Formatted Title
Use of a 3-D Conceptual Site Model for Rapid Site Assessment and Communication
Background/Objectives
The foundation for evaluating volatile organic compound (VOC) transport, fate, and potential exposure via the vapor intrusion pathway is a detailed conceptual site model (CSM). This vital component of site investigation is too often marginalized, oversimplified, or not updated with current information. Not only should the CSM be developed early in the life cycle of the site, but it should be updated regularly as site understanding, state of the science, and regulatory policies evolve. The CSM is a narrative and visual tool used to convey detailed site knowledge and understanding, thereby enabling site investigations to proceed with fewer slowdowns, as well as informing the need for a remedy and allowing for selection and optimization of the most appropriate remedial approach.
Approach/Activities
Due to technical and toxicological updates, it has become more difficult to screen out the vapor intrusion pathway, and site-specific evaluations have become more common. At a site in California, revised screening levels for volatile organic compounds in soil gas resulted in previously developed screening levels becoming no longer applicable. In response, lead regulatory agencies proceeded with a site-by-site approach, requiring selected sites to develop an updated CSM to evaluate site-specific risks related to vapor intrusion. An enhanced CSM was developed using 3-D visualization software to evaluate the potential risk to residential properties via the vapor intrusion pathway. This model utilized regional stratigraphy and geology, utility information, chemical concentrations in various media, and construction-specific information to depict how soil gas is likely to migrate through the subsurface. Through compiling and visualizing available geospatial information, limitations to soil gas migration became more apparent. This model can be utilized to identify data gaps and drive efficient and purposeful sampling. Refining the model and resolving data gaps provides valuable information on how site characteristics impact soil gas migration, and therefore streamline remedial needs determination and optimization.
Results/Lessons Learned
Development of a 3-D CSM was used to effectively communicate to regulatory agencies and stakeholders the reasonably complete understanding of the soil gas source, flow paths, and migration potential. Effective visual demonstration of the 3-D model resulted in rapid regulatory concurrence regarding subsequent investigation steps. Results of the model show how confining layers limit soil gas migration vertically and suggest that utilities are not acting as significant migration pathways. The presentation will include representative figures demonstrating how the model illustrates the CSM.