cis-1,2-Dichloroethene, trans-1,2-Dichloroethene, Vinyl chloride
Point of Contact:
Allied Signal FM&T
2000 East 95th Street
P.O. Box 419159
Kansas City , MO 64141-6159
Kansas City Plant,
A permeable reactive barrier (PRB) was installed in April 1998 at the U.S. Department of Energy's Kansas City Plant in Kansas City, MO. Contaminants of concern include 1,2-dichloroethylene (1,2-DCE) and vinyl chloride (VC). Maximum initial concentrations encountered at the site were 1,377 µg/L of 1,2-DCE and 291 µg/L of VC.
The Kansas City Plant site is underlain by alluvial sediments that range from 20-33 ft in thickness. Lower alluvial sediments are characterized by low plasticity clays that overlie basal gravels. The alluvial sediments are underlain by bedrock shale. The basal gravel is the most permeable unit and acts as a semi-confined aquifer. The hydraulic conductivity of the basal gravel is 34 ft/day, while the hydraulic conductivity of the overlying clay unit is 0.75 ft/day.
The PRB was constructed as a continuous trench measuring 130 ft long. Sheet piles were driven into bedrock to support the side walls. The resulting excavation was 6 ft wide. The first 6 ft of the trench above bedrock was filled with 100% zero-valent iron. The remainder of the trench was filled with 2 ft of zero-valent iron and 4 ft of sand. These differing thicknesses were used to compensate for the increased flow-through thickness required for the basal gravel unit. Approximately 8,320 ft3 of reactive iron was used in the permeable barrier.
Design costs were approximately $200,000. Design costs included pre-design site characterization done to obtain additional chemical, hydrological, and geotechnical data. Installation costs were $1,300,000. This includes construction, materials, the reactive material, and hazardous waste transportation and disposal.
Cleanup goals for the site are Maximum Contaminant Levels (MCLs)—70 µg/L for 1,2-DCE and 2 µg/L for VC. The VOC plume is predominant in the basal gravel unit.
Early in the project (1999), results of a sampling event indicated that all compliance wells were below MCLs but that concentrations were slowly rising in a sidegradient well. By April 1999, the contaminant concentration exceeded the MCLs. Based on further monitoring, it was estimated that the PRB captured and destroyed 97% of the contaminant mass that passed through or around the barrier during the initial 17 months of the demonstration.
Sampling conducted in 1999 detected vinyl chloride and 1,2-DCE in a well about 4 ft south of the wall at levels exceeding site clean up standards. Contamination in this well continued to increase throughout the remainder of the year. New wells installed 40 ft south of the wall
detected contamination by the same compounds but at levels below site clean up standards. Wells further south were free of contamination.
A pump test was conducted approximately 60 ft from the northern end of the wall in 1999 to determine the hydraulic conductivity of a former buried channel. It was thought that this channel may act as a conductive zone channeling contamination around (north) of the wall. Results of the pump test and data from newly installed wells in this area showed that the buried channel exhibited a conductivity very similar to that calculated from the original pre-design pump test (38 ft/day vs 34 ft/day). The buried channel was also found to be free of contamination and that upgradient (contaminated) ground water at the walls northern portion flowed into the wall with no bypass.
A detailed hydrogeologic investigation consisting of the installation and sampling of new wells, a pump test, and slug tests was conducted at the south end of the PRB in the Spring 2000. The pump test occurred in a well approximately 40 ft south of the wall. Results of this study identified a zone of high hydraulic conductivity (K>100 ft/day). Water levels also suggested a smear layer at the leading edge of the wall possibly created during construction.
Sampling of the barrier was discontinued in May of 2000. A new well installed about 180 ft downgradient of the wall in April 2000 exhibited contamination over site clean up standards. Regulatory authorities, however, required that an existing pumping well 140 ft downgradient of the wall resume pumping by June 1, 2000. The pumping well was restarted June 1, effectively rendering the PRB ineffective. Sampling of existing monitoring wells for VOC's in and around
the wall occurs once a year for regulatory permitting purposes.
An additional assessment of the wall is currently being conducted exploring
approaches to enhance performance.
The following are among lessons learned in this PRB installation:
- Detailed hydrogeologic characterization is required when designing PRBs. Sufficient pre-characterization for one type of containment system may not be sufficient for another.
- For sites where pump and treat had already been occurring, sufficient time must be allowed for the ground-water flow system and more importantly the contaminant plume to return to ambient non-pumping conditions.
- Installation of the continuous PRB did cause a redistribution of heads and a partial change in plume direction. The wall acted somewhat like an equalization tank redistributing heads. Flow gradient into the north end of the wall was about 4 times higher than at the south end. As a result, some of the ground-water flow at the south end was redistributed around the wall.
The cost and time required for constructing a continuous permeable reactive wall was estimated to be less than for constructing a series of impermeable wall and gate sections. The continuous wall was expected to be constructed with a one-pass deep trenching machine. However, the contractor had difficulties with the machine, which may have been due to the heavy, wet clay. The problems encountered resulted in utilization of conventional sheet pile construction of the permeable wall. This should actually benefit the long-term performance. For example, there was better opportunity during the installation process to verify intimate contact of iron placement with the bedrock surface; additional wall was created by the use of "Z" piles; and uniform, continuous placement of iron was visually verified.