Written by Pinal Patel and Mananki Patel, Staff Writers

On May 11, 2022, GeoPro Talks hosted a Webinar on the topic of Permeable Reactive Barriers. The speaker, Kevin French, is a VP at Vertex Environmental.

A Brief Introduction About the PRBs.

PRBs intercept and treat contaminated groundwater plumes (passive). PRBs Allow groundwater to flow through unimpeded but trap the contaminants and treat them. A PRB can be installed in the subsurface either by excavation of the soil or by injection of the barrier.  It can be excavated or injected.

Permeable Reactive Barrier

How a PRB works

The Permeable Reactive Barrier (PRB) relies on a passive technique, meaning it requires no external energy to force the contaminated liquid through the barrier. To allow the water to flow through the reactive material, the reactive barrier material must have a higher hydraulic conductivity than the surrounding contaminated soil. The PRB is then placed perpendicular to the direction of the water flow allowing the contaminated water to travel through the reactive material for the best removal results. Within the barrier the contaminants are either absorbed, chemically or biologically degraded. Once the chemical process is complete, the remediated water continues downstream via natural flow.

Advantages and disadvantages of PRBs.

One advantage of a PRB over other groundwater treatment methods is that they require no energy to operate.  Once installed, the natural flow of groundwater coupled with the physical/chemical/biochemical reactions in the wall neutralize the contaminants in the groundwater

Why would you need PRBs?

First and foremost, the benefit of a PRB is that it prevents off-site migration and reduces the risk to sensitive receptors: human health or ecology.

Difference in design between the old school and Modern PRBs

In the old school era generic assumptions were used. First step in the design is to dig a trench then mix up some ZVI (zero valent iron) and sand. After that Hope for the best if it is under design then it leads to failure and over-design is costly.  

Today PRBs are designed and installed in a better way through a specific process. This process includes different steps like ask, research, improve, imagine plan, create, test, and improve. Before the practice data are collected related to the Contaminant Type, Contaminant Concentration Depth of Water, Remedial Objective, and Sensitive Receptor. Site characterization added extra benefit to it, and it included data regarding groundwater flow direction, groundwater flow velocity and 2D contaminant distribution. 

PRB Alignment Profiling

Nowadays, High-Resolution Site Characterization (HRSC)such as laser induced fluorescence, membrane interphase probes, and hydraulic profiling tools are used to better understand subsurface conditions prior to the installation of a PRB.

 HRSC tools can detect Light Nonaqueous Phase Liquids (Laser Induced Fluorescence) Volatile Organic Compounds (Membrane Interface Probe) and permeability (Hydraulic Profiling Tool). 

HRSC has been used at 100s of sites across Canada. 

One form of PRB is Activated Carbon-Based injectables.  This type of PRB has been used in Canada since 2015. It is used for NAPL sites, more recalcitrant compounds, and high soil mass of contaminants. It is known as trap and treat technology. Firstly, contaminants are trapped or absorbed and then treated or degraded with the chemicals in the PRB or aerobic bacteria within the PRB. Bench scale testing is beneficial.

PRBs are no longer “old school” using rule of thumb and hope for the best. Advance science and technology have made the “old school” method obsolete today. It is now possible to more intelligently design, install and validate a PRB.