The first step in exposure assessment is typically the development of Conceptual Site Models (CSM). These models should include all potential exposure pathways that could theoretically occur at a site. The exposure assessment then discusses each of the pathways in detail and must provide adequate and defensible rationale for excluding a pathway from quantitative evaluation. Terra Mentis personnel collectively have conducted and reviewed hundreds of risk assessments and understand the conditions under which typically insignificant exposure pathways may become significant. For example, inhalation of VOCs released from soil and groundwater to ambient air is generally not considered an important exposure pathway, due to rapid dilution. However, dilution may be very limited in an excavation and even more so in a utility trench and the ambient air inhalation pathway may become significant for people working in excavations or trenches. A CSM is the foundation of the risk assessment process and identifies potential exposure pathways and the media that are impacted or that can act as a transport mechanism for the contaminants at the site. Region VIII guidance has proven to be extremely useful when preparing a CSM and typically allows for the identification of human receptors.
Data Quality Objectives (DQO) have been developed for a large variety of sites, including small gasoline stations and larger, multi-unit RCRA refineries and CERCLA sites. The adequacy of DQOs should reflect how the data will be used. Once data use is established, preparing and reviewing DQOs has many components and Terra Mentis' approach has been to systematically work through US EPA's DQO guidance process and other associated guidance documents.
The DQO process can be iterative. For example, a thorough statement of the problem requires a clear understanding of the nature of the contamination, source area, and study objectives. So, a preliminary screening approach to sampling may be necessary before a final problem statement can be developed. Problem statements for compounds that bioaccumulate will be more complex than statements for gasoline sites. A summary of the steps is provided below: