“Where is the NAPL” – Understanding NAPL Mobility for Risk Management Decision‐Making
Background/Objectives: Full-scale remediation of NAPL impacted sites to regulatory standards can often be technically prohibitive, costly, and provides marginal increases in protection to human health and the environment. With the advancement of NAPL investigation techniques, modeling, and mobility understanding, in many cases, NAPL can be managed in place allowing sites to be reused without costly remediation. Such an approach is being evaluated for the Toronto Port Lands Flood Protection and Enabling Infrastructure Project (PLFP) led by Waterfront Toronto. NAPL mobility is an important metric that should be included in the development of remedial and risk management strategies. Unlike many risk-based criteria which represent a defined toxicity and relatively easily measured value, metrics for NAPL mobility remain nebulous. NAPL mobility and stability are best understood using multiple lines of evidence including historical site data such as depth to NAPL, presence of NAPL in wells, spatial plume stability, and site-specific laboratory data such as total petroleum hydrocarbon (TPH) concentrations and NAPL saturation from undisturbed soil cores. This study provides an example of how these multiple lines of evidence can be combined to develop NAPL mobility criteria, which can then be used to manage NAPL in place in different areas of the PLFP site.
Approach/Activities: This presentation will demonstrate how existing geological, TPH, and laser-induced fluorescence (LIF) data can be used to provide a relatively high spatial data density for NAPL analysis. Two distinct LIF methods (UVOST and TarGOST) were utilized to distinguish between areas with different NAPL types, which typically have varying degrees of mobility. Due to inconclusive results from a previously completed NAPL transmissivity test, an undisturbed soil core study was conducted for further mobility assessment. NAPL information from existing site data were correlated with results from undisturbed soil core analysis. NAPL saturation data obtained from undisturbed soil cores were used to provide interpretation of NAPL mobility specific to soil and NAPL types. It is important to distinguish between mobile and stable NAPL, as it ultimately affects the selection of risk management measures. In particular, NAPL mobility was explored under typical groundwater gradients (via water drive test), conservative centrifuge testing, and spontaneous imbibition testing. By combining these data sets, NAPL mobility metrics (i.e., concentrations and soil types), above which NAPL may migratewere developed.
Results/Lessons Learned: Corollary analysis of undisturbed soil core results to other NAPL site characterization data was important for establishing a relationship between large historical data sets describing the NAPL distribution across the site and where those NAPL distributions may pose potential NAPL mobility risks. The NAPL mobility thresholds defined through this analysis provides a technical basis for remedy selection.