Describe the primary mechanism by which a permeable reactive barrier (PRB) treats contaminated groundwater without active pumping.

The primary mechanism by which a Permeable Reactive Barrier (PRB) treats contaminated groundwater without active pumping relies entirely on passive hydraulic flow and the specific chemical or biological reactions occurring within its reactive zone. A PRB is an in-situ, subsurface treatment zone filled with reactive materials, strategically installed to intercept a contaminant plume. Contaminated groundwater, driven by the natural hydraulic gradient (the natural slope of the water table), flows passively through the aquifer. The PRB is designed and placed directly across the natural flow path of the contaminant plume. A key aspect of its passive function is that the reactive material within the PRB is typically engineered to have a higher hydraulic conductivity – a measure of how easily water can flow through a porous material – than the surrounding native aquifer. This difference in hydraulic conductivity creates a preferential flow path, causing the contaminated groundwater to naturally divert *intoand *throughthe PRB, rather than flowing around it, without any need for external energy input for pumping or active hydraulic control. Once inside the PRB, the dissolved contaminants in the groundwater interact with the specialized reactive material chosen for the target contaminants. This interaction can involve several distinct treatment processes: Firstly, Redox Reactions, where contaminants undergo chemical transformation through the transfer of electrons. For example, zero-valent iron (ZVI) can reduce toxic chlorinated solvents, such as trichloroethene, into less harmful or non-toxic compounds. Secondly, Sorption, where contaminants either physically bind to the surface of the reactive material (adsorption) or are taken up into its matrix (absorption). Activated carbon, for instance, effectively adsorbs various organic pollutants, removing them from the water phase. Thirdly, Precipitation, where contaminants react within the PRB to form insoluble solid compounds that drop out of the dissolved phase and become immobilized within the barrier. This is common for heavy metals which can precipitate as mineral forms. Lastly, Biodegradation, where the reactive material creates a suitable environment (e.g., anaerobic conditions) or provides electron donors/acceptors that stimulate naturally occurring microorganisms to metabolically break down organic contaminants into benign byproducts like carbon dioxide and water. After undergoing these treatment processes within the reactive zone, the now-treated groundwater, with significantly reduced contaminant concentrations, passively exits the PRB and continues its natural flow downstream through the aquifer.