Environmental Forensic Investigation of a Catastrophic Hazardous Materials Transportation Incident: Integrating Air Dispersion Modeling, Combustion Chemistry, Environmental Sampling, and Exposure Assessment of Dioxin and PAHs
Project Overview
Major transportation accidents involving hazardous materials represent some of the most technically challenging environmental forensic investigations encountered by environmental consultants, regulators, and expert witnesses. Unlike conventional contaminated-site investigations, these incidents involve rapidly evolving release conditions, changing combustion processes, emergency response activities, and complex atmospheric transport mechanisms that influence contaminant generation, environmental fate and transport, deposition, and potential human exposure. As a result, comprehensive environmental forensic investigations require integration of environmental chemistry, contaminant fate and transport, air quality modeling, engineering analysis, toxicology, exposure assessment, and human health risk assessment to accurately reconstruct historical events and evaluate environmental consequences.
This case study describes a comprehensive environmental forensic investigation performed following a catastrophic hazardous materials transportation accident involving multiple railcars carrying industrial chemicals. The incident resulted in derailment, uncontrolled fire, prolonged thermal exposure of damaged tank cars, pressure relief device venting, and a subsequent controlled vent-and-burn operation implemented to reduce the potential for catastrophic vessel failure. These rapidly changing conditions created a scientifically complex environmental setting in which investigators were required to evaluate not only the chemicals originally transported but also numerous secondary combustion products generated during the fire.
Rather than simply determining whether hazardous chemicals had been released, the investigation sought to reconstruct the complete environmental history of the event. Primary technical questions included identifying contaminants generated during both the derailment and subsequent combustion, evaluating atmospheric transport and deposition, interpreting environmental sampling data, assessing regional background contamination, reconstructing historical exposure conditions, and determining whether multiple independent lines of scientific evidence consistently supported the observed contamination patterns. This multidisciplinary approach reflects the core principles of modern environmental forensic investigations and provides a scientifically defensible framework for environmental litigation, regulatory investigations, and complex environmental consulting projects.
To address these questions, the investigation incorporated historical document review, engineering reconstruction of the derailment sequence, emissions estimation, combustion chemistry, atmospheric dispersion modeling, meteorological analysis, environmental sampling review, particulate monitoring, contaminant fate and transport evaluation, toxicological literature review, and exposure assessment. Rather than relying upon any single analytical technique, the investigation integrated these independent scientific disciplines into a multiple-lines-of-evidence framework designed to evaluate whether each dataset independently supported the same environmental conclusions. Such multidisciplinary investigations are widely recognized as the most scientifically robust approach for evaluating complex environmental releases involving multiple contaminants and evolving emission conditions.
A distinguishing feature of this investigation was the extensive application of atmospheric dispersion modeling together with measured environmental data. Predictive air quality models were not interpreted independently but were evaluated alongside measured concentrations in soil and other environmental media, particulate monitoring data, engineering analyses, and contaminant chemistry. This integrated approach reduced uncertainty by allowing independent datasets to either corroborate or challenge one another, thereby strengthening confidence in the reconstructed environmental history. Because environmental forensic investigations frequently involve incomplete historical information, the investigation also incorporated sensitivity analyses to evaluate alternative emission scenarios, meteorological conditions, source geometries, and deposition assumptions to determine whether the principal scientific findings remained consistent under multiple technically reasonable scenarios.
Chemicals of Concern
The investigation evaluated both the transported industrial chemicals and the secondary combustion products generated during the incident. The principal transported chemicals included vinyl chloride, butyl acrylate, 2-ethylhexyl acrylate, and ethylene glycol monobutyl ether. Each compound possesses distinct physical and chemical properties influencing volatilization, atmospheric dispersion, environmental persistence, degradation, and potential human exposure. Among these, vinyl chloride received particular attention because of its high volatility, recognized carcinogenicity, and ability to contribute chlorine during combustion, thereby influencing formation of chlorinated combustion products under appropriate thermal conditions.
While evaluation of the transported chemicals was essential, the investigation recognized that the combustion products generated during the fire represented an equally important component of the environmental assessment. Consequently, substantial emphasis was placed on evaluating polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polycyclic aromatic hydrocarbons (PAHs), soot-associated contaminants, and fine particulate matter. Dioxins and furans are of particular environmental concern because of their persistence, resistance to degradation, tendency to adsorb strongly to soils and organic matter, and potential for long-range atmospheric transport while attached to airborne particulates. These characteristics make them important indicators in environmental forensic investigations involving combustion of chlorinated organic materials.
The investigation also evaluated polycyclic aromatic hydrocarbons (PAHs) generated through incomplete combustion of organic materials. Particular attention was given to carcinogenic PAHs such as benzo(a)pyrene, which serves as an important indicator compound for combustion-derived contamination. Unlike highly volatile compounds that dissipate relatively rapidly following atmospheric release, higher molecular weight PAHs preferentially adsorb to airborne particulate matter before depositing onto soils and other environmental surfaces, where they may persist for extended periods. Understanding the differing environmental behavior of volatile organic compounds, chlorinated combustion products, and particle-bound PAHs was essential for interpreting environmental sampling results, reconstructing contaminant fate and transport, and evaluating potential exposure pathways.
Finally, the investigation considered fine particulate matter (PM₂.₅) as both a regulated air pollutant and an important transport mechanism for combustion-derived contaminants. Fine particulates remain suspended in the atmosphere longer than larger particles, facilitating transport over greater distances before deposition. Because dioxins, furans, and many higher molecular weight PAHs readily adsorb to these particles, understanding particulate generation, atmospheric transport, and deposition was an integral component of the environmental forensic reconstruction and subsequent human health exposure assessment.
Methods
The environmental forensic investigation employed a multidisciplinary scientific methodology designed to reconstruct the environmental consequences of a catastrophic hazardous materials transportation accident. Rather than relying on a single analytical technique, the investigation integrated historical reconstruction, engineering evaluation, emissions estimation, combustion chemistry, atmospheric dispersion modeling, environmental sampling review, particulate monitoring, toxicological evaluation, contaminant fate and transport analysis, and human health risk assessment. This multiple-lines-of-evidence approach represents a cornerstone of modern environmental forensic investigations because it allows independent datasets to be evaluated collectively, reducing uncertainty while increasing confidence in scientific interpretations.
Historical Reconstruction and Engineering Evaluation
The investigation began with a comprehensive reconstruction of the incident chronology using engineering documentation, emergency response records, regulatory information, publicly available investigative materials, environmental monitoring reports, and technical literature. Particular attention was given to reconstructing the sequence of derailment events, fire progression, pressure relief device (PRD) venting, emergency response actions, and the subsequent controlled vent-and-burn operation. Establishing an accurate timeline was essential because contaminant generation, combustion conditions, and atmospheric releases evolved substantially throughout the incident. Evaluating these phases independently provided a more scientifically representative characterization of emissions than treating the event as a single continuous release.
Engineering analyses were also used to evaluate the physical processes governing chemical releases, including thermal loading of damaged railcars, vessel integrity, combustion dynamics, and pressure relief system operation. These engineering evaluations formed the basis for estimating source characteristics used during atmospheric dispersion modeling and were subsequently integrated with environmental sampling data to reconstruct contaminant generation and transport.
Combustion Chemistry and Emissions Estimation
Combustion chemistry represented a critical component of the environmental forensic investigation because many contaminants of concern were generated during the fire rather than being directly released from the transported chemicals. The investigation evaluated combustion conditions capable of producing chlorinated combustion products, including polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polycyclic aromatic hydrocarbons (PAHs), soot, and fine particulate matter. Particular consideration was given to combustion temperature, oxygen availability, residence time, cooling rates, and fuel composition, all of which influence the formation of combustion-derived contaminants.
Emissions estimation incorporated both the original transported chemicals and the secondary combustion products expected to form under documented fire conditions. Rather than assuming complete combustion, the investigation recognized that large industrial fires involve spatially and temporally variable combustion efficiencies, resulting in incomplete combustion and formation of complex contaminant mixtures. Consequently, emissions estimation was based upon engineering reconstruction, combustion science, chemical properties, and published guidance concerning combustion-derived contaminants.
Air Dispersion Modeling
Atmospheric transport and environmental deposition were evaluated using the U.S. Environmental Protection Agency’s AERMOD regulatory dispersion model. AERMOD was selected because it represents the EPA’s preferred regulatory model for evaluating near-field atmospheric transport and deposition of air contaminants. Model development incorporated representative meteorological observations, terrain characteristics, receptor grids, source geometry, emission rates, release temperatures, and contaminant-specific deposition parameters to estimate airborne transport and environmental deposition following the incident.
Recognizing that historical environmental reconstructions inevitably involve uncertainty, the investigation incorporated extensive sensitivity analyses to evaluate alternative modeling assumptions. Multiple EPA-recognized source representations—including point, area, volume, and line sources—were evaluated together with alternative release parameters, source dimensions, exit velocities, particle characteristics, and meteorological scenarios. Rather than relying upon a single model simulation, these analyses were designed to determine whether predicted contaminant transport and deposition remained consistent across multiple scientifically reasonable assumptions. This approach substantially increased confidence in the robustness of the modeled results.
Meteorological processing also received considerable attention because atmospheric stability, wind direction, wind speed, mixing height, and boundary-layer characteristics strongly influence contaminant dispersion. Multiple meteorological datasets were evaluated to determine their influence on predicted plume behavior and deposition patterns. This iterative modeling approach allowed investigators to evaluate uncertainty while maintaining consistency with accepted regulatory air dispersion modeling practices.
Environmental Sampling and Data Analysis
Environmental sampling data were compiled from multiple independent investigations to characterize contaminant occurrence within soil, surface water, sediment, air, particulate matter, and other environmental media. Rather than evaluating isolated analytical measurements, the investigation emphasized regional spatial patterns, concentration gradients, temporal trends, and consistency among multiple independent sampling programs. Evaluating contaminant distributions across the study area provided a more scientifically meaningful assessment than interpreting individual sample results independently.
Background contamination also received detailed evaluation because many combustion-derived contaminants—including PAHs and dioxins—may occur at measurable concentrations as a result of historical industrial activities, transportation emissions, residential combustion, and other anthropogenic sources. Consequently, multiple background datasets were reviewed to determine whether measured environmental concentrations reflected regional background conditions or were more consistent with atmospheric deposition associated with the incident. This comparative approach is fundamental to environmental forensic source attribution and helps distinguish incident-related contamination from pre-existing environmental conditions.
PM₂.₅ Monitoring and Exposure Assessment
Regional PM₂.₅ monitoring data were reviewed as an additional line of evidence supporting atmospheric transport analyses. Fine particulate matter was evaluated because many combustion-derived contaminants—including dioxins, furans, and higher molecular weight PAHs—adsorb to airborne particles that can remain suspended for extended periods before depositing onto environmental surfaces. Monitoring data were compared with documented fire activity, prevailing meteorological conditions, and modeled plume movement to evaluate temporal relationships between particulate concentrations and major combustion events.
Finally, the investigation incorporated a comprehensive exposure assessment integrating atmospheric modeling, environmental sampling, contaminant properties, and toxicological information. Potential exposure pathways included inhalation of airborne contaminants, inhalation of combustion-derived particulate matter, incidental soil ingestion, dermal contact with contaminated environmental media, and indirect exposure following atmospheric deposition. Because contaminants differed substantially in volatility, persistence, and environmental behavior, each chemical group was evaluated according to its unique physical, chemical, and toxicological properties. This contaminant-specific approach ensured that the human health evaluation reflected accepted principles of environmental chemistry, exposure science, and risk assessment rather than relying upon generalized assumptions.
Results
Integration of Multiple Independent Lines of Evidence
One of the defining characteristics of this environmental forensic investigation was the integration of multiple independent scientific disciplines into a unified analytical framework. Rather than relying exclusively on atmospheric dispersion modeling, environmental sampling, engineering analysis, or toxicological evaluation, the investigation synthesized evidence from historical reconstruction, combustion chemistry, meteorological analysis, PM₂.₅ monitoring, contaminant fate and transport evaluation, and human health risk assessment. Evaluating these datasets collectively allowed each line of evidence to independently support—or challenge—the others, resulting in a more scientifically robust reconstruction of contaminant generation, transport, deposition, and potential exposure than could be achieved using any individual analytical method alone.
The investigation demonstrated that each scientific discipline addressed a distinct component of the overall environmental question. Engineering analyses reconstructed the sequence of derailment events and emergency response activities. Combustion chemistry identified contaminants reasonably expected to form under documented fire conditions. Atmospheric dispersion modeling estimated contaminant transport and deposition. Environmental sampling characterized the occurrence and spatial distribution of contaminants in multiple environmental media. PM₂.₅ monitoring independently documented regional particulate transport, while toxicological evaluation provided context for interpreting the environmental significance of measured contaminants. When interpreted collectively, these independent datasets produced a coherent scientific reconstruction of the incident that was internally consistent across multiple analytical approaches.
Atmospheric Dispersion Modeling
Air dispersion modeling demonstrated that atmospheric transport was strongly influenced by changing meteorological conditions throughout the incident. Wind direction, atmospheric stability, mixing height, source geometry, release duration, emission characteristics, and particle behavior collectively influenced predicted contaminant transport and environmental deposition. Separate emission scenarios representing uncontrolled combustion, pressure relief device venting, and the controlled vent-and-burn operation were evaluated independently to account for changes in combustion conditions and source characteristics throughout the event.
One of the most significant aspects of the modeling effort was the evaluation of uncertainty through extensive sensitivity analyses. Rather than relying upon a single representation of the release, multiple EPA-recognized source configurations—including point, area, volume, and line sources—were evaluated together with alternative release parameters, source dimensions, emission rates, exit velocities, particle characteristics, and meteorological datasets. This systematic evaluation demonstrated that although predicted concentrations varied among individual simulations, the broader spatial distribution of modeled contaminant deposition remained generally consistent across alternative scenarios. Such consistency substantially increased confidence that the reconstructed plume behavior reflected the dominant physical processes governing atmospheric transport rather than artifacts associated with a particular modeling assumption.
Environmental Sampling and Spatial Patterns
Environmental sampling results were interpreted within the broader context of atmospheric transport rather than as isolated analytical measurements. Soil, sediment, particulate, and other environmental media collected during multiple phases of the investigation were reviewed to identify recurring spatial contamination patterns that could be compared directly with modeled deposition contours. Instead of emphasizing individual sample concentrations, the investigation focused on evaluating regional concentration gradients, contaminant distributions, and consistency among independent sampling programs, providing a more scientifically meaningful assessment of contaminant fate and transport.
The investigation demonstrated that elevated concentrations of combustion-derived contaminants—including dioxins, furans, and carcinogenic PAHs—occurred in geographic patterns consistent with atmospheric transport following the incident. Although contaminant concentrations varied spatially, the observed distribution generally corresponded with predicted deposition patterns generated by atmospheric dispersion modeling. The agreement between modeled deposition and measured environmental contamination provided an important independent line of evidence supporting the overall forensic reconstruction.
Background contamination also received careful evaluation because persistent combustion-derived contaminants may be present in the environment from historical industrial activities, transportation emissions, residential combustion, and other anthropogenic sources. Rather than assuming all detected contaminants were incident-related, the investigation evaluated multiple background datasets, spatial trends, contaminant profiles, and deposition gradients to distinguish regional background conditions from contamination consistent with atmospheric releases associated with the incident. This comparative evaluation represents a fundamental principle of environmental forensic source attribution.
PM₂.₅ Monitoring and Atmospheric Transport
Regional PM₂.₅ monitoring provided an additional independent dataset supporting evaluation of atmospheric transport during the incident. Temporal changes in fine particulate concentrations were compared with documented fire progression, prevailing meteorological conditions, and modeled plume movement to determine whether elevated particulate concentrations corresponded with major combustion events. Monitoring stations located at varying distances from the incident consistently documented changes in airborne particulate concentrations during periods of active combustion, providing valuable information regarding regional particulate transport.
Because combustion-derived dioxins, furans, and many higher molecular weight PAHs readily adsorb to fine airborne particulates, PM₂.₅ monitoring served as an important indirect indicator of particle-bound contaminant transport. Although particulate monitoring cannot independently identify specific chemical constituents, agreement between monitoring records, atmospheric modeling, engineering reconstruction, and environmental sampling substantially strengthened confidence in the reconstructed transport pathways and supported the interpretation of environmental deposition patterns.
Exposure Assessment and Environmental Significance
The investigation evaluated potential human exposure by integrating contaminant properties, atmospheric transport, environmental persistence, environmental sampling, and toxicological information into a comprehensive exposure assessment framework. Highly volatile compounds such as vinyl chloride were evaluated primarily with respect to short-term atmospheric transport immediately following release, whereas persistent contaminants—including dioxins, furans, and higher molecular weight PAHs—were evaluated for their tendency to adsorb to airborne particulate matter, deposit onto soils and other environmental surfaces, and persist following atmospheric deposition.
Rather than relying solely on modeled concentrations or isolated analytical measurements, the investigation integrated engineering analyses, combustion chemistry, atmospheric dispersion modeling, environmental sampling, contaminant fate and transport evaluation, PM₂.₅ monitoring, and toxicological information into a unified environmental forensic framework. This multidisciplinary approach demonstrated that independent scientific disciplines consistently supported the reconstructed contaminant transport pathways and environmental distribution patterns. Collectively, the results illustrate how multiple lines of evidence can provide a scientifically defensible basis for evaluating complex environmental releases involving hazardous materials transportation accidents and large industrial fires.
Conclusions
Large-scale hazardous materials transportation accidents require environmental investigations that extend well beyond conventional environmental sampling. As demonstrated by this investigation, accurately reconstructing the environmental consequences of a catastrophic release requires integration of engineering analysis, combustion chemistry, atmospheric science, environmental chemistry, contaminant fate and transport, exposure assessment, toxicology, and environmental data interpretation. Individually, each discipline provides valuable information; collectively, they establish a scientifically robust framework capable of reconstructing complex historical environmental events and evaluating contaminant behavior following large industrial fires.
One of the principal findings of this investigation was the importance of evaluating both the original transported chemicals and the secondary combustion products generated during the incident. While the transported hazardous materials represented the initial source of contamination, the investigation demonstrated that combustion conditions—including temperature, oxygen availability, fuel composition, residence time, and cooling rates—strongly influenced the formation of persistent combustion-derived contaminants. Consequently, understanding environmental impacts required evaluation of volatile organic compounds together with combustion products such as polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polycyclic aromatic hydrocarbons (PAHs), soot-associated contaminants, and fine particulate matter.
The investigation further demonstrated the value of integrating predictive atmospheric dispersion modeling with measured environmental observations. Air quality models provided a scientifically accepted means of reconstructing contaminant transport under changing meteorological conditions and estimating atmospheric deposition across the surrounding environment. However, the investigation did not rely exclusively on model predictions. Instead, modeled deposition patterns were systematically compared with environmental sampling results, particulate monitoring data, engineering analyses, and documented incident conditions. Agreement among these independent datasets substantially increased confidence that the reconstructed transport pathways accurately reflected the dominant physical processes governing contaminant dispersion and environmental deposition.
A significant strength of the investigation was its explicit evaluation of scientific uncertainty. Historical environmental reconstructions inevitably require assumptions regarding emission rates, combustion efficiency, release duration, source geometry, particle characteristics, and meteorological conditions. Rather than treating these uncertainties as limitations, the investigation incorporated extensive sensitivity analyses using multiple source representations, alternative emission scenarios, and varying modeling assumptions. Although individual concentration estimates varied among simulations, the broader spatial distribution of predicted deposition remained generally consistent across numerous technically reasonable scenarios. This consistency increased confidence that the principal findings were robust and not dependent upon any single modeling assumption or analytical technique.
The investigation also emphasized the importance of distinguishing incident-related contamination from regional environmental background conditions. Persistent combustion-derived contaminants, including dioxins, furans, and PAHs, may occur at measurable concentrations because of historical industrial activity, fossil fuel combustion, transportation emissions, residential heating, and other anthropogenic sources. Accordingly, interpretation of environmental sampling required evaluation of contaminant distributions, concentration gradients, regional background conditions, atmospheric deposition patterns, and multiple independent datasets rather than simple comparison of isolated analytical measurements. This comprehensive approach represents a fundamental principle of environmental forensic investigations and provides a scientifically defensible basis for environmental source attribution.
Beyond the specific circumstances of this project, the investigation demonstrates the increasingly interdisciplinary nature of modern environmental consulting. Questions involving industrial emissions, air quality modeling, groundwater and soil contamination, contaminant fate and transport, environmental chemistry, toxicology, exposure assessment, and human health risk assessment rarely can be answered using a single technical discipline. Instead, scientifically defensible environmental forensic investigations require integration of engineering, atmospheric science, chemistry, statistics, toxicology, and environmental data analysis within a transparent analytical framework capable of evaluating multiple competing scientific hypotheses. Such multidisciplinary investigations provide a strong scientific foundation for environmental litigation, regulatory investigations, environmental due diligence, remedial decision-making, and expert witness testimony.
Although every hazardous materials incident presents unique technical challenges, the investigative methodology employed during this project is broadly applicable to transportation accidents, chemical manufacturing facilities, petroleum refineries, industrial fires, medical sterilization facilities, wood treatment operations, municipal landfills, waste management facilities, and other complex environmental contamination investigations. By integrating historical reconstruction, atmospheric dispersion modeling, environmental chemistry, contaminant fate and transport evaluation, environmental sampling, and exposure science within a multiple-lines-of-evidence framework, environmental forensic investigations can provide objective, technically rigorous, and scientifically defensible evaluations that support informed environmental decision-making while maintaining the transparency and analytical rigor expected of modern environmental consulting practice.
This investigation was conducted under the direction of Dr. Paul Rosenfeld, an environmental scientist and environmental forensic expert with extensive experience in environmental exposure assessment, toxicology, air quality, environmental fate and transport, risk assessment, and environmental forensics. Through RoseyFields LLC, Dr. Rosenfeld provides scientific consulting, environmental forensic analysis, regulatory support, and expert witness services involving environmental contamination, human exposure, and public health. His technical oversight helped ensure that the methodologies, scientific interpretations, and conclusions presented in this case study are consistent with accepted scientific principles and current best practices.