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Permeable Reactive Barriers - ZVI Pilot Scale Case Study, Colorado

 ZVI PILOT SCALE

The following is a summary of The U.S. EPA’s findings of the pilot scale case study: (U.S. Department of Energy, 2005)

Background and Site Description

Cotter Corporation, located in Colorado, was a former uranium-ore milling site. The groundwater on this site was contaminated with both molybdenum (Mb) and uranium (U) with average concentrations of 4.8 mg/L of Mb and 1 mg/L of U. On site, groundwater flows through an alluvium containing unconsolidated sand, gravel, and silt. The bedrock is a mixture of clay, sandstone, and coal.  The saturated thickness varies seasonally from 0.5 to over 4 feet from the ground surface. The site is estimated to have 1 gpm of groundwater flow.

PRB Design

In June 2000, a pilot scale funnel and gate PRB was installed using ZVI as the reactive material. The dimensions of the PRB were 30 ft long and 7 ft deep.  In the direction of water flow, there was 2 ft of clean silica placed on either side of 5 feet of ZVI. Leading to a total width of 9 feet. The PRB was secured on either side with a vertical concrete wall. The funnel walls were 285 feet to the west and 85 feet to the east protected by a 36-mil layer of HypalonTM (a chemical, heat, and UV resistant synthetic rubber). Both the funnel and PRB were anchored 3 feet into the bedrock at the base. 

The construction process began with the removal of soil from the ground surface using open excavation technique to a depth of three feet into the bedrock. Next, the concrete side walls and Hypalon wing walls were constructed. The last steps were filling the ZVI and sand and then replacing the overburden soils. The groundwater was pumped away during the entire construction process.

Results

The figures below show the dissolved Mo and U concentrations at different points along the funnel and gate system, from November 2000 to October 2004.

Case study Colorado Figure 5Case study Colorado Figure 6

Figure 5 and Figure 6 from the report 

 

An excavation was done to assess the PRB because of the high down gradient values for both the Mo and U concentrations. The visual excavation showed a cementation of ZVI almost entirely throughout the 5 foot span. Further testing showed that U, Mo, and Ca were the minerals present on the ZVI. Solid uranium was present only from 0 to 0.5 feet from the front of the ZVI at concentrations as high as 3.7 mg/g. Solid Mo reached its highest concentration from 0 to 0.5 feet from the front of the ZVI, ranging from 0.13 to 4.05 mg/g.  Unlike uranium, Mo was found at slightly elevated concentrations up to two feet from the front of the ZVI. Calcium was present in high concentrations (up to 44.2 mg/g) from zero to 4 feet from the front of the ZVI.  The author considers Calcium as the likely cause of cementation and loss of hydraulic conductivity of the ZVI.

Case study Colorado ZVI cementation

A photo demonstrating the cementation of the ZVI from the report

 

Discussion and Conclusions

The mineral deposits that caused the cementation of ZVI greatly reduced the hydraulic conductivity. “By early 2004, chemical signatures indicate that most of the ground water was bypassing the ZVI or was flowing through preferential pathways and not being treated.” (U.S. Department of Energy, 2005) Possible recommendations to prevent these issues include easy PRB accessibility, acid flush to remove calcium deposits, and the installation of a pretreatment zone with gravel and ZVI (Bronstein, 2005). These suggestions may serve to extend the life of the PRB by prolonging or eliminating mineral deposits, which increase the hydraulic conductivity allowing the contaminated water to flow through the PRB.


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