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Soil vapor extraction (SVE) is a prevalent remediation remedy for volatile organic compound (VOC) contaminants in the vadose zone. To support selection of an appropriate condition at which SVE may be terminated for site closure or for transition to another remedy, an evaluation is needed to determine whether vadose zone VOC contamination has been diminished sufficiently to keep groundwater concentrations below threshold values. A conceptual model for this evaluation was developed for VOC fate and transport from a vadose zone source to groundwater when vapor‐phase diffusive transport is the dominant transport process. A numerical analysis showed that, for these conditions, the groundwater concentration is controlled by a limited set of parameters, including site‐specific dimensions, vadose zone properties, and source characteristics. On the basis of these findings, a procedure was then developed for estimating groundwater concentrations using results from the three‐dimensional multiphase transport simulations for a matrix of parameter value combinations and covering a range of potential site conditions. Interpolation and scaling processes are applied to estimate groundwater concentrations at compliance (monitoring) wells for specific site conditions of interest using the data from the simulation results. The interpolation and scaling methodology using these simulation results provides a far less computationally intensive alternative to site‐specific three‐dimensional multiphase site modeling, while still allowing for parameter sensitivity and uncertainty analyses. With iterative application, the approach can be used to consider the effect of a diminishing vadose zone source over time on future groundwater concentrations. This novel approach and related simulation results have been incorporated into a user‐friendly Microsoft® Excel®‐based spreadsheet tool entitled SVEET (Soil Vapor Extraction Endstate Tool), which has been made available to the public.

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Psge 2 of 2 ENGINEERING CHANGE NOTICE I. EM (use no. f r d p pg. 1) 1 604162 SDDlDD hncbbnd W i n Mteria operating Specification criridky Specification Conmptual Da8ign Report Equipmilt spec. const. spec. pmcummnt spec. Vendor infomation OM Manual FSAWSAR s.fay Equipment tist Radiion Work Permit Environmental impact Statomnt

Presumed to originate from 50.8 cm (20 in.) vapor header at juncture with 61.0 cm (24in.) header Auger soil sample read 3 Rad at contact See 241-AX Tank Farm Figure 1,241-AX Tank Farm Borehole Arrangement Plan View January 1968 Unknown, soil removed for disposal Transfer line at tank AX-103 400 m a & at the surface; 5R/hr at the pipe surface See 241-AX Tank Farm Figure 1,241-AX Tank Farm Borehole Arrangement PlanView April 1975 30,283 L (8,000 gal) Borehole 11-04-1 1 at 12.2 m (40 ft) depth Auger soil sample read 3 Rad at contact See 241-AX-Tank Farm Figure 1,241-AX Tank Farm Borehole Arrangement Plan View 1968-1975 9.5 x lo5 L (2.5 x lo5 gal) 216-A-40 Trench-121.9 m (400 ft) long by 6.1 m (20 ft) wide, located approximately 152.4 m (500 ft) west of the Radionuclide content unknown See 241-AX Tank Farm Figure 1,241-AX Tank Farm Borehole Arrangement Plan View 241-AX Tank Farm 1968-1974 unknown 216-A-41 Crib-3.1-m (IO-ft) by 3.1-m (10-ft) bottom surface, located approximately 152.4 m (500 ft) west and 61 m (200 ft) south of the 241-AX Tank Farm Small amounts of Beta, potentially slightly acidic See 241-AX Tank Farm Figure 1,241-AX Tank Farm Borehole Arrangement Plan View A-6 HNF-SD-HTI-TI-001, REV. 0 SESC-EN-RPT-002, Rev. 1 (formally known as) 5.0 REFERENCES