An engineered subsurface wall of reactive media installed across a contaminant plume so that groundwater flowing through it is passively treated. Common reactive media include zero-valent iron (for chlorinated solvents and Cr(VI)), apatite (for metals), and granular activated carbon (for PFAS and recalcitrant organics).
Permeable Reactive Barriers are installed by trenching, jet-grouting, or hydrofracturing reactive media into a wall geometry that intercepts the natural groundwater gradient. The plume passes through the wall under its own hydraulic head, the reactive media degrade or sorb the target contaminant, and treated water exits downgradient. PRBs eliminate the energy, capital, and operational footprint of pump-and-treat for plume-control applications.
Reactive media selection drives the design. Zero-valent iron PRBs have over 30 years of field record for chlorinated ethenes and Cr(VI). Apatite barriers immobilize lead, uranium, and other divalent metals. Activated carbon and ion-exchange-resin barriers are increasingly used for PFAS plume cutoff. Wall longevity is governed by reactive-media exhaustion, geochemical fouling, and hydraulic short-circuiting.
The honest cost question is PRB versus source removal, because a PRB treats the plume while the source keeps feeding it. A PRB makes sense when the source is already controlled, when the plume is dilute and slow-moving, when the site will be held for decades, and when the goal is containment rather than closure. Its costs are front-loaded (installation) plus decades of monitoring and eventual media replacement. Source removal with thermal desorption costs more per month of activity but ends the liability: once the source mass is destroyed and verified, the plume attenuates on its own and long-term monitoring obligations shrink with it. For a plume expected to run 30 or more years, discounted monitoring and media-replacement costs frequently approach or exceed the one-time cost of removing the source.
Nelson does not install PRBs and has no stake in selling one. The company's role at PRB-consideration sites is the alternative analysis: a firm cost and schedule figure for destroying the source with mobile thermal treatment, so the owner can compare a permanent endpoint against a perpetual management structure on equal terms.
Also known as: PRB, permeable reactive barrier, reactive barrier, passive treatment wall.
A permeable reactive barrier (PRB) is an underground wall of reactive material installed across a contaminated groundwater plume. Groundwater flows through the wall under natural gradient, and the reactive media (zero-valent iron, activated carbon, apatite) degrade or capture the contaminants without pumping.
A trench or injected zone filled with reactive media is placed across the plume's flow path. As groundwater passes through, chemical reactions destroy or immobilize the target contaminants, and treated water exits downgradient. No pumps or energy input are required during operation.
Typical design lives are 10 to 30 years, governed by reactive-media exhaustion, mineral fouling, and hydraulic performance. Most PRBs eventually require media replacement or rejuvenation, and all require long-term performance monitoring for as long as the source continues to feed the plume.
A PRB costs less upfront but carries decades of monitoring, maintenance, and media-replacement costs because the contaminant source is left in place. Source removal with thermal treatment costs more as a single project but destroys the liability and ends most long-term obligations. For plumes expected to persist 30 years or longer, the lifecycle costs are often comparable, and removal frequently wins.
When the source is already controlled, the plume is dilute and slow-moving, excavation or thermal treatment is physically impractical, the property will be held long-term, and containment rather than closure is the accepted regulatory endpoint.
In-Situ Chemical Oxidation (ISCO): treatment approach
In-situ chemical oxidation: injection of a chemical oxidant — permanganate, persulfate, or Fenton's reagent — directly into contaminated soil or groundwater to destroy dissolved-phase organic contaminants without excavation.
An ex-situ soil treatment process that heats contaminated soil to volatilize and separate organic contaminants from the soil matrix, after which the liberated vapors are captured and treated, usually in a high-temperature oxidizer.
A thermal desorption configuration where the heat source contacts the soil directly inside the rotary unit, delivering high heat-transfer efficiency and supporting higher treatment temperatures.
A thermal desorption configuration where heat is conducted through the wall of the treatment vessel without combustion gases contacting the soil, producing a concentrated, lower-volume vapor stream suitable for condensation and recovery.
Treatment that heats contaminated soil and groundwater in place, typically with electrical resistance, thermal conduction, or steam injection, to volatilize and recover contaminants without excavation. Used where excavation is impractical or where the contaminant zone sits beneath active infrastructure.