Story

Installation of EHC for VOCs Confidential Client Texas

Project: Installation of EHC for Treatment of Chlorinated Volatile Organic Compounds, Site in Texas

Summary

Adventus installed 1 ton (909 kg) of EHC into a trench and 3 tons (2,730 kg) of EHC into an open excavation at a site that has groundwater contaminated with a mixture of chlorinated solvents.

The Challenge

Two areas of contamination were targeted for treatment. The first was a dissolved plume migrating from a source zone through a thin permeable soil unit overlying bedrock at approximately 12 ft (3.7 m) below ground surface. The second was an open excavation containing groundwater, after contaminated soil had been removed. The client’s site was contaminated with a large range of Contaminants of Concern (COCs) at the two locations (Table 1).

The Solution

To treat these contaminants, EHC was selected for in situ treatment of the contaminants. Adventus recently adapted its patented combination of controlled-release solid carbon and zero valent iron (ZVI) particles to yield EHC, a solid material for stimulating reductive dechlorination of otherwise persistent organic compounds in groundwater.

Following placement of EHC into the subsurface, a number of physical, chemical and microbiological processes combine to create strong reducing conditions that stimulate rapid and complete dechlorination of organic solvents and other persistent compounds. A permeable reactive barrier (PRB) was constructed to intercept the dissolved groundwater plume (Figure 1). The PRB was constructed with an excavator by digging a 3 ft (0.9 m) wide trench down to the top of the bedrock, followed by filling the bottom foot with a mixture of EHC and sand (Figure 2). Only the bottom foot required filling because the groundwater table was less than a foot above the bedrock, and the permeable soil unit was also at this depth. The remainder of the PRB was backfilled with pea gravel. The bottom 1 ft (0.3 m) of the excavation was lined with sand and EHC (Figure 3), and the downgradient wall of the excavation was lined with a 3 ft (0.9 m) high, 3 ft (0.9 m) wide bench of sand and EHC.

The 3 ft (0.9 m) high bench would ensure that any water exiting the excavation at the downgradient side would come into contact with the sand and EHC mixture.

The Result

The installation was completed in early 2004. To monitor effectiveness of the installation, wells were placed upgradient and downgradient of the OPRB as well as within and downgradient of the excavation.

Figure 4 shows results at the excavation (shows complete VOC removal from the source area). Two pairs of upgradient and downgradient monitoring wells are being used to evaluate the effectiveness of the PRB for treatment of the dissolved plume. The data for the first upgradient-downgradient pair are provided in Figure 5, with the upgradient data on the left-hand side, and the downgradient data on the right-hand side.

Large reductions of all of the constituents present continue to be observed after 21 months of field data. Data for the second well pairing are shown in Figure 6, with lower overall reductions potentially due to flow heterogeneity, but consistent performance over time. Bench scale data for PCE and breakdown products were available from two sites other than this one. Using these data, a model was prepared to analyze the expected concentrations downgradient of the trench.

The modeled data were compared to observed concentrations in the field (Figure 7). The results show that the field data, after acclimatization, meet or exceed the concentrations drop expected based on both sets of bench data.

Timeline

The installation was completed in less than 2 days. EHC performance was observed quickly and for an extended period of time (approaching two years at the time of writing).