REMEDIATION SECTION

In-situ Remediation of
Contaminated Ground Water
Using a Passive Reactive Barrier

There are no silver bullets…but a Passive Reactive Barrier can hit your target at sites with contaminated ground water.

INTRODUCTION
Permeable reactive barriers (PRB) are an exciting new technology, the subject of continuing experimentation and research. Since 1994, numerous full scale and pilot scale systems have been constructed around the world at sites contaminated with dissolved chlorinated solvents, metals, and fuel hydrocarbons as the limits of the technology are explored.

BASIS
A PRB is essentially a trench filled with zero valence (rust free) iron. When ground water flows through the PRB, dissolved contaminants react with the iron. For example, chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE) are chemically degraded. The end products are iron chloride and ethane gas. Chlorinated ethanes, on the other hand, degrade via several pathways, one of which has vinyl chloride (VC) as an intermediate. PRB degradation of chlorinated solvents does not produce VC (a compound more hazardous than PCE or TCE). In the case of certain dissolved metals, such as hexavalent chromium, Cr (VI), the contaminant is reduced as the iron is oxidized. The resulting trivalent chromium is either precipitated or incorporated into chromium-iron hydroxides.

PRACTICAL CONSIDERATIONS
The best feature of a permeable reactive barrier is that there are no operating and maintenance (O&M) costs for many years after installation (e.g., electricity, labor, residual waste disposal). Obviously, monitoring of the ground water and periodic reporting will still be required, but eliminating traditional O&M cost items is a significant benefit. So how do we go about deciding if a PRB is the right choice? First, a technical analysis is performed. Will the dissolved contaminants react with zero valence iron? Will the resulting compounds satisfy regulatory goals for risk? REMEMBER: Not every compound must be degraded completely for total risk posed by the ground water to decline to acceptable levels. Second, the trench system is designed. Critical parameters are: the ground water plume cross-section (height and width), its horizontal flow rate, the type and concentration of contaminant(s), and the nature of the native soil. These are used to determine the length, height, and width of the trench and the number of pounds of iron per cubic foot of backfill used. The amount of iron per cubic foot is varied by mixing the iron particles with a medium to coarse sand prior to placement.

Most experienced remediation construction firms can construct a permeable reactive barrier using a variety of techniques such as an excavator, with a trench box or sheet piling for safety, or overlapping auger holes. In very transmissive sand and gravel aquifers, jetting of iron filings into the pore spaces between particles has been attempted, although this could change local water flow regimes, reducing the efficiency of water flow through the PRB. One additional consideration is that the technology is subject to a U.S. patent. The patent holder requires a site-specific licensing fee equal to 15% of the total cost to construct the PRB.

EXPERIENCE
Given the innovative nature of this technology, most projects to date have been funded by agencies of the federal government. The Payne Firm will oversee installation of a PRB over 800 feet long this summer and has included this technology as a remedial alternative in the Feasibility Study of an industrial facility contaminated with organic compounds. If you would like more information, please contact Jerry Lisiecki or Mike Woodruff at 513-489-2255 or toll free at 800-229-1443 or via e-mail at jbl@paynefirm.com or mlw@paynefirm.com.