A pugmill was used for S/S treatment of materials prior to placement.

Solidification/stabilization (S/S) treatment is being used as a remedial component to treat contaminated soils and sediments at the Brunswick Wood Preserving Superfund Site in Brunswick, Ga. This site was placed on Superfund's National Priorities List in 1997 and currently is undergoing remedial action under the designation Operable Unit One (OU1).

The site is an 84-acre former wood treating facility, operated from 1958 until 1991. Three major types of wood preservatives prevalent at the time were used at the site: creosote, which consists of many polycyclic aromatic hydrocarbons (PAHs), pentachlorophenol (PCP, which is associated with dioxin), and chromium/copper/arsenate (CCA).

Wood-treating activities were historically conducted at many areas of the site, including process areas, storage areas, the rail spur that ran along the southern end of the site, and several ponds or surface impoundments (IMs). On its western end, the IM-1 pond was used during the 1960s and later buried, while the IM-2 pond was where creosote processing was taking place at the time operations ceased in 1991. On the site's eastern end, the IM-4 and IM-5 spray ponds were used for spent creosote. Wood preservative chemicals were released to surrounding soils and surface waters as a result of these operations.

After a fire in early 1991, the EPA responded with a four-year removal action that included many activities. All but a few of the site structures were demolished and removed. Sludges were dewatered. Wastewater was treated. Drums and lab wastes were disposed off site. Poles, lumber, equipment and scrap metal was recycled or salvaged. And contaminated soils/sediments were excavated and stockpiled onsite in four encapsulated waste cells, each covered by a geomembrane. The EPA's removal costs were approximately $12 million.

In 1997 and 1998, the state of Georgia conducted its own removal action at the former wood treatment site, during which the three largest cells, primarily containing soils/sediments impacted by creosote and PCP, were disposed off site. The state's removal costs were approximately $18.5 million.

The selected remedy for OU1 utilized a containment strategy to encapsulate contaminants remaining onsite. The chosen remedy components included the construction of subsurface barrier walls around the old creosote ponds, which were located on the eastern and western ends of the site. These consisted of trenches that would need to be filled with a soil/bentonite mix and keyed into the weathered limestone aquitard, located 50 to 65 feet below grade.

The proposal called for engineered caps to be placed over the areas contained within the barrier wall. Groundwater outside each of the cap/wall footprints would be treated in-situ with chemical oxidation to enhance natural degradation of contaminants. The caps would include subcaps consisting of approximately 75,000 cu. yards of solidified/stabilized (S/S) materials from the creosote impoundments, Burnett Creek, site soils above the performance standard of 1 part per billion set for dioxin, and the remaining waste cell onsite (which contain CCA-impacted soils). The S/S remedy component could be applied to these subcap materials.

S/S treatment

S/S-treated material mixed to a consistency of damp soil.

S/S treatment involves mixing a binding agent, commonly portland cement, into contaminated soil, sediment, sludge or waste. Successful treatment is accomplished through physical changes to the treated material and, often, chemical changes to the hazardous constituents themselves.

S/S treatment operations may be conducted either ex- or in-situ. During ex-situ treatment, the waste material is first excavated then mixed with the S/S binding agent, commonly with excavators or in pugmills. Ex-situ treatment can be performed entirely on site by a mobile treatment plant transported to the site. During in-situ treatment, contaminated material is mixed with S/S binding agents while the material remains in place , commonly with excavators, horizontal rotary soil mixers or deep soil mixing augers.

S/S treatment technology contributes to greening their remediation processes through sustainable development of a contaminated property. While immobilizing hazardous constituents, the technology also can improve the construction properties of the treated materials, enabling their reuse.

Treatment

S/S treatment at the site was applied to 11,000 cu. yards of surface soils above the dioxin standard of 1 ppb; pond sediments of 55,000 cu. yards; 8,000 cu. yards of materials from the remaining waste cell, and 600 cu. yards of Burnett Creek sediments, for a total volume of approximately 75,000 cu. yards.

These materials were excavated and staged for treatment. An excavator was used to pre-blend the staged material with a 10-percent addition by weight of fly ash. The pre-blended material was then run through a 2-inch screen to remove oversized material. After screening, the material was processed through a pugmill to mix a 10-percent addition of portland cement with the material. Water was added during pugmilling to result in a damp soil-like consistency.

The S/S treatment reduced the permeability of the material for use in the cap while improving its structural properties, which will contribute to the long-term protectiveness of the remedy and facilitate possible future site redevelopment.

Performance standards for the S/S treatment included: (1) unconfined compressive strength (UCS) of at least 100 psi at 28 days, (2) hydraulic conductivities of less than 1x10-6 cm/sec by the falling-head permeability test, and (3) no significant increased leaching of contaminants of concern by the Synthetic Precipitation Leaching Procedure. Quality control tests included UCS tests on every 500 cu. yards of treated material, and permeability and leachability tests on every 1,000 cu. yards. Full-scale mixing began after an initial pilot scale session lasting several weeks, which verified the achievement of performance standards using the mix design.

Reuse of S/S-treated material in capping system

S/S-treated materials were placed, layered and then compacted with a bulldozer. The final cap was at least 3 feet thick and then covered with at least 2.5 feet of soil in order to support proper vegetation growth.

The remedy includes the construction of caps over the former creosote impoundments on the eastern and western ends of the site. The caps will extend significantly beyond the pond footprints of these former impoundments.

The footprints were established for an informal groundwater standard of 1,000 ug/L of total semi-volatile organic compounds to capture groundwater contamination within the barrier walls. The capping system consisted of subcaps, geosynthetic clay liners, and a 2.5-foot thick vegetative soil layer. S/S treated material was used as the construction material for the subcaps. Freshly mixed S/S-treated material was discharged from the pugmill into dump trucks and then transported to the former impoundment areas, where the material was dumped and then spread and compacted by bulldozer. Successive layers of the material were placed to form subcaps of at least 3 feet thick over the pond footprints themselves. Future plans call for a geosynthetic clay liner to be installed on top of the subcaps, and a 2.5-foot soil layer to be placed on top of the geosynthetic clay liner as a vegetative soil cover.

The use of S/S treatment technology in the remedy will contribute to the future redevelopment options of the property. The strength of the capped areas resulting from the cement-treated subcaps will support the replacement of a former rail spur across the western wall/cap, while the eastern wall/cap will accommodate a potential concrete parking area.

Onsite management of the 75,000 cu. yards of S/S-treated material avoided nearly 4,200 dump truck roundtrips to transport the material to off-site disposal and replacement fill. PE