Case Studies

Case Study: Soil Vapor Extraction Modeling

Client:  Engineering Company, on behalf of a Confidential Industrial Client
Location:  Southern California ​

Issue:  Risk of Vapor Intrusion (VI) from an old redeveloped site with chlorinated solvents required sub-slab remedial action. A large Engineering company hired Mutch Associates to develop a three-dimensional SVE model to simulate and optimize a full-scale system.

Action:  Developed a three-dimensional soil vapor flow model of the site using MODFLOW SURFACT, a modeling code that simulated air-water systems. Used the model to estimate the optimal location of SVE wells for the full-scale SVE system.

Site Overview:

Volatile organic compounds (VOCs), including tetrachloroethylene (PCE), trichloroethylene (TCE), and other compounds were present in groundwater, and soil vapor in a relatively deep water table condition (70 feet). The site was redeveloped with multi-family homes. The local Department of Toxic substances required the installation of a sub-slab remediation system. The best solution for this problem was defined to be a soil vapor extraction system.

Mutch Associates was retained to simulate and optimize the number and locations of wells for the full-scale SVE system, based on a previous pilot SVE test conducted by the Engineering firm. The key goal of the project was to determine the optimal number and location of SVE wells to maintain a downward gradient, eliminating the VI pathway and therefore any risks to the homes.

Figure 1: Simulated vacuum (top) and air flow velocity (bottom) – Pilot SVE Test.

Figure 2: Simulated versus measured vacuum (top) and vacuum versus distance from SVE well (bottom).

Results:

Our Soil Vapor Extraction Model was initially calibrated with vacuum readings measured during the week-long pilot SVE test. Figure 1 (left diagram) shows the model-calculated vacuum contours. The diagram on the right of this figure presents contours of air flow velocity. Figure 2 (left) shows the calculated versus observed vacuum readings, with excellent match. Figure 2 (left) present a model simulated vacuum versus the observed values, as well as a vacuum versus distance from the SVE pilot well on the right.

Design Optimization:

SVE systems ROIs are typically determined using straight line fitting with vacuum versus distance plots. The model, however, suggested that the ROI was much larger, thanks to a vacuum tailing effect vacuum with distance. A better way to determine ROI is to use air flow velocities. A typically used criterion is to determine the areas with velocities greater than 0.01 cm/s as effective for SVE systems. Using this criterion, the ROI increased by from 70 feet to over 100 feet.  This allowed a significant reduction in the number of SVE wells and piping infrastructure in the full-scale SVE system, without any loss of effectiveness. The original design based on a straight line approximation included 28 SVE wells. The modeling showed that an effective system could be implemented with only 13 wells, a 54% reduction compared to the original system. Significant cost savings were achieved.

Figure 3: Simulated velocity contours (m/s) and air flow velocities. Red arrows indicate downward flow velocity, demonstrating that the VI pathway is effectively eliminated with the system. The final number of SVE wells was 13, 54% less than the original design.