When environmental practitioners hear the term Permeable Reactive Barrier (PRB), likely an “iron wall” most often comes to mind. This is not surprising since iron PRBs have a long history – they were installed as early as 1995 to treat dry-cleaning solvents.
PRBs intercept and treat contaminated groundwater plumes. They are passive and sustainable systems, requiring no energy to operate. Contaminated groundwater flows through unimpeded. Treated groundwater emerges downgradient.
For more than 20 years many attempts have been made to create a PRB for the treatment of petroleum hydrocarbons (PHCs), but until recently each technique has had serious limitations.
For example, microbes can be stimulated to feed on PHCs in groundwater by creating a “curtain” of oxygen, either chemically or mechanically. However, neither of these approaches are truly passive, as ongoing inputs are required. Also bacteria growth can lag in conditions of variable groundwater flow conditions (i.e. a spring melt).
For difficult to excavate sites, this was problematic. Yes, contaminant mass reduction and containment could be achieved using more traditional technologies such as pump and treat and vapour extraction systems. But accumulating operational costs and diminishing returns over time with these systems often leads to demands for a longer term solution for a migrating PHC plume.
The case study below underscores this point. Design considerations and monitoring results of a PRB installed to manage PHCs are also presented.
Case Study: A PRB for PHCs at a Service Station
Groundwater beneath an operating service station was impacted with PHCs that was difficult to access due to concrete rubble buried to 3 m below ground. There was a sensitive wetland located directly adjacent to the service station.
Pump and treat and soil vapour extraction systems had been operating at the site for years. Chemical oxidants were also injected over two events. These technologies were successful in removing separate phase fuels and reducing contaminant mass, but after many years, high concentrations of PHCs remained in the dissolved phase.
Recent (2015) analysis revealed groundwater with up to 10,000 µg/L of PHCs with impacts continuing to migrate offsite into the wetland. Impacted groundwater resided from approximately 7.5 m below ground surface (mbgs) to the top of the bedrock, which ranged from 10 to 16 mbgs.
Due to the concrete rubble, direct excavation of the contamination and/or installation of a traditional groundwater barrier would be challenging, disruptive, costly and ecologically unsound.
Site conditions and constraints pointed to the need for an injectable PRB to manage the dissolved phase PHCs.
PRB Material Selection
Once the physical characteristics of a site are well understood (geology, hydrogeology, groundwater elevations and seasonal variabilities, confining layers etc.) the dimensions and placement of a PRB can be determined.
When selecting the reactive material to be emplaced within the PRB there are two over-riding considerations:
- Sufficient quantities of reactive material must be placed within the PRB to satisfy the residence time required for contaminant removal
- Reactive materials must remain in place and active for the intended life cycle of the PRB (i.e., no wash-out)
This last point is deceptively simple but merits further thought. If your PRB material washes-out (i.e., is soluble or remains in the water column), not only will you lose the functionality of your PRB, but perhaps worse, you could be in violation of regulatory permitting by allowing an injected substance to migrate across a property boundary.
In recent years there have been a number of proprietary formulations brought to market that are designed to help manage dissolved-phase PHC plumes.
Granular activated carbon (GAC), traditionally used in the waste water treatment industry, is well known for its capacity to adsorb and hold organic contaminants, such as PHCs, within its porous matrix.
Today, GAC is recruited for remediation duty via injections into subsurface aquifers for contaminant adsorption.
In our opinion, the best GAC formulations for groundwater remediation contain sufficient concentrations of particle-sized activated carbon (i.e. not “liquid carbon”). This high loading rate is achieved using an injectable high mass slurry formulation. The solids within these slurries then stay at the location where they are injected. This satisfies the two critical design elements identified for materials in a PRB: sufficient dosing and no wash-out.
Recovering contaminant mass from the dissolved phase is a great first step. But it’s important to go one step further and actually degrade the adsorbed PHCs. A new technology (to Canada) allows for both the adsorption and the treatment of the dissolved PHCs. The technology is referred to as a “Trap and Treat” concept. It ensures PRBs can be installed for PHC plume capture and treatment over a long period of time.
BOS 200® is the brand name of the “Trap and Treat” material specifically designed for PHCs. The formulation provides a high dose of slurry injectable activated carbon (in particle sizes that will not wash-out of the PRB) plus nutrients and microbes to degrade the PHCs that are adsorbed. As the microbes (introduced or endemic) degrade the PHCs the adsorption sites on the carbon are freed up thereby regenerating the GAC.
Because the properties of BOS 200® met the design specifications for an injectable PRB, it was the material of choice at the site in question.
Injectable PRB Installation & Results
Pilot-scale injections of BOS 200® were completed at the site in June of 2015. This was followed by a full-scale injection in October of the same year. Approximately 12,500 Kg of BOS 200® was injected in 60,000 L of slurry (12 to 24% wt.) over the course of 10 days. Results of the monitoring program up to August 2016 are presented in the following chart.
Downgradient Monitoring Well Displaying Significant and Sustained Decrease in PHC Concentrations After Injections of BOS 200®
From the data a clear and significant drop in dissolved phase PHCs was immediately observed in the downgradient monitoring well. This quick response is attributed to the “Trap” component of the BOS 200®. The “Treat” component then ensures the PHC contaminants are degraded rather than being released to downgradient receptors over time (once the adsorption sites on the activated carbon are saturated).
At this site, the PRB is effective at intecepting the dissolved phase PHCs, preventing off site migration and protecting the wetland. The data collected to date shows effective performance up to one year later. In fact, the BOS 200® formulation is expected to remain active for many years.
Pump and treat, soil vapour extraction systems and chemical oxidation were used for free-product (LNAPL) removal and PHC mass reductions.
A PRB for managing PHC was successfully installed via injections of BOS 200® at an operating petroleum service station. Immediate (“trap”) and long term (“treat”) reduction of PHCs resulted. The Trap and Treat® PRB finished the remediation work, providing a long-term passive solution.
There is also another relatively new technology being used in PHC PRB applications that makes use of the low solubility of chemical oxidants, such as potassium persulphate, to create slow-release oxidant PRBs. Bruce Tunnicliffe will be presenting on the topic of PRBs for PHC remediation at SMART Remediation in Vancouver on March 2. If you can’t make it to this event, but envision an application for this technology at a site you are managing, Bruce can be reached directly at : (519) 653-8444 x 304.
Until next time: Excellence is Your Choice!
Bruce, Kevin, Mike, Nathan, Pat, Kyle & the rest of the Vertex Team!