A Pilot Study Using HRCTM to Enhance Bioremediation of CAHs.
ABSTRACT: The effect of Hydrogen Release Compound (HRCTM) in enhancing the anaerobic bioremediation of chlorinated aliphatic hydrocarbons (CAHs) was observed in a single well pilot study. HRC is a proprietary, environmentally safe, food quality, polylactate ester formulated for the slow release of lactic acid upon hydration. Microbes metabolize the lactic acid releases by HRC and produce hydrogen, which can be used by reductive dehalogenators to dechlorinate CAHs. The objective of the pilot study was to enhance anaerobic reductive dechlorination by reducing the redox potential while providing increased levels of dissolved hydrogen.
For the pilot study, eleven four foot long canisters containing HRC were placed into a five inch monitoring well containing trichloroethylene (TCE), cis-1,2-dichlorethylene (cis-1,2-DCE), and vinyl chloride (VC). Initial TCE, cis-1,2-DCE, and VC concentrations in the well were 148 ppb, 2,030 ppb, and 471 ppb, respectively. After one month of treatment, dissolved oxygen was depleted from the well and highly negative redox levels were achieved. Data from the final monitoring event, taking place approximately three months after HRC installation, indicated reductions of TCE, cis-1,2-DCE, and VC concentrations of 96%, 98%, and 96%, respectively. Based on these results, a full scale application of HRC is being planned.
A 3-month pilot study testing the efficacy of HRC (manufactured by Regenesis, San Jan Capistrano, California) in enhancing anaerobic bioremediation of chlorinated aliphatic hydrocarbons (CAHs) was conducted at a site in Florida. HRC is a proprietary, environmentally safe, food quality, polylactate ester formulated for the slow release of lactic acid upon hydration. Microbes metabolize the lactic acid releases by HRC and produce hydrogen, which can be used by reductive dehalogenators to dechlorinate CAHs. Contaminants of concern include TCE, cis-1,2-DCE and vinyl chloride. The contaminant plume, contained in a fine to medium grained sand aquifer, measured 120 feet in length by 60 feet in width (see Figure 1). Due to a flat gradient, groundwater velocity is estimated to be less than 0.1 foot per day.
MATERIALS AND METHODS
Application. For the pilot study HRC was packed in four-foot long, three and one half diameter PVC canisters designed to be inserted into existing monitoring wells. Each canister contained several hundred small holes to allow for the contact of HRC with groundwater passing through the well. On March 30, 1998, eleven canisters were placed vertically into a five inch recovery well RW100 (see Figure 1) covering a vertical distance of approximately 45 feet.
FIGURE 1. Tetrachloroethene plume map identifying HRC pilot study well (RW100) location.
Monitoring Program. For sampling purposes a polyester tube was attached to the canister assembly at a vertical distance of 24 feet. The monitoring program included sampling of the following: 1) EPA method 8010 (Chlorinated VOCs), 2) Gases (methane, ethene, ethane), 3) Nutrients, electron acceptors, inorganics, 4) Alkalinity, TOC, and conductivity, 5) Volatile acids, 6) pH, DO, ORP and temperature, 7) Microbial (total anaerobes & sulfate reducing bacteria). Baseline sampling was performed just prior to the installation of the canisters and at two, four, eight, and twelve weeks following installation. The final sampling event took place on June 22, 1998.
RESULTS AND DISCUSSION
Following approximately 82 days of treatment with HRC, reductions in TCE, cis-1,2-DCE, and VC were 96%, 98%, and 99%, respectively. These reductions are graphically presented in Figure 2. TCE, cis-1,2-DCE, vinyl chloride, acetic acid, lactic acid, propionic acid, sulfate, and total anaerobic plate count data are presented in Table 1. The absence of DO and highly negative redox levels confirmed the existance of a highly reduced environment required for HRC to be effective. Sampling data strongly support the effectiveness of HRC in this single well application. Due to the production of lactic acid breakdown products acetic acid and propionic acid, it is evident that the lactic acid released from HRC was successfully metabolized leading to the production of hydrogen and the subsequent reductive dechlorination of the chorinated compounds. All contaminant concentrations in the well were decreased substantially over the twelve week period of the test, supporting the effectiveness of HRC in this pilot scale application. Based on these results a full-scale application of HRC on the site is being planned.
TABLE 1. 12 week pilot study sampling data