Client: Confidential
Location: Northern Montana
Project Overview: Trihydro conducted a risk assessment focused on potential migration of BTEX vapors into residences situated above a dissolved gasoline plume. The plume area incorporated approximately 80 acres under a residential neighborhood including an elementary school and a high school. Trihydro conducted indoor air sampling during several seasons and worked with state regulators to develop a program to distinguish between potential plume-related and alternate sources (smoker's, material storage, cleaning supplies, etc.) within the residences.

The primary populations evaluated were school children and residents. Exposure of school maintenance workers, teachers, and construction workers were also evaluated. Background ambient air sampling was conducted in, and upwind of, the town and the results incorporated into the evaluation. An extensive subsurface investigation (soils and groundwater) was also conducted to collect data for both plume definition and risk assessment analyses. Due to its high profile, and the essential element of sampling in people's homes, the project involved considerable interaction with the public through open meetings, phone contacts, public newsletters, and close collaboration with the regulatory agency.

Client: Confidential
Location: Various locations throughout the U.S.
Project Overview: Over the past eight years, Trihydro has conducted 15 human health risk assessments for a client that operates solvent processing and handling facilities throughout the country. The risk assessments have been conducted as part of the remedy evaluation process (CMS phase) of RCRA corrective action. The sites we've evaluated have been diverse in size, location, potential for and routes of exposure, nature and degree of impacts, and regulatory complexity. Some have involved potential contaminants from neighboring properties, and consideration of offsite data. The assessments have been performed in accordance with EPA's Risk Assessment Guidance for Superfund, as well as applicable state and regional guidance (e.g., Arkansas, California, Illinois, Kentucky, EPA regions 4, 5, 6, and 9). Typically, these risk assessments have involved the following components:

• Statistical evaluation of background soil concentrations for metals and incorporation of background data into the risk evaluation;
• Evaluation of data quality, representativeness, potential bias, and spatial distribution to determine suitability of the data set for use in risk assessment;
• Development of site conceptual model and identification of complete exposure pathways and potentially exposed populations;
• In general, evaluated scenarios have included inhalation, ingestion, and dermal exposure to chlorinated VOCs and SVOCs and consideration of construction workers, industrial workers, residents, and trespassers;
• Use of fate and transport modeling to determine or support exposure point concentrations;
• Quantification of risk and uncertainty evaluation;
• Collaboration, discussion, and meetings with regulatory risk assessment technical staff regarding overall approach and technical details; and
• Integration of the risk assessment results into the corrective action approach involving remediation and institutional and engineering controls.