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Permeable Reactive Barriers - ZVI Pilot Scale Case Study, Montana


The following is a summary of Richard Wilkin’s findings of the pilot scale case study: (Wilkin et al., 2009)

Background and Site Description

This case study followed a pilot scale PRB design in East Helena Montana. This site was the location of a lead smelter that operated for over 100 years from around 1888 to 2001. The ground water under the site was contaminated with arsenic, selenium, lead, cadmium and zinc. There were also reports of plumes of arsenic and selenium that have travelled offsite. The contaminated groundwater traveled through unconsolidated alluvial deposits containing cobbles, gravel, sand and silt. Fine-grained volcanic ash tuff deposits lie beneath this alluvial deposit. The saturated thickness of the alluvial deposits ranged from 4.9-5.8m. 

Case study Montana EPA fig 1

Figure demonstrating the contaminant plume and the recommendation of the PRB location taken from the report (EPA, 2006)


PRB Design

The pilot study used a granular iron reactive medium PRB with a length of 9.1m, a depth of 13.7m deep and a width of 1.8-2.4m. It was installed over a three day period using bio-polymer slurry methods and modified excavating equipment for deep trenching. The trench was backfilled with the granular iron from 13.7m-6.1m, with the remaining height being filled with sand. In order to make sure that the slurry didn’t affect the permeability of the wall, it was necessary to degrade the slurry with a flushing process. This process took approximately three days. 

Case study Montana EPA fig 2

A figure demonstrating the trench construction and excavation taken from the report (EPA, 2006)


The top of the granular iron was >2m above the maximum groundwater level observed. However the bottom of the granular iron was approximately 1m above a layer of ash tuff, meaning the PRB was a “hanging wall” design. This hanging wall was destroyed to examine the effects of bypass processes.

The EPA estimated the construction cost at $325,000. (EPA, 2006)



A monitoring network of wells were placed upgradient of the PRB, within the PRB, and down gradient of the PRB. Ground water samples were collected at 12 months, 15 months and 25 months downgradient. “After over 2 years of monitoring a pilot-scale, zerovalent iron PRB, results indicated arsenic concentrations >25 mgL-1 in wells located hydraulically upgradient of the PRB. Within the PRB, arsenic concentrations were reduced to 2 to <0.01mgL-1.” (Wilkin et al., 2009) This showed that in the region with the PRB there was ~99% removal of arsenic from the groundwater. There were also no observed changes in the hydraulic conductivity of the PRB that would indicate corrosion and a precipitation build-up.  

The results of the arsenic concentrations versus the side view of the PRB are shown in the following figure.

Case study Montana Fig 11

A demonstration of the flow of contaminants through the PRB


The spike in arsenic in the region where the PRB was not anchored into the ash showed that the groundwater was flowing beneath the PRB. This problem can be fixed with proper analysis of the subsurface and anchoring the PRB into an impermeable boundary forcing the arsenic to go through the PRB.


Discussion and Conclusions

The case study concluded that ZVI is effective in treating arsenic contaminated groundwater as long as the geochemical and hydrological conditions were properly analyzed. The study also mentioned that even though ZVI is accurate in removing arsenic from solution, it has a finite capacity. The flux evaluations in this study showed that the ZVI is highly effective if the amount of arsenic loadings are below 5 g As m-2d-1. Therefore analysis of the amount of arsenic flowing with depth needs to be accounted for in the design requirements of the PRB.


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