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Permeable Reactive Barriers - ZVI Full Scale Case Study, Massachusetts

ZVI FULL SCALE

The following is a summary of Peter Richard’s findings of the full scale case study: (Richards, 2008)

Background and Site Description

A full scale case study took place at a contaminated site in Needham, Massachusetts. This site had a plume of chlorinated solvents resulting from an electronics manufacturer that was migrating toward two public water-supply wells. The primary chlorinated solvent was trichloroethene (TCE) had as high  as 1,100 μg/L concentrations. The goal of the PRB was to reduce the contaminates to 5 μg/L (the maximum contaminant level) or less.

The PRB was installed in 2001, and data was collected from October 2001 to 2007.

 

PRB Design

The PRB was designed in two sections due to the variability in the TCE concentrations across the site. Zone A was designed to be placed at the location with the highest concentrations in the plume (from surface to depth of 38 feet) while Zone B was designed to be keyed-in to the bedrock surface (from surface to depth of 55 feet). Due to Zone A having higher concentrations of contaminants it had a width of approximately 1.7 feet of iron whereas Zone B had a width of 0.5 feet of pure iron.

Case study Massachusetts Exhibit 2

A cross section of the PRB showing the two types of zones taken from the report

 

Results

Case study Massachusetts Exhibit 7 A figure demonstrating the reduction of the TCE in percentage in comparison between two wells. 

 

The data results showed that the PRB is effective in removing, on average 80 percent of the TCE from the groundwater. In comparing upgradient and downgradient well data the PRB reduced the TCE concentration to 5 μg/L, the goal value of the contamination.

There were however, two locations where at depth the concentration of TCE downgradient showed little to no improvement when compared to upgradient. These high concentrations indicated either a design or construction issue with the PRB. These flaws can possibly be described by: higher permeability values than predicted, higher TCE concentrations in an area that doesn’t have adequate ZVI width, a construction related issue, or improper anchoring to the bedrock. The author analyzed each of these possible contributing factors to determine that a construction related issue was the most likely contributing factor. Analysis of the different parameters can be found in more detail in the paper.

The two primary sources of error that the author contributed to these high concentration values were cobbles being present or a flaw in the trench excavation. A boulder was encountered but not removed by the bedrock which could possibly lead to flow that does not come into contact with the ZVI. However, the author also notes that there was a slurry “failure” during which the slurry had to be enhanced with higher viscosity fluid and elevated pH levels. Boreholes were taken after the ZVI was put in place to determine if the width and mixing of the ZVI was accurate with depth. There were no direct indications of a trench failure, but a micro failure could have occurred leading the author to conclude that a construction-related issue was the most possible reason for higher TCE levels at the bedrock downgradient.

 

Discussion and Conclusions

The author concluded that the ZVI remediated most of the TCE concentrations in the plume, with the exception of the two areas discussed in the previous section. The author notes that the PRB would have had higher TCE contamination removal if the PRB was properly constructed as designed.

               

 

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