PRB’s are most known and associated with the treatment of contaminated groundwater. The concept is that ground water is re-directed or funneled through a permeable treatment or reactive zone that contains materials that will filter or react with contaminants – only allowing ‘clean’ water to exit the barrier. When low-permeability materials are used to direct groundwater flow – this is often referred to as a Funnel & Gate type PRB.
In a sediment remediation application this same concept can be utilized in a number of different configurations.
Below is a graphic that represents what we term a Vertical Funnel & Gate – because the flow of groundwater up through the sediment moves upward or in a generally vertical direction through the treatment gates or permeable reactive materials. As is shown, the low-permeability AquaBlok® cap re-directs the groundwater to the permeable treatment gates.
An alternative approach is what we refer to as a Horizontal Funnel & Gate. This approach is favored in situations where a shoreline seep zone is impacting sediment only a short distance from the upland area of concern. The general approach here is to place a permeable treatment or reactive layer of material over the sediment – which will act as a higher permeability preferential pathway for groundwater. This permeable treatment layer is then covered by a low-permeability layer of standard AquaBlok material. The goal is to provide a pathway for groundwater to follow that has a higher permeability than the underlying sediment. When the impacted groundwater follows this pathway, contaminants are removed by the reactive material, prior to the water discharging from under the low-permeability layer into the water body.
Fully Permeable Reactive Barriers can also be constructed at the shoreline to intercept potential upland contamination migration. A conceptual model of such a PRB is shown in the graphic below. To design the reactive layer, it is generally considered necessary to have good information regarding groundwater flux, contaminant concentrations and the adsorption kinetics of the reactive materials. Once this information is obtained, then it is typical that the layer is modeled to determine the thickness and likely break-through time, based on a set of design assumptions.
An example of project where an upland seep (arsenic) was addressed using a Horizontal Funnel & Gate approach is provided in the I-475 Installation Profile.