Title: Introduction Text: The primary objective of the Interim Corrective Measure (ICM) at Solid Waste Management Unit 12 (SWMU 12) Naval Weapons Station (NWS) Charleston, South Carolina was to prevent the continued migration of the contamination from the source area to a downgradient marsh. Additional site cleanup objectives included the following: Preserve natural resources Minimize impact to landscape Minimize operation and maintenance Utilize green (ecologically derived) processes when practical Visual Description: Aerial map of Solid Waste Management Unit 12 showing distribution of contaminants in source area, high dissolved area, and low dissolved area (fringe).

Title: Site Treatment Zones Text: Contamination at the site consists of three primary zones. Source - high concentrations in low permeability materials (potential DNAPL ganglia) High Dissolved Concentrations Low Dissolved Concentrations - at fringe and tail of plume Several factors went into the selection process of the remedial actions, including the site history, setting, groundwater contamination levels, and other issues. The remedial actions chosen to meet the cleanup objectives were: engineered phytoremediation for the source zone; native phytoremediation and MNA for the low dissolved concentration zone; and a permeable reactive barrier (PRB) consisting of zero valent iron (ZVI) reactive medium for the high dissolved concentration zone. Visual Description: Illustration of contamination site treatment zones: engineered phytoremediation, native phytoremediation and MNA, and PRB.

Title: Phytoremediation: Technology Overview Text: Phytoremediation is a soil and groundwater treatment technology that uses vegetation to remove, contain, or reduce the toxicity of contaminants. It can be implemented by using existing plant-life at the site or by establishing a selected plant or community of plants. The technology exploits the natural hydraulic and metabolic processes of plants such as: phytostabilization; phytodegradation; rhizosphere degradation; hydraulic control; and phytovolatilization. Since preserving natural resources and minimizing the impact to the landscape were two of the main objectives of this project, utilizing the mature lowland forest for phytoremediation and natural attenuation downgradient of the source area became part of the remedy design for the site. In addition to utilizing the existing trees on site, an engineered phytoremediation plot was constructed in the source zone area. Visual Description: Graphic animation of phytoremediation. Illustrations of bioremediation, contaminant, root uptake, plant uptake accumulation and volatilization are shown.

Title: Engineered Phytoremediation Plot Text: To treat the contamination without encouraging the downward migration of these solvents into the lower more permeable formation in the source area, loblolly pine trees were installed to create a mechanism for direct uptake, phytovolatilization and improve soil structure that will enhance biodegradation in the newly formed rhizosphere. The source zone groundwater is biologically degrading naturally through electron donors supplied by the abiotic generation of acetate from 1,1,1 Trichloroethane (1,1,1-TCA) and the naturally occurring carbon in the aquifer sediments. Visual Description: Photos of the installation of loblolly pine trees.

Title: Native Phytoremediation/ MNA Text: During phytoremediation, dissolved contaminants are extracted from groundwater into the root system and up into the tree. The contaminants are then mineralized and/or transpired from the tree (e.g. volatilized into the atmosphere). Both tree cores and sap meters were used to measure the effectiveness of phytoremediation at SWMU 12. Tree cores were sampled to track the presence of contaminants and sap meters were used to measure the seasonal transpiration rate and the trees' influence on contaminant attenuation. Preliminary results indicate that the trees have a considerable impact on the movement of the groundwater plume throughout the year and help to reduce migration of contaminants. More data from this site will be available later in 2004. Visual Description: Photo of sap meters in trees.

Title: Roots in the Primary Contamination Zone Text: The root hairs collected from the primary contamination zone, and the data collected from the tree cores and the sap meters indicated the direct uptake of groundwater from the native trees downgradient of the source zone. The mature lowland forest incorporates direct uptake for phytovolatilization and is part of the attenuation processes beyond the PRB. Taking advantage of the naturally occurring passive processes (MNA and phytovolatilization) allowed the project team to enhance the existing processes using low energy techniques (PRB and engineered phytoremediation) such that treatment of the plume should be complete prior to discharge to the freshwater marsh. In addition, impact to the landscape was minimized as all treatment processes are part of the natural environment or are below ground. Visual Description: Photos of root hairs 8.5 - 9.0 ft. below land surface.

Title: Permeable Reactive Barrier Text: An Interim Corrective Measure (ICM) was needed to minimize the continued migration of contaminants toward the downgradient marsh. Several factors went into the selection process including the site history, setting, groundwater contamination levels, and other issues. After conducting a bench-scale study, a permeable reactive barrier (PRB) consisting of zero valent iron (ZVI) reactive media was selected for implementation at SWMU 12. A permeable reactive barrier, in its simplest form, is a trench built across the flow path of a groundwater plume. The trench is filled with a suitable reactive or adsorptive medium that removes the contamination from the groundwater, thus protecting downgradient water resources or receptors. The treatment media is selected for its ability to clean up specific types of contaminants. The following slides provide case study information from the design and installation of the PRB at SWMU12 in 2002. Video clips of key steps in the process and lessons learned are also highlighted. Visual Description: Graphic animation of permeable reactive barrier. Contaminant spill, contaminant plume, groundwater flow, permeable reactive barrier and clean water are illustrated.

Title: Bench-Scale Study Text: A bench-scale study was conducted to evaluate the effectiveness of the Zero Valent Iron (ZVI) media in treating chlorinated volatile organic compounds (VOCs) to meet cleanup objectives. The study also assessed the potential impact of lactate (an electron donor source) on ZVI performance. The lactate, or a similar substrate may be injected during implementation of the final remedy (post-PRB installation) to enhance bioremediation. The study results indicated that ZVI reductive dechlorination can completely treat the anticipated VOC concentrations in the groundwater. During the bench-scale study, the ZVI performance was not affected by the presence of lactate at concentrations of as high as 1,000 mg/L. The necessary residence time for the PRB was also estimated based on reaction kinetics measured in the lab, VOC concentrations, and site hydraulics. Visual Description: Photo of a technician performing a bench-scale study.

Title: Text: The iron used in PRBs is typically elemental iron (or Zero-Valent Iron (ZVI)) derived from scrap cast iron. A particle size range of -8+50 mesh has been used in most applications. In general, finer iron particles provide a larger surface area, and therefore greater reactivity. Larger iron particles typically provide a higher hydraulic conductivity, and therefore more effective groundwater plume capture. In the -8+50 mesh-size range, the iron is expected to exhibit optimum reactive and hydraulic properties.

Title: PRB Design Text: The PRB was designed to cost-effectively achieve the remedial action objective, while considering the site-specific conditions and constraints, as well as minimizing the impact to the landscape. Groundwater flow modeling and contaminant transport modeling were performed to evaluate potential site conditions and constraints that may affect the PRB performance. The PRB was designed to be a reactive wall extending from approximately 3.5 feet below ground surface (bgs) to approximately 32 feet bgs, and spanning approximately 130 feet. The design also called for soil backfill from 3.5 feet bgs to the ground surface. Visual Description: Cross sectional view of the sub-surface layers around the PRB.

Title: SWMU 12 Site Setup Text: Prior to conducting any subsurface work, the following items must be considered: Coordinating the base digging permit from the Navy Coordinating the underground injection control (UIC) permit Utilities Badging requirements for personnel and subcontractors Potable water, sanitary facilities, electrical hookups Mobilizing necessary equipment Visual Description: Photos of equipment at the SWMU 12 site.

Title: Site Preparation Text: Site Preparation activities at SWMU 12 NWS Charleston included: Establishing work zones Removing and relocating the power pole and several trees that were located adjacent to the proposed layout of the PRB Removing the existing fence and gate Removing and relocating the weather station that was located near the proposed layout of the PRB Site clearing and grubbing Building a pad to raise the ground surface to 5 feet above the water table Visual Description: Video of site preparation activities at the SWMU 12 NWS Charleston.

Title: Barrier Installation Text: The PRB installation trench was excavated using a biopolymer slurry for trench support. The ZVI and sand were mixed on site and added to the trench one section at a time. The PRB was designed to consist of three sections as follows: 40 feet in the center with 100% ZVI 40 feet in the middle with 50:50 ZVI:sand by weight 50 feet in the end with 20:80 ZVI:sand by weight. Visual Description: Photo of trench excavation for barrier installation.

Title: Guar Gum Slurry Preparation Text: The biopolymer slurry was prepared on site in a biopolymer slurry mixing plant. Prior to the slurry preparation, the tank was flushed with a bleach-water solution to sterilize the tank. The slurry was prepared by mixing guar gum and water in the mixing plant. The pH of the slurry was continually checked and adjusted, if necessary, by the addition of soda ash. Once trenching activities began, the slurry was pumped into the trench to support the trench walls during excavation and until the iron/sand is added to the trench. The slurry properties were continually monitored during the construction activities. The slurry was tested for pH, viscosity, and density. Visual Description: Video of guar gum slurry preparation.

Title: Trench Excavation Text: The PRB trench was excavated from the ground surface to an approximate depth of 32 feet below ground surface (bgs). The trench excavation began at the northeast end of the PRB and was continuously excavated to reach the southwest end of the PRB. The guar gum bioslurry was pumped into the trench, as needed, throughout the excavation in such a way as to continually displace the excavated soils and maintain the slurry level at approximately 1 foot below ground surface. Spoils generated from the excavation were stockpiled and dried in designated waste staging areas. Visual Description: Video of trench excavation.

Title: Addition of Zero Valent Iron Text: The reactive materials (ZVI and sand mixture) were prepared, as required, for the placement of the reactive material in the completed sections of the PRB trench. The mixes were proportioned by weight of sand and the ZVI (e.g. middle section = 50:50 ZVI and sand). The sand was proportioned using a loader scale for loading into the ready-mix truck. The weight tickets on the bagged iron were used to ensure proper weight of the ZVI in the mix. The ZVI and sand were mixed thoroughly into a homogeneous blend. Samples of the prepared mixes were routinely collected and analyzed for the sand:ZVI proportions by weight. When the mix was ready, batches of reactive material were added to the trench. Visual Description: Video showing addition of zero valent iron.

Title: Backfilling the Trench Text: After completion of the reactive material placement over the entire length of the PRB, a liquid enzyme breaker was introduced through slotted PVC pipes (installed during reactive material placement), and circulated through the trench. This process was performed to ensure complete degradation of the slurry. After the completion of the PRB, the remainder of the PRB trench was backfilled with clean imported sand, and soil. The final backfilled grades were graded to smoothly transition into the surrounding grades and to ensure that no ponding of surface water would occur over the PRB location. Visual Description: Video showing backfilling of the trench.

Title: Monitoring Well Placement Text: The PRB monitoring system consists of a total of eight well clusters and two shallow monitoring wells. Each well cluster includes one shallow monitoring well and one deep monitoring well. Visual Description: Graphic of monitoring well placement. Monitoring network and PRB reactive material distribution are shown.

Title: Lessons Learned Text: The original excavation at the NWS, Charleston collapsed. The site engineer describes several factors that may have contributed to the collapse of the trench. Some of these factors may have been: The use of heavy equipment around the edges of the trench Heavy rainfall may have loaded the soils around the trench Heavy rainfall brought the water table up 0.8 feet, which reduced the head of the slurry over the water table The trench was excavated in one complete section, without the use of end stops, which could have greatly increased the load across the trench The excavation was open for about 36 hours without the addition of iron The collapse of the trench cannot be attributed to one specific factor. In this case, many factors may have caused the collapse. The collapsed trench was backfilled, and a second trench was installed approximately 25 feet downgradient from the original placement. The second PRB was constructed in 20-30 foot sections by trenching and backfilling with reactive material in one day. The installation of the second trench was completed without any major setbacks. Visual Description: Video of site geologist presenting lessons learned.

Title: Contact Information Text: For more information about the Charleston Case Study, please contact: NFESC POC (805) 982-1656 PRTH_NFESCT2@navy.mil