Potential risks to human health and the environment are major drivers behind most corrective action decisions. Understanding the risks associated with contaminated sites is essential for determining appropriate responses that minimize exposure to environmental contaminants and address the true concerns. Identifying and characterizing both actual and potential (or percieved) risks associated with environmental contamination provides the foundation for developing appropriate corrective action and site management strategies. Risk assessment is the process of evaluating and quantifying existing or potential adverse impacts resulting from exposure to contaminants.
Personnel from THWA have extensive experience in risk assessment and risk-based decision-making and provides services and project support in all aspects of risk evaluation, exposure assessment, and toxicological evaluation. We focus on technically sound, scientifically defensible evaluation of site-specific risks and using the results in making practical decisions regarding management of these risks that protect human health and the environment and minimize costs. By considering the nature, degree and extent of contamination, site characteristics and current and future land use, our risk assessments realistically characterize the level of concern and degree of urgency represented by contamination at a site and help identify appropriate mitigation approaches. This enables our clients to make informed decisions that achieve regulatory compliance and facilitate commercial transactions in the most cost-effective manner.
Many regulatory agencies have developed a “cook-book” approach for risk evaluation that, while providing a standardized process, often results in unrealistically conservative risk estimates. While these approaches are adequate for many sites, they typically use unrealistic, overly simplistic or inappropriate characterizations of the contaminant sources, transport and fate processes, and receptors. In our experience, most environmental consultants can “plug and chug” through the calculations the cook-book type calculations, but have no real understanding of the meaning of the results or how to use them.
THWA personnel have developed or reviewed numerous risk assessments for a wide variety of sites and contaminants under many different regulatory programs. Most risk-based evaluation approaches in different regulatory settings are fundamentally similar. THWA uses state-of-the-art information and methods to develop sound risk assessments for complex situations and procedures consistent with U.S. EPA and state regulatory guidance. Typically, a tiered approach to risk evaluation is used that adapts the sophistication of the analysis to the complexity of the site. Additionally, THWA personnel’s experience in the regulatory environment allows us to work effectively with regulatory agencies and to bring a unique perspective on reconciling risk with ultimate remedial objectives such as restoration.
Risk from environmental contamination is largely determined by actual or potential exposure to contaminants that can occur through a variety of exposure or migration pathways. The potential for exposure is controlled by the physical and chemical properties and behavior of the contaminants under the hydrogeologic and geochemical conditions at a site. THWA leverages its expertise in hydrogeologic evaluation, environmental geochemistry, contaminant transport and fate and environmental modeling to produce realistic exposure estimates.
THWA is familiar with issues related to contaminants with uncertain toxicity and methods for evaluating them. Additionally, THWA has extensive experience with a vast array of public databases and other sources of toxicological literature and information. Our firm will compile and evaluate available information related to toxicity and exposure to constituents of potential concern and attempt to develop prudent criteria to represent toxicity based on the best available data. A particular area of expertise is with polychlorinated dibenzo-p-dioxins and dibenzofurans, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). GRI personnel are faculty members with the Graduate Toxicology Program at the University of New Mexico.
Associates of THWA were involved in the development of U.S. EPA’s Soil Screening Guidance and strategic planning for development of the Superfund risk assessment protocol. Our experience in the regulatory arena includes development of regulations and guidance to implement risk-based approaches to site remediation, such as Wisconsin’s guidance on risk-based soil cleanup levels for polycyclic aromatic hydrocarbons (PAHs), regulatory review of risk assessments and cleanup level determinations, and training and consulting for state regulatory agencies on technical and policy issues for implementing risk-based corrective action (RBCA) programs. Personnel from THWA have developed and provided training in risk assessment, risk-based decision-making and exposure assessment modeling to several hundred environmental professionals and state regulators.
SERVICES
- Human health risk assessment
- Ecological risk assessment
- Transport and fate analysis and modeling
- Exposure assessments
- Cleanup level determination
- Risk-based screening
- Risk-based corrective action (RBCA) analysis
- Toxicology evaluations
This work was conducted at a politically sensitive site and involved evaluating potential risks associated with residual total petroleum hydrocarbon (TPH) contamination from a diesel fuel release to determine whether the site warranted further active remedial measures. Potential exposure pathways included migration in groundwater and vapor-phase transport to outdoor air and buildings. Residual contamination was characterized as predominantly TPH-diesel and TPH-gasoline with no associated breakdown of composition. TPH measurements are method-dependent and the available analytical results could not be directly related to specific compositional groupings used in the Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG The TPHCWG consisted of representatives from industry, government, and academia, including the Gas Research Institute, several major petroleum companies, the American Petroleum Institute, the Association of American Railroads, several state governments, the U.S. Environmental Protection Agency, the Department of Defense, the University of Massachusetts, and private consulting firms) or Massachusetts Department of Environmental Protection approaches.
THWA performed a thorough evaluation of the site characterization data and relevant available literature characterize the site hydrogeology, distribution and concentration relationships of petroleum hydrocarbon constituents in soil and groundwater, and to determine appropriate and reasonable parameter values for use in contaminant transport and fate evaluation. The available literature and information on the toxicology of fuel hydrocarbon mixtures and hydrocarbon fractions were reviewed and evaluated to determine the most appropriate representation of the contaminants at the site for risk assessment. THWA then extensively reviewed and evaluated available information and literature on TPH analytical methods, fuel mixture compositions, and fuel mixture weathering to develop an appropriate breakdown of TPH measurements from the site into hydrocarbon fraction concentrations based on mass proportions. Exposure assessment modeling was performed using representative values for physico-chemical and transport parameters of the hydrocarbon fractions developed by the TPHCWG. The Johnson and Ettinger (1991, Heuristic model for predicting the intrusion rate of contaminant vapors into buildings. Environmental Science & Technology, 25:1445-1452) vapor transport model was adapted to accommodate TPH fractions.
The results of the risk assessment clearly demonstrated that there were no human health risks associated with current land use at the property and that potential human health risks associated with future land use scenarios were minimal and associated with very unique conditions that could be readily mitigated. Based on the risk assessment, no additional active remediation was warranted.