Mitigation and Remedy of Groundwater Arsenic Menace in India

Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision Documentthat impact contaminant concentrations. Increases in pH favor the precipitation of carbonates ofcalcium and iron as well as insoluble metal hydroxides. Decreases in Eh drive reduction ofmetals and metalloids with multiple oxidation states. Finally, an increase in the partial pressure ofhydrogen in subsurface systems supports the activity of various chemotrophic organisms thatuse hydrogen as an energy source, especially sulfate-reducing bacteria and iron-reducing bacteria.Arsenate ions bind tightly to the iron filings, causing the ZVI to be oxidized to ferrousiron, aerobically or anaerobically in the presence of water. The process results in a positivelycharged iron surface that sorbs the arsenate species by electrostatic interactions (Su and Puls,2001). In systems where dissolved sulfate is reduced to sulfide by sulfate-reducing bacteria,arsenic may be removed by precipitation of insoluble arsenic sulfide (As2S3) or co-precipitatedwith iron sulfides (FeS).PRBs can be constructed by excavating a trench of the appropriate width andbackfilling it with a reactive medium. Commercial PRBs are built in two basic configurations: thefunnel-and-gate and the continuous wall. The funnel-and-gate uses impermeable walls, forexample, sheet pilings or slurry walls, as a "funnel" to direct the contaminant plume to a "gate(s)"containing the reactive media, while the continuous wall transects the flow path of the plumewith reactive media (EPA, 1998).PRBs are a passive treatment technology, designed to function for a long time with littleor no energy input. They produce less waste than active remediation, as the contaminants areimmobilized or altered in the subsurface. PRBs can treat ground water with multiple contaminantsand can be effective over a range of concentrations. PRBs require no above ground equipment,except monitoring devices, allowing return of the property to economic use during remediation.PRBs are best applied to shallow, unconfined aquifer systems in unconsolidated deposits, as longas the reactive material is more conductive than the aquifer (EPA, 2001a,b ).The technology relies on the natural movement of ground water; therefore, aquiferswith low hydraulic conductivity can require relatively long periods of time to be remediated. Inaddition, PRBs do not remediate the entire plume, but only the portion of the plume that haspassed through the PRB. Because cleanup of ground water contaminated with arsenic has beenconducted at only few sites, the long-term effectiveness of PRBs for arsenic treatment has notbeen demonstrated fully (EPA, 2001a,b).5.3.2 PhytoremediationPhytoremediation is an in-situ technology applicable to contaminated soil and groundwater. It is designed to use plants to degrade, extract, contain or immobilize contaminants in soil,sediment or ground water (Zhang et al., 2001). Typically, trees with deep roots are applied toground water and other plants are used for shallow soil contamination. This technology tends tohave low capital, operating and maintenance costs relative to other arsenic treatmenttechnologies because it relies on the activity and growth of plants.NIH & CGWB 95