The site consists of saprolite and residual soils with a thickness of 6 to 35 meters over competent rock. The saprolite consists of stiff to hard clays and silts, as well as fine to medium sands. Bedrock is generally slightly to moderately weathered with shallowly angled fractures.
Targeted Environmental Media:
 Fractured Bedrock
Most of the chloroform plume was downgradient of an existing ground-water extraction system. The extraction system was determined to be ineffective and was ultimately shut down.
Major Contaminants and Maximum Concentrations:
- Chloroform (1,900 µg/L)
No technologies selected.
- Bioremediation (In Situ)
- Reductive Dechlorination (In Situ Bioremediation)
Comments:
The primary factors that affect cost include the injection material, number of injection points, method of injection and frequency/number of injection events. A reliance on direct push technology has generally limited the number of applications in bedrock environments.
This application involved the treatment of a chloroform plume, via injection of sodium lactate solution. Existing extraction and monitoring wells were converted into injection wells in order to reduce costs. The injection was accomplished by gravity flow and sodium lactate (highly miscible and mobile) was used to maximize the treatment area and minimize the number of injection points. The material costs for sodium lactate also resulted in significantly lower costs.
Two rounds of injection were implemented over a period of 21 months beginning May 2003. Prior to injection, the first 90% of the solution was diluted with water to 10% of the initial concentration. The purpose of dilution was to reduce the lactate concentration and help the indigenous organisms adapt to a new food source. The final 10% of the solution was injected without dilution. This concentration slug at each injection point would serve as a limited continuing source of lactate and was intended to provide a degree of extended time release.
None provided
After the first injection event, the biogeochemical conditions changed rapidly. This yielded reducing conditions and reduced chloroform concentrations to non-detectable at several locations. As the lactate was metabolized and migrated, chloroform concentrations rebounded at some locations and remained low at others. Approximately 15 months after the first injection, a second injection was completed.
After two injection events, chloroform was non-detect in seven of the thirteen injection wells, as well as being non-detect in four of the seven area monitoring wells.
Additional data will be collected through 2005 to determine the ultimate effectiveness of the current treatment program. However, current data indicates that chloroform is being degraded.
The advantages of using existing wells and gravity flow injection as well as highly mobile sodium lactate addresses the most costly elements of injection projects: injection point construction and pressurized pumping. Further, use of existing wells enables the lactate material to reach bedrock, which frequently cannot be achieved by typical direct push technologies. Effective dechlorination has been shown to be achievable within fractured bedrock while maintaining controls on the cost of injection activities.
Reference:
Dahlgren, Bryon; David S. Woodward; Mark S. Heaston. 2005. Low Cost Reductive Dechlorination of Chloroform in Fractured Bedrock Aquifer. The Eighth International In Situ and On-Site Bioremediation Symposium, Baltimore, Maryland. June 6-9.
|
