The Use of Ultrasound for the Rejuvenation of Granular Iron Permeable Reactive Barriers 2004-01-2592

Permeable reactive barrier (PRB) technology involves construction of a permeable barrier or wall of appropriate treatment materials below ground across the path of a groundwater contaminant plume. PRBs containing granular iron, which destroy chlorinated solvents such as perchloroethene (PCE) and trichloroethene (TCE), have saved the government and private industry millions of dollars in remediation costs over the past ten years. In most in-situ applications and low flow rate above ground applications, the iron should last for 10 to 15 years or more before any rejuvenation is needed. It has been suggested that, over time, mineral precipitates could cause a decline in hydraulic conductivity and/or reactivity of the iron medium. Ultrasound may represent a cost effective means of removing these precipitates and therefore extending the lifetime of iron treatment systems, but questions remain regarding this approach, mainly related to the radius of influence of the ultrasound probe and the persistence of any positive changes resulting from ultrasound. This article presents two field evaluations of the ultrasound technique. The results at an in-situ pilot-scale PRB showed a radius of influence of the ultrasound pulse of about 2 ft. Results from an above ground treatment system showed a lack of significant changes in VOC degradation rates after ultrasound treatment attributed to the fact that the treatment system had not yet been running for a long period of time (i.e. The iron reactivity had not been adversely affected by mineral precipitation). A similar radius of influence was measured in the above ground treatment system. Although valuable information was obtained from these trials concerning practical application of the technology (instrumentation requirements, etc.), additional field trials are needed to better evaluate the cost effectiveness of ultrasound approaches.