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PERMEABLE REACTIVE BARRIER
CASE HISTORIES |
GROUNDWATER REMEDIATION SYSTEM
Old Hickory, Tennessee
 Groundwater remediation at an active chemical plant in Old Hickory, Tennessee involved the phased construction of two Permeable Reactive Barrier (PRB) walls for the insitu treatment of contaminants. Reactive barrier walls are passive treatment systems that permit groundwater flow through a specific treatment medium. Contaminants in the groundwater react with the installed medium and are either degraded, precipitated or absorbed, depending on the type of contaminant and the medium used.
The two-foot wide walls were 850 and 475 feet long, with maximum depth of excavation being 32 feet and 49 feet respectively. Construction was accomplished using the Biopolymer (BP) slurry trench method and keyed two feet into the underlying clay stratum. In phase 1 of the project, construction of the 850-foot wall, trenching was performed with a 330 Caterpillar hydraulic excavator capable of working to depths of 40 feet. A LS7400 Link Belt excavator with extended dipper stick was mobilized for the second trench. Excavated materials were placed directly into tandem dump trucks and transported to a spoils area at the site.
Immediately after trenching began, BP slurry was pumped into the cut to near ground surface level to maintain sidewall stability. During trenching, depth of the excavation was verified by “sounding” with a weighted tape.
Prior to placement of the iron-sand treatment medium, temporary PVC “development wells” were placed into the excavation on 50-foot centers to induce the hydraulic flow of groundwater towards the completed barrier. To prevent component segregation, the iron-sand mixture was tremie-placed from ready-mix trucks into the bottom of the excavation through the stabilizing slurry. The level of the iron-sand mixture was determined by continuous sounding.
Although, the BP slurry is biodegradable, an enzyme breaker was circulated through the PRB from the development wells to expedite the degradation process. The broken slurry was discharged at the top of the PRB and allowed to filter back through the reactive barrier. The broken slurry permeated into the in situ soils, reducing the slurry level in the trench and thereby facilitating capping.
After the completion of the iron-sand placement, the top of the trench was backfilled to grade with low permeability fill using conventional earth moving equipment.
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BIOTRENCH CONSTRUCTION
McGregor, Texas
 Ammonium perchlorate (AP) is an oxidizing agent that has been used in solid fuel rocket propellants since the 1940s. Soluble in groundwater, and known to affect the thyroid hormone function, this contaminant has been identified in the water supply systems of a number of States since improved analytical methods, first introduced in 1997, lowered the perchlorate detection level from 400 to 4 g/L.
Following closure of a former Naval Weapons Industrial Reserve Plant in McGregor, Texas, the Department of the Navy was advised by the Texas Natural Resource Conservation Commission (now Texas Commission on Environmental Quality) that AP had been stored, used, and disposed of at the site. A pilot study implemented by the Navy’s Southern Division to evaluate measures to protect down gradient surface watercourses and Lake Belton, a primary drinking water source, from offsite migration of perchlorate-contaminated groundwater included cutoff and collection, exsitu treatment, and insitu biotrench treatment.
A report published by the Southern Division indicated that less than three weeks after the pilot biotrenches were completed, perchlorate concentrations in the trench water had been reduced from 27,000 ug/L to less than 20 ug/L, well within the Texas drinking water standard of 22 g/L. Given these results, coupled with low installation costs and operating and maintenance costs estimated to be 95% lower than other treatment systems, insitu bioremediation was selected as the primary treatment method. Subsequently, Remedial Construction Services, L.P. (RECON) was awarded a contract to install seven new biotrenches, totaling 2,555 linear feet, in another area of the site.
Biotrench Construction
A biotrench consists of a zone of reactive organic and other material, installed in the path of a contaminated groundwater plume. The groundwater flows through the reactive trench under its natural gradient, creating a passive treatment system. As biodegradation occurs in the trench, the contaminants are reduced to non-hazardous compounds.
The 2.5-foot wide trenches were excavated to depths ranging from 10 to 19 feet below grade and keyed into the underlying competent rock stratum. The organic backfill, consisting of 60% drain rock, 20% mushroom compost, and 20% wood chips by volume, was delivered in bulk and combined on site. During mixing of the mushroom compost and wood chips, soybean oil was manually sprayed over the mixture at a ratio of ½ ton per “dry” ton. Finally, drain rock was blended into the mixture before it was placed in the trench. With backfilling complete, geotextile was placed in the excavation, a clay cap was constructed in 8-inch lifts, and reserve topsoil was spread and graded.
Several 2-inch PVC riser pipes were installed in each trench, extending to a surface trench vault. The pipes were connected to a mobile soybean oil injection system, constructed to hold a 520-gallon soybean oil tank connected to a permanently mounted, motor driven pump.
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