Bioremediation in Bedrock
- By Chuck Wehland
- May 01, 2003
Fuel releases from underground storage tanks (USTs) and associated delivery equipment typically form contaminant plumes that migrate both horizontally and vertically from the source zone. When the resulting fuel release impacts groundwater within fractured bedrock strata, the cleanup of these contaminants can be very difficult. Remediation of groundwater in fractured bedock is very challenging, but in situ bioremediation can have significant advantages over other remediation technologies and can effectively treat dissolved contaminants if correctly applied. Specifically, an in situ bioremediation system must be designed with the unique characteristics of fractured bedrock zones firmly in mind. Advantages with this type of treatment include cost-effectiveness (especially when compared to other technologies) and application flexibility to account for site-specific fracture variations and heterogeneities.
This article discusses the problems associated with bedrock groundwater treatment, and summarizes the application of the in situ technology for bioremediation of a truck stop gasoline release into fractured bedrock. Using this bioremediation approach, dissolved benzene, toluene, ethylbenzene and xylenes (BTEX) and methyl tertiarybutyl ether (MTBE) reductions below regulatory levels were achieved in shallow and deep groundwater zones within the first nine months of system operation.
Understanding how water flows -- and does not flow -- through fractured rock is important for assessing the groundwater resources of fractured bedrock aquifers and predicting the movement of hazardous chemicals if contamination occurs.
Fractured Bedrock Strata
Underground fractures often serve as major conduits for the movement of water and dissolved chemicals through rock formations. Understanding how water flows -- and does not flow -- through fractured rock is important for assessing the groundwater resources of fractured bedrock aquifers and predicting the movement of hazardous chemicals if contamination occurs. The low permeability and highly heterogeneous nature of fractured rock formations often make this assessment difficult. Additionally, in terms of remediation of a contaminant release, even after a site characterization is completed, the treatment options are often restricted. Several treatment limitations are often realized in fractured bedrock environments. First, despite extensive site characterization, the true interconnectivity of fractures often remains unknown. Therefore, accessibility to contamination for treatment purposes can be limited. Second, in some fracture zones, the hydraulic conductivity may vary drastically from one area to another, complicating remedial system design. Third, the fractures typically create preferential pathways for fluid flow that reduce the ability of standard remediation systems to contact and/or capture dispersed contaminants. As a result, sparge-type treatment systems are rendered largely ineffective by this characteristic of fracture geology. Finally, infiltration is often reduced due to the lack of an extensive fracture network.
Groundwater impact in fractured bedrock is an especially important issue when dealing with gasoline releases, which often contain both benzene and MTBE. Benzene is readily soluble in groundwater and is a regulated contaminant of concern because of its carcinogenicity, while MTBE is of special concern because of its unique physical/chemical properties. Specifically, MTBE is highly soluble in water (20 times more soluble than benzene) and is considered more resistant to biodegradation. Because of its tendency to form large contaminant plumes, MTBE typically migrates further than other gasoline constituents, making it a prime threat to public and private drinking water wells. While MTBE is not currently classified as a carcinogen, its health effects are unknown, and it causes significant odor and taste problems in groundwater.
Sparge systems designed to strip volatile gasoline-range contaminants from groundwater are largely ineffectual in fractured bedrock (the injected air/oxygen finds preferential fracture pathways, restricting influence to the remaining plume area). This limitation also extends to sparge-type systems that are implemented to provide dissolved oxygen for biological contaminant degradation -- these systems have difficulty tranferring dissolved oxygen to the bulk of the plume area. This same limitation also applies to oxygen release chemicals.
Pump and treat and multi-phase extraction systems can capture groundwater and dissolved contaminants in fractured formations, but are often limited by an understanding of fracture connectivity and the variable groundwater flows through a fractured zone. However, utilization of this technology in tandem with in situ bioremediation system technology can encourage and support active in situ biological degradation of fuel contaminants.
How the DO-IT Technology Works
Bioremediation is the process of using bacteria and other biological enhancements under controlled conditions to convert organic compounds (including petroleum hydrocarbons) to carbon dioxide, water and energy for cell production. The enzyme-catalyzed Dissolved Oxygen In Situ Treatment (DO-IT) process is a specialized bioremediation technology that optimizes this process by using proprietary biological products in combination with a highly specialized in situ oxygenation equipment platform to obtain rapid reduction of petroleum contaminants, including proven degradation of MTBE. The DO-IT process is successful because it is a complete system; the technology includes application of all the primary components necessary to support the bioremediation process, including petroleum-specific enzymes and bacteria, nutrients and extremely high levels of dissolved oxygen.
Because of its tendency to form large contaminant plumes, MTBE typically migrates further than other gasoline constituents, making it a prime threat to public and private drinking water wells.
With most in situ bioremediation efforts, contaminant degradation is usually limited by the amount of available dissolved oxygen (the electron acceptor in aerobic biological processes). Therefore, the DO-IT technology includes a unique equipment platform, the Super-OxTM, which contains a specialized pure-oxygen mixing process that generates high-dissolved oxygen water at concentrations of approximately 40 parts per million (ppm). These dissolved oxygen (DO) levels are not only four times greater than what conventional systems can provide, the dissolved oxygen is also very stable; due to the proprietary oxygen-mixing technology, the steady-state dissolved oxygen half-life is greater than 20 days. This allows the oxygenated water to influence a large plume area, resulting in faster, more complete contaminant degradation. The Super-Ox injects this oxygenated, biologically-enhanced water into the subsurface to support continuous microbial activity.
By utilizing the water as the carrier for nutrients, bacteria and high concentrations of dissolved oxygen, the in situ technology can support microbial activity even in a fractured bedrock formation. While fracture connectivity is still a limitation, the treatment water can follow the same fracture pathways of the original contaminant release, and it is not bound to preferential pathways. By implementing this technology as a closed-loop injection/extraction application, the induced groundwater gradient will also cycle oxygenated treatment water throughout the fractured matrix, ensuring dissolved oxygen delivery to a greater portion of the contaminant plume. Furthermore, well-placed injection wells and/or trenches can maximize contact between the oxygenated treatment water and the dissolved contaminants in various fracture zones. This layout provides constant microbial contact and support, improves in situ treatment efficiency and decreases overall treatment time.
A Cleanup at an Active Truck stop Site
A gasoline release from a fuel dispensing island located at a truck stop site, which has an ongoing business, impacted shallow groundwater at approximately six to eight feet below the ground surface. At the site, silt overburden is underlain by shallow fractured limestone bedrock, and due to a sloping bedrock horizon, groundwater intersects both the overburden soils and the fractured zone. Previous attempts at air sparging yielded only marginal contaminant volatilization and very limited oxygen transfer for microbial support. As a result, the DO-IT system was applied to maximize in situ microbial degradation of the dissolved BTEX and MTBE contaminants.
The contaminant plume covered approximately 16,000 square feet, with five main groundwater monitoring wells (MW-1, -2, -3, -4, -7 and -5) exhibiting benzene and MTBE contamination. Benzene concentrations prior to in situ application ranged as high as 13,000 parts per billion (ppb), while MTBE concentrations as high as 1,000 ppb were present. The hydraulic zone of fluctuation, or "smear zone," was approximately two to three feet annually. Cleanup goals for this offsite plume were the Pennsylvania Statewide Health Standards (SHS), which include a five ppb benzene limit and a 20 ppb MTBE limit.
Groundwater (and soil) monitoring was the independent responsibility of the environmental consultant on the project. Groundwater sample collection was performed quarterly for most monitoring points, and all sample analyses were completed by an accredited, licensed environmental laboratory.
Sparge systems designed to strip volatile gasoline-range contaminants from groundwater are largely ineffectual in fractured bedrock.
Extracted groundwater from a series of extraction wells was used to supply water for the closed-loop recycle system, and existing air sparge wells, as well as several shallow injection laterals, were used for water injection. The Super-Ox oxygenation equipment was housed in a heated, weatherproof equipment shed located adjacent to the piping infrastructure. The extraction wells fed groundwater into the Super-Ox system, which treated, oxygenated, biologically-enhanced and re-injected this water into the subsurface via the injection array.
This layout allowed for both hydraulic control of the dissolved-phase contaminant plume and continuous recycling of oxygenated treatment water throughout the site. Monitoring wells installed within the fractured bedrock zone were used to measure remedial progress. No injection into any monitoring wells was performed, ensuring that representative groundwater data could be collected throughout treatment.
An initial biological inoculation with pre-activated hydrocarbon-degrading bacteria and nutrients was performed in June 2002. Since that time, the DO-IT system has performed automatic oxygenated water injection into the injection array on a scheduled basis. Water samples are collected monthly to monitor various parameters that affect bioremediation, including DO, pH, oxygen reduction potential, nutrients, microbial plate counts and specific inorganic compounds. The resulting data is used to verify biological degradation as the primary contaminant removal mechanism and to make regular system adjustments to maximize microbial activity.
Successful Treatment Results
Significant degradation of the benzene and MTBE has been achieved within the first five months of system operation. Currently, the DO-IT technology has resulted in average benzene and MTBE reductions of 90 percent and 95 percent, respectively (see associated graphs). As of now, benzene and MTBE levels in all wells except MW-2 are below the Statewide Health Standards for groundwater. Site completion is anticipated by mid-2003 (a total treatment time of approximately 12 months).
With the system, MTBE is being rapidly and successfully degraded. The groundwater sampling results from this site show ongoing, complete degradation of dissolved-phase MTBE. This MTBE degradation with the in situ process has been verified in both laboratory and field studies and is being successfully utilized on numerous in situ projects.
In situ bioremediation using the DO-IT process can be a successful remediation technology for groundwater treatment in fractured bedrock formations. Successful applications of this technology consistently achieve fast, complete treatment of dissolved-phase benzene and MTBE compounds. Requirements for effective application of the in situ process at fractured bedrock sites include:
- A reasonable understanding of site geology and hydrology. In addition to plume characterization, information regarding groundwater flow patterns, flowrates and localized hydraulic conductivites allow for successful design and implementation of a remedial strategy.
- A powerful, complete bioremediation platform that can provide high levels of dissolved oxygen for continuous support of subsurface microbial kinetics. Automated process support from the turnkey Super-Ox equipment is ideal for supplying ongoing site-wide groundwater oxygenation and contact.
- A well-conceived monitoring plan to provide ongoing data that can be used to make appropriate system adjustments throughout treatment.
This technology process is a proven bioremediation system that can be used as a primary remediation component or in conjunction with other technologies to cost-effectively reach regulatory treatment goals.
This article originally appeared in the May 2003 issue of Environmental Protection, Vol. 14, No. 4.
This article originally appeared in the 05/01/2003 issue of Environmental Protection.