Passively Enhanced In-situ Biodegradation of Chlorinated Solvents
Harding Lawson Associates (HLA) conducted a field demonstration to evaluate in-situ enhanced anaerobic biodegradation of chlorinated solvents using a hydrogen-release compound (HRC). The technology is designed to enhance the natural biodegradation process for a more rapid and complete degradation of the organic contaminants to non-toxic compounds (e.g., ethylene). This article presents the results from a field demonstration that was conducted over an eleven-month period.
Technical ApproachAnaerobic biodegradation of chlorinated solvents such as perchloroethene (PCE) and trichloroethene (TCE) require highly reducing conditions to stimulate anaerobic bacteria to dechlorinate the solvents. The technical approach was designed to provide a carbon or electron donor source to create the conditions necessary to enhance anaerobic biodegradation.
HRC is a lactic acid ester. When delivered to the subsurface, lactic acid, which has been shown to be an effective electron donor, is released continuously into ground water. Because this material can be injected directly into the subsurface, it can be used to deliver nutrients passively and enhance active biodegradation of chlorinated volatile organic compounds (cVOCs).
HLA conducted a field demonstration to evaluate the performance of HRC using a system designed to recirculate ground water and create a treatment cell. The objectives of this test were to evaluate the rate and extent of chlorinated solvent biodegradation using HRC as the electron donor source and also to gather design data such as how long HRC would last and the extent of biodegradation along the ground water flow path.
Case StudyThe site of the study is situated in a historically industrial section of Watertown, Mass. The general soil profile consists of approximately 13 feet of sand and gravel over approximately 7 feet of silty sand; then glacial till (an aquitard) is encountered. Ground water occurs at approximately 8 feet below land surface and is contaminated with several chlorinated solvents, including PCE, TCE and degradation products characteristic of natural biological reductive dechlorination.
In the field demonstration, ground water is extracted from three down-gradient wells and injected into three wells 17 feet upgradient. Five 2-inch PVC monitoring wells are positioned between the injection and extraction wells to monitor the progress of the biodegradation process. A ground water recirculation rate of 0.25 gallons per minute (gpm) establishes a single recirculation cell of about 30 feet in diameter.
The system was in full operation for eight months with a relatively constant recirculating flow rate of 0.25 gpm. HRC canisters were placed into the three injection wells to passively deliver electron donor. VOC and other data, which measured redox conditions and the concentration of electron acceptors (dissolved oxygen, ferrous iron, sulfate, nitrate, methane) and electron donors (total organic carbon (TOC), volatile acids), were collected over the time course of the study.
The pump was shut down and HRC remained in the injection wells to act as a barrier. Data were collected - approximately two months after the pump was shut down - to evaluate rebound within the treatment cell and to evaluate the effectiveness of HRC as a barrier.
Results from the field demonstration indicate that under anaerobic conditions, a significant reduction in the concentration of all cVOCs was observed. The average TCE concentration at the beginning of the study was 9,900 ug/L (parts per billion), and after 206 days levels were reduced to less than 10ug/L. Initial PCE concentrations of approximately 740 ug/L were reduced to less than 1 ug/L. Dichloroethene (DCE) levels, which started at an average value of 2,500 ug/L, remained relatively constant for the first six months, then decreased to less than 100ug/L. Vinyl chloride levels rose from an average 250 ug/L to 3,000 mg/L after the first six months, then decreased less than 100 ug/L. Ethene was detected in the down-gradient monitoring wells indicating complete biodegradation of the cVOCs. The reduction in total mass was calculated to be 97 percent, based on the average concentration of cVOCs in the treatment cell.
It can be seen that reductive dechlorination is not apparent until 50 days after the testing had started. After 50 days, redox levels had dropped from +100 mv to less than -50 mv across the entire cell corresponding to the time when reductive dechlorination was first observed. Redox levels were maintained within a range of -150 mv to -50 mv over the remaining duration of the study.
TOC and volatile acid data were collected throughout the study from the injection well. TOC levels were detected at a concentration of 1,200 mg/L after two months and remained constant after that. Lactic acid levels were first detected in the range of 70 mg/L and increased to approximately 800 mg/L. The concentration of TOC and volatile acids dropped off dramatically by the time ground water reached one down-gradient monitoring well, with TOC at approximately 20 mg/L and little to no lactic acid detected. Other volatile acids such as propionic and acetic acid were sporadically detected at levels between 1 mg/L and 15 mg/L at all down-gradient monitoring wells. Although TOC and volatile acid levels were significantly lower downgradient of the injection well, it was apparent that cVOC biodegradation was continuing based on the continued cVOC reduction observed along the flow path.
Elevated sulfide levels were detected in the injection well and two of the monitoring wells. Dissolved iron levels were elevated across the cell, but there was no observed increase in methane. These results suggest that sulfate reduction and iron reduction were the predominant conditions favored across the treatment cell.
The recirculating pump was shut down and the HRC containing canisters remained in the injection wells. CVOC data were collected approximately two months after the shutdown. These analytical results showed that there was only a limited rebound in the concentration of cVOCs within the treatment cell.
Under the conditions of this field demonstration, HRC polymer (though almost gone) still was present in the canisters after one year.
CostsThe field demonstration was conducted for a cost of less than $30K (including analytical and monitoring). The estimated cost to expand this program to full-scale using a passive design is less than $50K. HRC injections are proposed across a source area of 100 feet by 40 feet and would be delivered in holes using direct-push techniques. The only other project costs beyond the $50K would include those required for long term monitoring (sampling, analytical and reporting costs). A monitoring program for this site would be estimated to cover a time period of two to five years and would be incorporated as part of a natural attenuation remedy. The analytical program would require collection of VOC samples as well as some natural attenuation parameters to monitor redox conditions and geochemical parameters. A second HRC injection is expected to be needed after one year, but costs may be less than the original $50K as the concentration of volatile organics will likely be lower after the first year of treatment.
ConclusionsSpecific conclusions that can be drawn from the results at the field demonstration:
- Complete biodegradation of PCE, TCE and daughter products was demonstrated.
- DCE and VC biodegradation rates were rapid under HRC-enhanced anaerobic conditions.
- Sulfate-reducing and iron-reducing conditions appeared to be the predominant microbiological conditions across the treatment cell.
- A lag period, which likely corresponds with the establishment of low redox conditions, appears to be required before biodegradation is enhanced.
- Enhanced biodegradation was observed at locations 17 feet downgradient of the injection points, indicating HRC affected conditions beyond the area immediately surrounding the injection well.
- There was little rebound observed within the treatment cell after the recirculating system was shut down, suggesting significant reduction of residual cVOCs.
This field demonstration showed HRC can be used to enhance cVOC biodegradation passively and is a cost effective remediation technology option. ND