In Situ Biogeochemical Stabilization (ISBS) of Creosote/Pentachlorophenol NAPLS Using Permanganate
A growing number of non-aqueous phase liquid (NAPL) removal technologies have been promoted commercially over the past few years, including: thermal-enhanced recovery, surfactant flooding, steam- and temperature-enhanced extraction and others. It is our experience that the effectiveness of the NAPL removal is site-dependent and inversely related to the NAPL saturation. Saturations decline with each gallon on NAPL removed from the formation until their residual saturation is reached, where by definition NAPLs can no longer flow to a well or trench. Although some technologies may be capable of recovering a small percentage of the residual NAPLs, it is technically impracticable to recover a significant percentage of the remaining NAPLs.
At wood treating and related sites, hydrocarbons may be present in the subsurface as NAPLs.. These NAPLs tend to be a long-term source of dissolved-phase organic plumes in groundwater. NAPLs that are present in the subsurface at saturations significantly above their residual saturation can be removed by enhanced recovery technologies; however, NAPLs that are at or below their residual saturations are trapped in the formation and are not recoverable.
An alternative approach to residual NAPL recovery is in situ NAPL management. In situ chemical oxidation for source area stabilization entails the use of a permanganate chemical oxidizer that is flushed through an aquifer zone containing residual NAPLs. The oxidant is not meant to remove NAPL mass entirely. Rather, as the oxidant migrates through the targeted source area, (bio)geochemical reactions between the organic constituents of interest (COIs) and the oxidant cause the destruction and stabilization of NAPL via a two step process: i) oxidation, and ii) dissolution. The biochemical oxidation processes destroy COIs present in the dissolved phase, thereby increasing the dissolution of COIs from the NAPL into the groundwater. The more water soluble, lower-molecular-weight NAPL constituents are then released and chemically oxidized at a proportionally higher rate, thus leading to a 'hardening' or chemical 'weathering' of the residual NAPL mass. The selective removal of the more labile constituents causes a net increase in the viscosity of the NAPL, yielding a more stable NAPL source that is less susceptible to dissolution processes. In addition, the oxidation reaction precipitates manganese dioxide (MnO2) and results in the formation of a chemical 'shell' which further isolates the 'weathered' NAPLs. As such, the flux of COIs into the dissolved phase is decreased, allowing natural attenuation processes to more effectively manage COI plumes.
Proof of Concept
In 1997 scientists, now at Adventus, conceptualized the ISBS technology and began working on basic technology research and development in collaboration with scientists at the University of Waterloo. In 2002, pilot scale field studies were initiated at an operating wood treatment facility in Denver, Colorado (Kopper's Inc., Superfund Site) where 24,050 gallons of 3% aqueous permanganate (KMnO4) solution were injected into 13 locations within a defined test area (75 x 95 x 10 ft deep). Performance monitoring was conducted for 6 months to evaluate the ability of ISBS to stabilize the free-phase NAPL residuals and enhance the natural attenuation processes by: i) mitigating the migration of NAPL; ii) reducing the concentration of COI in the dissolved phase; iii) decreasing the mass of NAPL residuals (source reduction); and iv) reducing the flux of COI from NAPL residuals (especially true with MnO2 precipitate). Field data showed rapid and complete stabilization of NAPL. In addition, mass was reduced by 10 to 79% (Table 1) and the flux of COI was reduced by 56 to 99% (Table 2).
In December 2003, full scale application of ISBS technology at the Denver Site was approved by State of Colorado (CDPHE) and Federal (US EPA Region VIII) regulators, following their rigorous peer review (EPA Research Laboratories) of all existing data. Full scale application was completed in May 2004. A total of 82,553 gal of KMnO4 @ 30 g/L Solution (3%) was added to 44 injection points and 9,789 gal were applied two three trenches (90 gal / LF trench) yielding a total 10,495 kg KMnO4 for 4.5 g KMnO4 / Kg soil.
As applied to the Kopper's Inc. Superfund Site, permanganate application had an effective radius of influence of approximately 15 ft. There was a discernible decrease over time in the thickness of LNAPL for only those monitoring piezometers located within the permanganate treatment area. Changes in NAPL thickness were not observed outside of the treated area suggesting that NAPL migration did not occur. Flux reduction should have a significant beneficial affect thereby accelerating the contraction of the dissolved phase COI plume at the Kopper's Inc. Superfund Site.
The cost of full scale field implementation of the ISBS technology was approximately $200,000. As summarized below, this represents a very cost-efficient, effective alternative for managing NAPL sites.
Materials and Injection $11-15/cy
Drilling / Trenching $2-5/cy
TOTAL = $15 - 25/cy
Data exist to demonstrate that the ISBS technology will effectively stabilize various chlorinated solvents including perchloroethylene (PCE), trichloroethylene (TCE) and carbon tetrachloride (CT); stabilization of hydrocarbons (creosote) and chlorinated pesticides (i.e., pentachlorophenol) has also been documented under field conditions. In-house data show that technology will also stabilize certain metals via the enhanced precipitation reactions. Since January 2004, we have proposed the use of ISBS technology for in situ source management relatively large, complicated sites in South Carolina (creosote), Florida (creosote), Pennsylvania (chlorinated solvents), California (chlorinated solvents), New Jersey (chlorinated solvents), West Virginia (chlorinated solvents), and New York (multiple MGP sites).