Permeable reactive barrier
Sorption and precipitation are potentially reversible and may thus require removal of the reactive medium and gathered products in order to continue with remediation.
Microorganisms commonly facilitate such redox reactions, exploiting contaminant degradation as a means to obtain energy and materials for cell synthesis.
Reactive barriers containing oxygen-releasing compounds have been used successfully to stimulate aerobic biodegradation of monoaromatic hydrocarbons.
However, clay's low permeability means it cannot be used in flow-through PRBs,[3] but have been proposed for use in slurry walls, landfill liners, and containment barriers.
Most importantly, by modeling the flow, the hydraulic capture zone width (HCZW) and the residence time can be determined.
The HCZW is the width of the zone of groundwater that will pass through the reactive cell or gate (for funnel-and-gate configurations).
Note that reaction will only occur when contaminants, either dissolved in the groundwater or as DNAPL, come into contact with the iron surfaces.
The funnels are non-permeable, and the simplest design consists of a single gate with walls extending from both sides.
The main advantage of the funnel and gate system is that a smaller reactive region can be used for treating the plume, resulting in a lower cost.
[11] PRBs are typically installed by digging a long trench in the path of the flow of the contaminated groundwater.
After the trench has been filled with reactive material, soil will typically be used to cover the PRB, thus eliminating visibility from the surface.
[13] Mendrel technology involves vertically driving a long hollow beam deep into the ground.
[13] This methods utilizes injected fine-grained iron into fractures below the surface that were created using controlled applications of high pressure.
This method can treat plumes to a depth of 100 feet, but the treatment zone is relatively low in the proportion of iron.
Depending on assumptions of controlling factors, longevity estimates can differ by an order of magnitude (e.g. 10–100 years).
[14] A field-scale application of PRBs in groundwater remediation consisted of a treatment zone formed by excavating an area isolated by sheet piles, refilling the hole with a mixture of granular iron and sand, and removing the sheet pile to leave an in situ, permeable, iron-bearing treatment zone.
The contaminants, chlorinated ethylenes (PCE and TCE), were removed, leaving, for the most part, fully dechlorinated groundwater (little vinyl chloride was observed).
The first field-scale implementation of PRB was in Sunnyvale, California, at the site of a previously operating semi-conductor plant.
Granular metal was chosen as the reactive media after laboratory testing using contaminated water from the site.
[13] In 1996 a 46 m long, 7.3 m deep, .6 m thick PRB was installed at a Coast Guard Facility near Elizabeth City, NC.
The goal of this PRB was to remediate a contaminant plume of trichloroethylene (TCE) and hexavalent chromium (Cr (VI)).
Data from quarterly monitoring, tracer testing, and iron cell coring have been used to determine the effectiveness of the site.
[18] Until about 2000, the majority of groundwater remediation was done using "conventional technologies" (e.g., pump-and-treat systems), which have proven costly to meet applicable cleanup standards.
