Prior to pilot testing, TCE was present in groundwater at concentrations reaching 12,000 µg/L in an onsite well and 330 µg/L in an offsite commercial well in the north plume area. The site is situated in Piedmont metamorphic and granite rocks that have weathered into a 20- to 50-ft layer of residual soil (saprolite) near ground surface. Hydraulic conductivity of the saprolite is approximately 1x10-4 cm/s. A more permeable zone of partially weathered rock (PWR) exists beneath the saprolite and above unweathered bedrock, at a typical depth of 50-60 ft bgs. Impacted groundwater exists primarily in the PWR, which has an average hydraulic conductivity of 1x10-2 cm/s (30 ft/day) and an average groundwater flow rate of 100 ft/yr in the offsite plume area. A bifurcated TCE plume in the unconfined PWR aquifer extends approximately 1,800 feet west beneath the facility and nearly 1,700 feet northeast into adjacent offsite properties (Figure 4).
Pilot tests lead to full-scale ISCO using sodium permanganate in fractured bedrock
The Phase I pilot test was conducted using an existing recovery well and a single monitoring well located approximately 30 feet from the recovery well. The initial field injection employed one gallon of a 40% NaMnO4 solution mixed with the monitoring well water to produce a 2.5% solution that was injected into the subsurface through the well. Post-injection testing of water quality parameters such as pH, temperature, dissolved oxygen, and ORP indicated that oxidation of TCE by NaMnO4 had occurred. Direct correlations between the lowest TCE concentration and an increased chloride concentration (3:1 ratio) and the highest ORP measurement provided additional evidence of TCE oxidation.
Phase II pilot testing was conducted using an existing offsite monitoring well screened in the PWR and located on commercial property with the highest offsite TCE concentration in groundwater (330 µg/L). Four additional PWR monitoring wells were installed upgradient of the existing monitoring well at 20-ft intervals. A total of 7,780 gallons with an average NaMnO4 concentration of 210 mg/L was injected into a single well during four events in May 2001. Over the next three months, four additional 500-gallon injections were completed with NaMnO4 concentrations of 5,600 mg/L. Phase II results indicated that TCE concentrations in each of the monitoring wells decreased significantly after the NaMnO4 injections.
Overall pilot results indicated that no daughter products had formed, oxidant demand was minimal (less than 10 mg/L), and TCE concentrations were reduced to below the maximum contaminant level (5 µg/L) in three of the five wells. In addition, NaMnO4 was highly persistent in the aquifer (more than one year), no screen fouling was observed, and metal mobilization did not occur. The results of pilot-scale ISCO prompted refinement of the site conceptual model to include a better understanding of fracture porosity in the PWR.
Current full-scale ISCO implementation has built upon lessons learned during the two pilot tests, the most important of which was the need to use relatively small injection volumes (only a fraction of estimated pore volume) in order to minimize displacement of treated groundwater. Full–scale operations began with the installation of 12 new wells, (8 injection and 4 monitoring wells) in the north off-facility portion of the plume. The first series of semi-annual injections was conducted in February 2003. Single injections ranging from 250 to 500 gallons in volume were accomplished through gravity feed of a 2% NaMnO4 solution to the screened zone of eight wells. Similar injection events have been conducted semi-annually since October 2003 and are scheduled to continue at least through the end of 2011. The extended treatment duration is needed mainly because of owner restrictions regarding the quantity and location of injection wells on private property in the north plume area.
Spring 2008 sampling results indicate that the areal extent of the northeast plume has decreased approximately 30% since full-scale ISCO began (Figure 4). The maximum TCE concentration has declined to 120 µg/L, and two of the ISCO monitoring wells now meet the TCE target of 5 µg/L. Mann-Kendall statistical analysis of TCE concentrations in five wells within the treatment area shows a statistically-significant decreasing trend in three wells and no significant trend in the other two. Reduction of TCE concentrations within the plume also is suggested by decreased TCE concentrations measured in the nearby Jail House Branch and its tributaries (Creeks A and B, Figure 4). Continued plume reductions beneath impacted properties are expected to lead to several property delistings from the mandated remediation program as early as 2010. ISCO project costs at Tenneco Automotive total approximately $525,000, to date, including $85,000 for the two-phase pilot testing, $170,000 in capital costs, and $45,000 for annual O&M that includes periodic injections. Development of the site conceptual model, which included adjacent onsite and offsite areas addressed by other remedies, incurred an additional $1 million.