SSP&A is part of multi-firm team developing remedial alternatives and strategies for RI/FS and post-ROD activities.
SSP&A was retained in 2006 to provide support in the evaluation, design and optimization of a large groundwater remedy at the 200-ZP-1OU of the USDOE Hanford Site. A 5-square-mile plume, which extends to 600 feet below land surface; an estimated 800 to 1,000 tons of carbon tetrachloride were released, together with trichloroethylene, hexavalent chromium and tritium. SSP&A developed a modeling strategy to support an RI/FS and remedial design and developed a step-wise strategy to evaluate the relative benefits of alternate remedies, focusing on combined pump and treat and MNA. The first step involved the development and application of a simple superposition modeling approach written by SSP&A, calibrated to over 60 years of water-level data at dozens of wells that recorded the response of the aquifer to discharged wastewater. Calibration was complemented by use of the program PEST. Advective-dispersive particle-tracking, undertaken with a code developed by SSP&A, illustrated that the calibrated superposition model reproduced contaminant patterns throughout the area of interest. As a result, the superposition model was used to develop and provide a cost-basis for the remedy alternatives described in the FS. Successful completion of the RI/FS led to agreement between the U.S. EPA and the Washington State Department of Ecology, and signing of the first final Record of Decision (ROD) for a groundwater OU at the Hanford Site.
SSP&A has completed numerous groundwater flow and contaminant transport simulations for all Contaminants of Concern (COCs), in support of the Remdial Design/Remdial Action (RDRA) work plan, 30%, 60% and 90% designs. These simulations included evaluations of the sensitivity of influent concentrations and masses, and attainment of the remedial action objectives, to reasonable variations in pumping rates and contaminant transport properties. The computer codes MODFLOW, MODPATH, and MT3DMS were used for these simulations. To resolve the dry-cell problem in MODFLOW, SSP&A implemented a variably-saturated flow equation within MODFLOW, with an asymmetric matrix solver and Newton-Raphson linearization scheme that has been benchmarked against MODFLOW-SURFACT.
SSP&A also developed an ensemble approach for assessing the performance of the various Hanford pump and treat remedies that combines numerical flow, pathline and transport modeling; multi-event water-level mapping combining kriging and analytic elements; and, analysis of COC trends. Based on this approach Hanford annual reports now incorporate Capture Efficiency maps and Capture Frequency maps to depict the likely extent of hydraulic containment developed by each pump and treat system.