Bioremediation on the Fast Track
An expanding groundwater treatment is moving accelerated natural attenuation into the mainstream
- By Bryan W. Vigue, Stephen S. Koenigsberg, Ph.D.
- May 01, 2002
Bioremediation -- cleaning up soil and groundwater contamination with indigenous bacteria -- has been a holy grail for property owners and consultants, since it can eliminate active pumping systems, which include associated operations and maintenance, as well as treatment and disposal costs. However, the natural pace of unamended bioremediation or "leave-alone" natural attenuation tends to be too slow and uncertain to be practical. Two formulations, Oxygen Release Compound (ORC® ) and Hydrogen Release Compound (HRC®), are speeding up these processes and permitting accelerated natural attenuation for certain types of groundwater pollution. Many of the contaminants generated from leaking underground storage tanks, industrial/manufacturing processes, farming, landfills and military activities can be degraded with these compounds.
A magic bullet it's not, but the combination of two versatile compounds has been achieving results at literally thousands of sites that are impacted with a host of groundwater contaminants. ORC is specifically geared toward removing aerobically degradable contaminants, particularly petroleum-based fuels and fuel constituents which include gasoline; diesel; kerosene (a heating oil); benzene, tolvene, ethylbenzene and xylene (BTEX); methyl tertiary butyl ether (MTBE, a fuel oxygenate); ethers and others. HRC was developed to address the widespread chlorinated contaminant problem associated with degreasing operations like those involved in computer chip manufacturing, dry cleaners, the aircraft industry, military operations and energy industries. HRC anaerobically degrades:chlorinated ethenes (e.g., tetrachloroethylene (PCE), trichloroethne (TCE) and vinyl chloride), chlorinated aromatics (e.g., pesticides, chlorobenzenes, and furans), nitroaromatics (including explosives and dyes), inorganics (e.g., nitrates and perchlorate) and heavy metals (e.g., chromium, arsenic, etc.).
The approach is to speed up the existing natural processes that are responsible for the degradation of biodegradable contaminants and effectively remove them from groundwater and soil. Upon injection into the subsurface, ORC releases a slow and consistent supply of oxygen that results in the degradation of a broad range of petroleum hydrocarbons through the oxygen-dependent activities of aerobic microbes. HRC works in similar fashion, through the controlled release of hydrogen, to accelerate the anaerobic bioremediation of chlorinated compounds via hydrogen-hungry microorganisms. In short, these products feed and grow existing microbial populations beyond the capabilities of the natural system, which enables the reduction and elimination of contaminants. Often the end result is complete degradation of pollutants and the production of harmless end products, such as ethene, ethane, carbon dioxide and water. Both products are non-toxic food-grade materials.
These two products, which together have been applied at a combined total of 7,000 domestic and international environmental projects, offer environmental engineers a tool to restore, improve and protect the environment in a way that conserves both time and money. At the latest International Conference on In Situ and On-Site Bioremediation in California June 2001, environmental consultants, engineers and scientists presented a total of 41 different papers on the use of ORC and HRC to treat petroleum hydrocarbon and chlorinated solvent contamination.
BTEX Remediation, Vancouver, Wash.
At the Fourth Plain service station in Vancouver, Wash., leaks in underground petroleum storage tanks and supply pipelines resulted in a large plume of the BTEX chemicals - i.e., the volatile monoaromatic hydrocarbons benzene, toluene, ethylbenzene and xylene -- which are commonly found together in gasoline and other petroleum products. Concentrations in the core of the contaminant plume were as high as 23,600 micrograms per liter (µg/L), and there was a prominent isopleth of concentrations in excess of 10,000 µg/L.
Soil and groundwater investigations of the aquifer, which consists primarily of sand and gravel, indicated that these contaminants were being slowly attenuated by natural biodegradation and that this natural process could be enhanced by providing additional oxygen. An air sparging system was initially proposed to provide the extra oxygen, but this plan met strong opposition from the residents of nearby homes due to the visual and physical obtrusiveness of such systems. Enhanced bioremediation using ORC was proposed, and then was selected by the lead contractor because it was expected to reduce the mass of contaminants in the aquifer by more than 50 percent in six months, requiring a smaller capital investment and operating expense in comparison to the initially proposed pump-and-treat (P&T) system.
This site was remediated under the state of Washington's Independent Remedial Action Program (IRAP), which allows site owners to manage site cleanups independently. Under the IRAP, managed by the Washington State Department of Ecology, the state does not provide oversight or direction to site owners on the appropriateness of their remedial approaches. Instead, the state provides a letter requiring no further action once the owner can demonstrate that cleanup levels have been met and that the site no longer represents a threat to human health and the environment. Because the aquifer at the site was a source of potable water and was situated immediately up-gradient from a surface-water body, the cleanup levels were five µg/L for benzenes (the Model Toxics Control Act (MTCA) cleanup level) and 95 µg/L for BTEX.
ORC was applied up-gradient of the plume through fifteen soil borings, each boring being backfilled with approximately 60 pounds (lb.) of ORC slurry. After 150 days, the total mass of the BTEX plume had been reduced by 58 percent. The isopleth of >10,000 µg/L concentrations, which had encompassed 52,500 square feet (ft2), had decreased to 1,500 ft2.
The client's report stated that the cost of ORC application was $40,000, while a conventional pump-and-treat system would have been about $250,000. Use of ORC thus represented a savings of $210,000. Equally important from the client's point of view, the choice of ORC contributed positively to relations with the adjacent homeowners, while allowing the remediation objective to be achieved. The site has now been closed.
PCE/TCE/DCE/Vinyl Chloride Cleanup, Sunnyvale, Calif.
Manufacturing operations at a former microwave electronics plant in Sunnyvale, Calif., left substantial amounts of PCE, TCE, cis-1,2-dichloroethene (DCE) and vinyl chloride (VC) in the soil and groundwater. In 1996, CDM, an environmental consulting company, began evaluations of the effectiveness of a pump-and-treat (P&T) remediation system that had been implementated nine years before. At the time, it was anticipated that the P&T system would have to continue operationing until 2015.
CDM concluded that the P&T system was no longer cost effectively removing contaminant mass and began to investigate alternative approaches. After evaluating several alternative technologies, and considering on site-specific conditions, the consultant selected enhanced anaerobic bioremediation with HRC. HRC was applied at 27 location points in and near the primary contaminant source and another 16 points downgradient of the source area.
Reduced nitrate and sulfate concentrations following the injections of HRC, coupled with the high influx of hydrogen into groundwater, resulted in conditions favorable for complete reductive microbial dechlorination of the contaminants to ethene. Decreases in PCE and TCE concentrations resulted in regulatory approval to shutdown the expensive and outdated groundwater P&T system.
MTBE and BTEX, North Carolina
At a U.S. Coast Guard support facility on North Carolina's Pasquotank River, where JP-4 and JP-5 jet fuels are stored and handled, leaks in an underground storage tank and transfer lines released MTBE and BTEX into the groundwater.
The site lithology - silty clay overlying strata of silty sand and coarse sand - suggested the use of accelerated natural attenuation, which would allow a cleanup to proceed without interference to facility operations. ORC was injected into the source area and the dissolved plume area with direct-push technology. The initial injection design included 18 points in the source area, using 35 lb. of ORC per point, and 13 points in the plume area, using 22 lb. of ORC per point. Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) analytical results were used to estimate the total oxygen required per injection event.
Post-treatment monitoring of the aquifer showed increased biological activity and substantial decreases in dissolved MTBE and BTEX concentrations. Three months after initial treatment, the MTBE concentration had fallen from 510 µg/L in the source area and 390 µg/L in the plume area to below quantifiable limits in both areas, and dissolved MTBE mass had been reduced 100 percent.
BTEX concentrations, recorded before treatment at 745 µg/L in the source area and 1,100 µg/L in the plume area, were reduced to a minimum of 14.3 µg/L in the source area after two months after treatment and 87.5 µg/L in the plume after three months.
Following initial treatment, a slight apparent rebound in BTEX concentrations was observed, particularly the benzene component, possibly due to undetected residual adsorbed petroleum mass which had migrated below the runway apron and had not been removed during tank closure. A secondary treatment was applied, in which 33 lb of ORC was placed into 16 injection points within the plume.
Subsequent monitoring confirmed that intrinsic remediation was occurring at a rate sufficient to protect down-gradient receptors. Site closure has been obtained from the North Carolina Department of Environment and Natural Resources, Division of Environmental Management. Total costs for the ORC were $26,200.
Perchlorate, Hollister, Calif.
At an ordnance facility in Hollister, Calif., a groundwater aquifer was found to be contaminated by a variety of compounds, the most significant being perchlorate, the primary ingredient of solid rocket propellant, which from a health standpoint can disrupt thyroid function; hexavalent chromium (Chrome-6), a known human carcinogen; and the refrigerant 1,1,2-trichloro-1,2,2-trifluoroethane (Freon-113), an environmental pollutant that contributes to depletion of atmospheric ozone. Perchlorate was detected at levels of 1,600 to 7,500 µg/L, Chrome-6 at levels of 15 to 210 µg/L, and Freon-113 at levels of 180 to 430 µg/L.
The aquifer was composed of a medium to fine silty sand, with groundwater flowing northwest at a velocity of approximately 0.07 ft/day. In a pilot study application, 660 lb. of HRC was applied to the site through a 25-point injection grid at a rate of six lb. per vertical foot. The grid covered an approximate area of 1,200 ft2 and an application thickness of 15 feet. Points within the grid were spaced five feet apart on center.
Significant reductions in the various contaminants have occurred through 79 days of monitoring. Perchlorate concentrations have decreased from 7,100 µg/L at baseline to 860 µg/L at day 79, a reduction of 88 percent. Chrome-6 concentrations have decreased from 160 µg/L at baseline to non-detect levels (< 1 µg/L) at day 79, a reduction of over 99 percent. Freon-113 concentrations have decreased from 270 µg/L at baseline to 21 µg/L at day 79, a reduction of 92 percent.
Decreasing sulfate levels and increasing TOC concentrations are positive indicators of the enhancement of reductive dechlorination and the presence of HRC within the aquifer system. Monitoring at this site is ongoing.
PCE, Cedarburg, Wis.
The Cedarburg Drycleaner site in the Milwaukee suburb of Cedarburg, Wis., was contaminated with high levels of the chlorinated hydrocarbon perchloroethene (PCE), which can cause kidney and liver damage in humans. To remediate the PCE, environmental consultant Montgomery Watson Harza (MWH) applied 240 lb of HRC to the site via Geoprobe direct-push injection at 20 lb. per hole in 12 holes.
After analyzing the contaminants and their breakdown products, MHW noted an increase in lactic and other metabolic acids (acetic, propionic and pyruvic) at the HRC delivery points and at downgradient locations. Additionally, groundwater pH analysis showed a shift to the acid side and indicated that redox (reduction-oxidation reaction) levels had dropped steadily. Within five months, the remediation team had noted an 80-percent mass removal with good field mass balances and concluded that "the risk to groundwater was significantly diminished."
The $31,000 research project garnered an Honor Award for MHW at the 2000 Engineering Excellence Competition held by the Wisconsin Association of Consulting Engineers, as well as National Finalist status in the 2000 Engineering Excellence Competition of the American Consulting Engineers Council.
In a separate development, profiles of other sites successfully treated with HRC were recently posted on the Web site of the State Coalition for Remediation of Drycleaners (SCRD), a U.S. Environmental Protection Agency (EPA)-sponsored association of states with established drycleaner remediation programs, including Alabama, Florida, Illinois, Kansas, Minnesota, Missouri, North Carolina, Oregon, South Carolina, Tennessee and Wisconsin (www.drycleancoalition.org/profiles/). In addition, Oregon has designated HRC as a "presumptive remedy" under that state's Dry Cleaning Fund.How It Works
Oxygen Release Compound (ORC), introduced in early 1995, is a formulation of magnesium hydroxide that is purposefully designed to slowly release molecular oxygen for extended periods of up to 12 months when contacted with water. It is the oxygen that drives the metabolism and population growth in aerobic, contaminant-degrading microbes. ORC is supplied in 30-pound buckets as a fine powder that is typically mixed with water to form an injectable slurry for groundwater applications. The preferred application method is direct-push injection, using a portable pump and direct-injection probe.
Hydrogen Release Compound (HRC) is a polylactate ester designed for the slow release of hydrogen when contacted with water for extended periods of up to 18 months. It is a viscous, honey-like substance, also supplied in 30 pound buckets and also applied by direct injection into the subsurface. Hydrogen is produced as the result of anaerobic microbial activity acting upon a lactic acid supply provided by the breakdown of HRC. The additional hydrogen produced in this microbial driven process is substituted for harmful chlorine atoms -- through a process known as reductive dechlorination -- which effectively disarms or reduces them to harmless end products, such as ethane or ethane. This process also includes the use of a portable pump and direct-injection probe.
Both products can cause dramatic reductions -- sometimes as high as 90 to 100 percent -- in the concentration of a long list of major contaminants including gasoline (BTEX and MTBE), diesel, kerosene, perchlorate, certain pesticides, nitroaromatic explosives such as trinitrotoluene (TNT), degreasers like perchloroethene (PCE), trichloroethene (TCE) and heavy metals like hexavalent chromium (Chrome 6).
Enhanced Bioremediation, Bioenrichment and Bioaugmentation
Generally, biodegradation refers to any process by which microbial organisms transform or alter (through metabolic or enzymatic action) the structure of chemicals introduced into the environment. Bioremediation is the deliberate use of biodegradation to manage or eliminate site contamination; this often involves adding nutrients or microorganisms to the environment to increase the rate at which biodegradation occurs.
Enhanced bioremediation, also called accelerated bioremediation or biostimulation, seeks to speed up and/or intensify the process by various means. ORC and HRC are, in effect, biostimulation technologies, which supply oxygen and hydrogen - molecules which are not nutrients in themselves, which are vital to microbial processes. Biostumulation typically does not involve the addition of new non-indigenous "bug" species, but rather enhances the growth of existing, indigenous microbial colonies to levels capable of achieving effective accelerated contaminant degradation.
The term bioenrichment is sometimes used to refer to the enrichment of a culture with oxygen or hydrogen to stimulate microbial growth, but it is more commonly and more correctly used to describe the addition of nutrients, such as phosphorus, nitrogen, vitamins, etc., to a contaminated environment to stimulate and support the growth of the microorganisms capable of biodegradation. This process is also called nutrient enrichment.
Bioaugmentation, also known as microbial seeding, refers to the addition of microorganisms to the existing native contaminant-degrading population. In practice, it involves the purchase and addition of so-called contaminant-specific microbes to the subsurface environment. Often, these microorganisms are already present in groundwater aquifers, but not in the numbers necessary to degrade excessive contaminant levels (due to limiting factors in the aquifer). However, if the environment in which these microorganisms are placed is not capable of supporting their populations, (e.g., if oxygen or hydrogen levels are not high enough and maintainable), the populations will die off and the money spent will be simply wasted. Increasingly and in the future, bioaugmentation is branching out to include manipulation of the strains of bacteria used, through isolation, selective breeding and genetic engineering.
The term bioaugmentation is also sometimes mistakenly used as a synonym for the enhanced bioremediation processes described above.
This article originally appeared in the 05/01/2002 issue of Environmental Protection.