Remediation of TCE NAPL in active Alleyway Arlington, Texas - Case Study

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Courtesy of TRS Group, Inc.

Project Summary

TRS Group, Inc. (TRS) completed a standard Fixed Price Remediation (SFPR) of trichioroethene (TCE) in soil in Arlington, TX using electrical resistance heating (ERH) in 2012, The system incorporated 19 electrodes with horizontal vapor recovery (VR) wells to treat a soil volume of approximately 4,155 cubic yards (yd3) located between two active buildings. Active subsurface heating extended from approximately 5 feet below grade surface (ft bgs) to just below the bedrock surface,

The goal of the ERH remediation was to removal NAPL and reduce the average soil concentratbn of TCE to 10 mg/kg. Post ERH soil sample analyses indicated a removal efficiency of 95.5%.

Background

The treatment region was located in a narrow aiie} between Building 2 and Building 6, see Figure 1. The ERH treatment area measured approximately 5,100 square feet in surface area. The targeted heating zone ranged from approximately 5 ft bgs and extended to between 25 ft, to 32,5 ft bgs as determined by the bedrock interface, The average depth was 27 ft bgs and the resulting treatment volume was approximately 4,155 yd3.

The contamination was identified during due diligence sampling supporting a pending property transaction, however the source is unknown.

In September 2007, prior to ERH treatment, soil samples collected from locations within the treatment area indicated TCE concentrations up to 2,700 mg/kg at 25 ft bgs. The average pre-ERH TCE concentration was 83 mg/kg.

The lithology consisted of 23 to 24 ft of clay and silty clay overlying shale. In some locations, the bottom foot or two of unconsolidated material was more-permeable silty sand. Groundwater at the site was reported to range from 3 ft to greater tha 20 ft bgs.

System Construction/Operations

The ERH system consisted of 19 vertical electrodes with co-located horizontal vapor recovery wells to prevent unnecessary entrainment of groundwater. The electrodes were completed to variable depth intervals to the weathered shale and six electrodes were installed two feet into the weather shale bedrock. The top of the conductive interval started at 7 ft bgs.  Subsurface temperatures were measured at five temperature monitoring points (TMPs) with thermocouples spaced at 4-foot depth intervals through the heated volume. The ERH system consisted of a custom 500 kW power control unit (PCU) and condenser/cooling tower unit, and a 25 horsepower blower for vapor recovery Vapor treatment consisted of two 2,000 pound granular activated carbon vessels (VGAC) installed in series.

System operations began in the fall of 2011 following a two-week start-up. Operations were complete in less than four months, TRS applied 699,000 kWh of energy to compelte the remediation. The average rate of energy applied was 230 kW, The TCE NAPL/groundwater boiling point of 85°C was reached after 45 days of operations during which the rate of greatest mass removal occurred, Groundwater boiling was reached after 90 days of operations, Based on the vapor concentration data collected, TRS estimates that approximately 2,047 pounds of VOCs were physically recovered from the subsurface by the application of ERH.

Interim soil sampling analytical results showed that 68% of the treatment area had reached the cleanup goals, TRS removed 13 of 19 electrodes from service after 13.5 weeks in order to concentrate energy on the remaining two areas with elevated TCE concentrations. After another 3 weeks of ERH opeations, the remaining 6 electrodes were taken off line.

Results

Soil sampling and analysis showed that average concentrations of TCE in soil decreased over 95,5% in the treatment volume following the application of ERH,   The average post-ERH Freon 113 concentration for all soil samples in the treatment volume was 0,05 mg/kg, which correlates to a 99,9% reduction. Figure 2 illustrates the average TCE and Freon concentrations in soil pre and post ERH remediation.

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