Formatted Title
Ongoing Use of Fate and Transport Modeling to Support a Successful Natural Attenuation Remedy for PCP in Groundwater: A 10-Year Perspective
Background/Objectives
In 1999, an analytical groundwater chemical fate and transport model predicted attenuation of a pentachlorophenol (PCP) plume in an aquifer beneath a mill complex. The model predicted PCP plume attenuation below the cleanup objective (method reporting limit of 0.5 µg/L) in 12 years. In 2011, a fate and transport model using MODFLOW/MT3D was completed and predicted plume attenuation in 65 years. State regulators expressed concern about the new cleanup time because it implied the existing natural attenuation approach was not protective of the aquifer and downgradient public water supply users. Integral evaluated the MODFLOW/MT3D model and found multiple issues with model construction and the input parameters used. Integral was then tasked with building a robust and defensible model for PCP plume attenuation predictions to support stakeholder decision-making.
Approach/Activities
Initially, Integral conducted a detailed refinement of the conceptual site model (CSM), including 1) understanding the multi-layered unconsolidated, alluvial sediments at the site underlain by volcanic bedrock, 2) understanding the key regional and local hydrogeologic factors affecting the groundwater flow system, 3) estimating the initial PCP source, remediation timeline, and contaminant loading rates on groundwater, and 4) assessing changes in the 3-dimensional PCP plume distribution over time. Based on the revised CSM, Integral constructed and calibrated a numerical fate and transport model, and utilized a robust, existing groundwater quality data set to assess the model’s accuracy and sensitivity to key parameters. Finally, PCP plume attenuation times were predicted for different portions of the plume, with an emphasis on the more distal portions that were closest to offsite public water supply wells.
Results/Lessons Learned
Model simulations in 2013 demonstrated that the site PCP plume would attenuate within about 30 years (i.e., by 2040–45). State regulators conducted a comprehensive model review and accepted the existing natural attenuation approach for the PCP plume based on the new modeling efforts. In 2019, a review of PCP concentration trends at key distal plume wells versus the 2013 model predictions demonstrated slightly faster than predicted attenuation. In 2019, the groundwater data suggested that plume attenuation to below 0.5 µg/L was likely to occur before 2040. This paper presents an additional 5 years of groundwater PCP data for comparison to the 2013 model plume attenuation predictions. Also presented is a discussion of the tension between balancing reduction of model input uncertainties versus required degrees of precision and accuracy, while keeping in mind the purpose of the model—to demonstrate protectiveness and thereby gain stakeholder consensus.