Permeable Reactive Barrier (PRB)
- The PRB (treatment wall) is 100% granular iron, 2 ft wide, 152 ft long, begins 4-8 ft below ground surface (bgs) and extends to 24 ft bgs
- The PRB consists of 450 tons of granular zero-valent iron
RCRA Corrective Action - part of an Interim Corrective Measure
|EPA Point of Contact:|
Robert S. Kerr
Environmental Research Center
Nat. Risk Mgmt. Research Lab.
P.O. Box 1198
Ada, OK 74821
|State Point of Contact:|
North Carolina DENR
Hazardous Waste Section
401 Oberlin Rd., Ste. 150
Raleigh, NC 27605
|USCG Project Manager:|
Jim Vardy, P.E.
U.S. Coast Guard
CEU Cleveland Env. Engr.
Elizabeth City, NC 27909
Chlorinated solvents and heavy metals - Maximum concentrations detected during initial investigations included TCE (>4,320 mcg/L) and hexavalent chromium (Cr+6 (>3,430 mcg/L))
Spills and leaks to the subsurface through floor drains and holes in building floor
Type/Quantity of Media Treated:
- 2.6 million gallons (estimated) treated
- DNAPL suspected in groundwater at the site
- Groundwater is found at 6 ft bgs
- The PRB is located in 1 aquifer at the site; this aquifer is influenced by a nearby surface water
- Hydraulic conductivity ranges from 11.3 to 25.5 ft/day
Purpose/Significance of Application:
Use of PRB to treat groundwater contaminated with TCE and hexavalent chromium; extensive sampling conducted to evaluate PRB.
Regulatory Requirements/Cleanup Goals:
- Cleanup goals for this site are primary drinking water standards, with the following specific cleanup goals for the aquifer down-gradient of the wall: TCE (5 ug/L) and Cr+6 (0.1 ug/L). A secondary goal of the PRB is to contain the contaminated part of the plume up-gradient of the reactive zone.
- Cr+6 concentrations were below the cleanup goal in all down-gradient monitoring wells in November 1996 and September 1997 sampling events. However, TCE concentrations were above the cleanup goal in four of the six down-gradient wells in September 1997.
- A pilot study performed in 1994 and 1995 was successful at demonstrating the effectiveness of the PRB technology at this site; these results lead to the selection of PRB as the remedy for this RCRA corrective action.
- The data indicate that the TCE plume may not be contained; however, the reason for the elevated TCE concentrations in some down-gradient wells has not been confirmed.
- Estimated costs for PRB were $585,000 ($500,000 in capital and $85,000 in O&M), which correspond to $225 per 1,000 gallons of groundwater treated.
- According to the USCG site contact, by using a PRB, the USCG will save nearly $4,000,000 in construction and long-term maintenance costs, when comparing PRB with a typical pump and treat system.
The Support Center, Elizabeth City (SCEC), is a USCG facility providing support, training, operation, and maintenance associated with USCG aircraft. The facility included an electroplating shop which operated for more than 30 years, ceasing operation in 1984. In December 1988, a release was discovered during demolition of a former plating shop. Soil excavated beneath the floor of the former plating shop was found to contain high levels of chromium. Subsequent investigations indicated that the groundwater had been impacted by chromium and chlorinated solvents. Multiple sources were suspected of having contributed to the groundwater contamination. A full-scale PRB was constructed as part of an Interim Corrective Measures (ICM) associated with a voluntary RCRA Facility Investigation (RFI), with the electroplating shop identified in the facility's RCRA Part B permit as a Solid Waste Management Unit (SWMU).
The PRB used at this site consists of 450 tons of granular zero-valent iron keyed into an underlying low conductivity layer at a depth of approximately 22 ft bgs. The required residence time in the treatment zone has been estimated as 21 hours, based on a highest concentration scenario. The average velocity through the wall was reported as 0.2 to 0.4 ft/day. Analytical data from the first year of full-scale operation show that the cleanup goal for Cr+6 has been met, but not the goal for TCE. Several possible reasons are provided for the elevated TCE levels.