The Ends Report
Permeable reactive barrier passes 10-year test
16 June 2010 14:59 BST
Academics have assessed the performance of Europe’s oldest zero-valent iron permeable reactive barrier. They have concluded that the technology is a reliable long-term remediation method for groundwaters contaminated with chlorinated solvents.
Last month ENDS reported that a team of academics has evaluated the long-term performance of a technology used to remediate groundwater contaminated with trichloroethene (TCE). Their focus was Europe’s oldest commercially installed zero-valent iron permeable reactive barrier (ZVI PRB), which comprises CL:AIRE Technology Demonstration Project (TDP) 3.
Installed in December 1995, by Keller Ground Engineering and Golder Associates, it treats historical contamination beneath Nortel Networks’ site at Monkstown, Newtownabbey, near Belfast. The project was one of the first to be reported by Contaminated Land: Applications in Real Environments (CL:AIRE) as Technology Demonstration Project (TDP) 3.
Researchers led by Queen’s University Belfast found that the barrier was still remediating TCE after ten years of operation. But they recommended modifications to extend its life.
The barrier is an unusual design intended to treat a concentrated and localised plume of the chlorinated solvent. Installed to a depth of 12.2 metres, it comprises a collection well and cut-off wall feeding a steel column reaction vessel which is filled with iron filings that treat the TCE. The treated groundwater flows out via a distribution well.
TCE forms a dense non-aqueous phase liquid (DNAPL) in aquifers, making it difficult to remediate. But zero-valent iron-based barriers have been used to tackle TCE and other halogenated hydrocarbons at a number of sites worldwide, avoiding the need for expensive pump-and-treat groundwater remediation.
Reactive barriers require no energy input and are virtually invisible, as they are buried underground. It is now an established technology, but little evidence has been gathered until recently on its long-term effectiveness, despite the barriers being designed to work for decades.
In a ZVI PRB, iron filings react with the chlorinated hydrocarbon to remove chlorine, leaving simple hydrocarbons such as ethene or ethane, which are readily biodegraded.
Assessment of the Monkstown barrier’s performance in 1996-2000 found that the concentration of TCE in the groundwater was reduced.
The latest study considered performance during the barrier’s second five years of life. Researchers investigated whether the TCE was still being effectively dehalogenated, examined the iron for corrosion and mineral precipitation, and assessed microbial communities and their impact on remediation.
Samples of groundwater were taken twice a year from wells located around and within the barrier. Tests found that remediation had indeed continued, as the chemical analyses were similar to those of the 1996-2000 samples.
Debra Phillips of Queens University Belfast explained: “PRBs are designed to work into the decades and our research found that the Monkstown barrier is doing just that. The latest samples from 2006 show that, even ten years after installation, the barrier was still remediating TCE. What we thought about the long-term performance of PRBs appears to be true.”
But Dr Phillips and her collaborators also found that significant mineral precipitation had occurred within the upper portion of the PRB where the TCE-contaminated groundwater enters the iron filings. Calcium and iron carbonates, iron sulphides, and iron oxides and hydroxides had all formed in the barrier, slowing the flow of contaminated groundwater through it.
She recommended the iron filings in the upper portion of the barrier be replaced to increase its remediation efficiency and prolong its life.
Dr Phillips concluded: “This research shows that a ZVI PRB can remediate TCE over a ten-year period, but the findings also show the need to regenerate portions of the ZVI, in order to extend its effective life to 20 years or more.”
Picked up originally also by VerticalNews (18 & 22/06/10) and Highbeam Research (Ecology, Environment & Conservation) automated news, and the UK Ends Report (Land & Ecology news 16 June 2010; http://www.endsreport.com/)
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Title Permeable reactive barrier passes 10-year test [The Ends Report] Date 16/06/2010 Persons Debra Phillips, Keith Dickson, Jason Ahad, Brad Newton, Trevor Elliot, Bob Kalin