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PERMEABLE REACTIVE BARRIER
TECHNOLOGY OVERVIEW |
TABLE OF CONTENTS
- INTRODUCTION
- HISTORY
- METHODS
- CONSIDERATIONS
- CONCLUSION
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INTRODUCTION
Permeable Reactive Barriers (PRB’s) can provide cost-effective, long-term solutions for many groundwater contamination problems. The barriers are constructed underground to intercept groundwater flows and to provide preferential flow paths through reactive materials. As groundwater moves through the reactive materials, contaminants are treated and transformed into harmless by-products. |
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HISTORY
The fact that certain chemicals change their form when exposed to metals has been long known. However, application of the chemistry to address environmental remediation problems has only occurred in the past twenty years. Laboratory studies have shown that many common contaminants, including chlorinated hydrocarbons and reducible metals, are rapidly degraded or modified when exposed to reactive materials such as zero-valent iron (ZVI). As a result, numerous full-scale reactive barrier walls utilizing ZVI technology have been constructed in the past decade throughout the United States and Europe. |
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METHODS
In some cases, PRBs can be installed by traditional excavation and backfilling methods. However, in most circumstances Biopolymer (BP) trenching can be very useful for the construction PRBs. The method eliminates the need for shoring and dewatering and allows flexibility of design and quick installation.
BP trenching involves excavating a narrow trench that is kept full of a “biodegradable slurry”. The slurry exerts hydraulic pressure against the trench walls and acts as shoring to prevent collapse. BP slurry will not affect the final permeability of the in-situ soil or ZVI backfill materials.
BP excavations are typically constructed with hydraulic excavators, with widths varying from 2 to 3 feet. (Excavation depths in excess of 100 feet are possible with this technique.) Polymer is combined with water in a colloidal mixer and the resulting BP slurry mixture is pumped through a pipe to the excavation site as needed to keep the excavation full.
After sufficient excavation has occurred, sand and ZVI or other reactive materials, such as chelators, sorbents, or microbes are proportioned, mixed and placed into the excavation; typically through a tremie pipe. (It is acceptable practice to increase the PRB thickness by mixing the reactive material with sand to enhance hydraulic conductivity, facilitate construction, and/or to minimize construction costs.) Once the excavation and backfilling are complete, an enzyme breaker can be added to the trench to expedite bio-degradation of the slurry and the reestablishment of permeability.
The BP trench technique can also be used to construct infiltration and recharge galleries that can increase the effectiveness of PRBs or for the construction of other in-situ groundwater treatment systems, such as carbon filters and air sparge trenches. In addition, slurry walls can be used to create “funnels” to channel groundwater flows through the reactive material “gates”.
In certain circumstances, reactive materials can also be placed by less invasive techniques, such as soil mixing. Single or multi-axis drilling equipment can be used to rotate and advance a specially designed mixing tool into the soil. As the mixing tools advances, reactive materials that are suspended in BP slurry is injected into the soil through the tool. The resulting columns are overlapped to create a treatment zone. This procedure may be advantageous, particularly when cost for disposal of excavated soil is high. |
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CONSIDERATIONS
With proper design and construction, PRBs can be effective for many years without requiring material replacement or maintenance. In contrast, adsorption treatment processes are limited by the loading capacity of the adsorbent material. |
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CONCLUSION
PRBs are an effective solution for many groundwater contamination problems. In addition, special construction techniques such as BP trenching and soil mixing can be used to make PRBs more attractive. |
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