Or…Removing a Stain on the Environment
For decades tetrachloroethylene (a.k.a. perchloroethylene, PCE or perc) was favoured as a dry cleaning fluid due to its strong degreasing power, relatively gentle action on fabrics, and non-flammable nature. In time, scientists came to understand the health and environmental impacts of perc releases into soil and groundwater. Perc itself is a suspected carcinogen. In the subsurface it tends to degrade to more toxic breakdown products including, trichloroethylene, dichloroethylene and vinyl chloride (VC) – a known carcinogen. These breakdown products are also more mobile and volatile, leading to serious concerns about vapour intrusion into indoor occupied spaces and off-site migration of these contaminants. Complicating matters, perc is denser than water. Accumulations of significant mass will sink through the soil and down past the water table, rendering the contaminants much less accessible for remediation. The result is that many older dry cleaning facilities carry environmental liabilities that present significant liabilities and obstacles to refinancing or rezoning.
In recent years, in situ chemical reduction (ISCR) technologies have proven extremely powerful for remediating the legacy of dry cleaning operations. Its power lies within these three critical advantages:
(i) ISCR goes with the flow – most subsurface environments tend toward anaerobic and reducing conditions. ISCR formulations drive subsurface reducing conditions even lower and often also provide a source of food to kick anaerobic degrading bacteria into high gear.
(ii) ISCR creates an unfriendly environment for contaminants – in fact, direct contact with the contaminants is not always necessary. ISCR sets up an environment which physically destabilizes contaminant molecules, leaving them susceptible to direct chemical breakdown and/or consumption by the soil microbes.
(iii) ISCR is an Inject-and-Leave approach – often one dose is enough. It’s more passive, often less expensive and less disruptive than alternatives including excavation and shoring, soil vapour extraction, and other in-situ treatment methods.
The ISCR approach is also particularly favoured at sites where back-diffusion is anticipated to cause on-going recontamination of groundwater. That is, within geologies consisting of tight silty or clay soils or bedrock. Once the proper conditions are established via ISCR, any desorbing contaminants are continuously degraded.
A bench scale test conducted at Colorado State University to demonstrate the effect of back diffusion.
- Iron (zero valent or ferrous iron)
- Fermentable carbon sources (e.g. emulsified vegetable oils, lactates, lecithin, molasses)
- Trap and TreatTM – a combination of activated carbon and iron
- Dehalococcoides microbes – a mixture of bacteria possessing the enzymes to fully degrade PCE without stalling at VC
Some sample projects to illustrate these concepts:
1. At a site in Brantford, Vertex recently mixed zero valent iron (ZVI) into subsurface soils using specially-designed soil mixing equipment (a Lang Tool). Super sacs of ZVI were blended in-situ at a pre-determined ratio to knock out dry cleaning solvent impacts in soils. On-site QA/QC measurements confirmed the design distribution of ZVI in the subsurface was achieved.
Approximately 4,000 metric tonnes of soil was successfully treated in place rather than being excavated and sent for off-site disposal. Additionally, proper conditions were established for the on-going reduction of contaminants in groundwater.
2. At a site in Mississauga, Vertex mixed ZVI with backfill material in an excavation to ensure the degradation of PCE and breakdown products in groundwater. The remedial design was enhanced by the installation of horizontal wells (HWs) during backfilling. Fermentable substrates were then injected into the HWs and also into temporary vertical injection points to stimulate anaerobic bacteria to degrade residual groundwater impacts.
3. At a former dry cleaning facility in Waterloo, Vertex injected an organic carbon substrates and ZVI along the downgradient property boundary to depths of up to 11 metres below grade. The resulting injectable permeable reactive barrier (PRB) established a reducing zone that degrades contaminants, preventing off-site migration. Pre- and post-injection groundwater results from the worst-case monitoring well on the downgradient property line are shown below:
|Parameter||Worst-Case Monitoring Well Results (µg/L)|
Several post-injection monitoring events shown that all groundwater results now meet the MOECC generic and site-specific clean-up objectives.
Releases of dry cleaning fluids into the subsurface can be effectively managed with the right choice of modern technologies. ISCR provides a solution that works with the naturally existing geochemical conditions, often eliminating the need for highly disruptive alternatives. The results speak for themselves: we have noticed a steady increase in the application of ISCR technologies over the past several years. We believe this is mainly because ISCR offers an efficient, cost-effective, long-term solution for managing liability at legacy dry cleaning facilities.
Until next time: Excellence is Your Choice!
Bruce, Kevin, Mike, Nathan, Pat, Kyle & the rest of the Vertex Team!