long-range transport potential and contaminant concentrations observed in remote regions. The continent-scale mass balance calculated by the model provides a sound basis for evaluating long-range transport potential of organic pollutants, and formulation of continent-scale management and regulatory strategies for chemicals.

Risk assessment for environmental contamination: an overview of the fundamentals and application of risk assessment at contaminated sites

Many jurisdictions throughout Canada and the United States allow the decision to proceed with an active site cleanup, an evaluation or monitoring program, or site closure to be determined from the results of a risk assessment. Risk assessment can protect both human health and the environment while providing significant savings to industry and government by reducing unnecessary environmental expenditures and allowing for the more efficient allocation of resources. Risk assessment is a dynamic process which is evolving as new methods for contaminant fate and transport modelling, more complete toxicological data, and more standardized statistical methods become available. Principal methodologies for conducting risk assessments are provided by the United States Environmental Protection Agency (USEPA), the American Society for Testing and Materials (ASTM), and the Canadian Council of Ministers of the Environment (CCME). Risk assessment methodologies include both "forward" risk calculations and "reverse" risk calculations. Uncertainties inherent in risk assessment methodologies can be difficult to quantify and are dealt with by incorporating conservative assumptions throughout the risk assessment process. Institutional controls may be required to ensure the continued validity of assumptions. There is a trend toward the increased application and acceptance of risk assessment, although Canada remains well behind the United States. To ensure the expanded use of risk assessment in Canada, more training for both regulators and environmental professionals, increased public awareness, and the adaptation of the risk assessment methodologies being applied in the United States to reflect the environmental policies and technical concerns of interest across Canada will be required.Key words: risk assessment, petroleum, risk-based corrective action, site remediation, remediation objectives, Canada, institutional controls, ethics.

Comparison of injection well patterns for remediating benzene‐contaminated groundwater

Ten alternative injection well patterns and a no‐action plan were evaluated for bioremediating benzene‐contaminated groundwater. Evaluations were made for a silty‐sand aquifer using a contaminant fate and transport model. Wells injected water containing 8 mg/L dissolved oxygen. The study's objective was to rank the alternative schemes based upon mass of contaminant removed and area of residual groundwater contamination. Interior configurations with injection wells at areas of high benzene concentration outperformed the other remediation schemes. Interior wells supply oxygen quickly to areas of a plume where it is most needed. Alternative configurations rely upon groundwater to transport the oxygen, an inefficient process. No action was the least effective strategy.