Virtual Site Investigation to Evaluate Conceptual Site Model Development at DNAPL-Impacted Sites

The development of a conceptual site model (CSM) is essential to support decision making for the investigation and remediation of contaminated sites, including sites impacted by dense nonaqueous phase liquids (DNAPLs). Dynamic investigation strategies using high-resolution site investigation tools can be used to increase the accuracy, and increase the precision (reduce the uncertainty) of a CSM, while balancing life-cycle costs. However, the evaluation of CSM accuracy is limited by the hydrogeological and biogeochemical complexities inherent in subsurface systems. This study simulated DNAPL migration, dissolution, and reactive transport to create three high-resolution "virtual sites" composed of 0.4-1 billion parameter values each, which were investigated by teams of leading practitioners. Use of these virtual sites allowed key hydrogeological and biogeochemical parameters to be known by the model developers but not the investigation teams. Comparison of the CSMs developed by these practitioners to the simulated values across multiple metrics (e.g., dissolved concentrations, DNAPL mass, and mass discharge) showed that 92% of those metrics were estimated within a factor of 10 and 63% were within a factor of 3 of the true (simulated) values. Furthermore, estimates of dissolved phase plume footprint were often more accurate than estimates of DNAPL source footprint, and the approaches used for investigation and data analysis were both important for CSM development. Overall, none of the site investigation approaches used to develop the CSMs resulted in the highest accuracy for all metrics, which demonstrates the need to establish best-practices that draw from multiple approaches and vary depending on investigation objectives.