Persistence Pays Off

Persistent bio-oxidation is an innovative remediation strategy for achieving successful site closure for low-level contaminated sites

Most companies that deal with environmental liabilities usually manage a broad array of projects and sites with varying degrees of contamination. While the remediation and cleanup options for many properties can be readily identified, environmental decisions for some sites can be somewhat difficult. Specifically, there are numerous properties throughout the United States that contain low levels of regulated compounds or contaminants, but whose scope will never warrant a full-scale remediation system. For example, gasoline stations often have localized areas of contamination where soil and/or groundwater exhibits low levels of benzene, toluene, ethyl benzene and xylene (BTEX) or methyl tertiary butyl ether (MTBE) (usually from customer drive-offs that spill fuel, or accidental overfills of an underground storage tank). While the scope of contamination is limited, the constituent concentrations exceed regulatory guidelines, and thus the sites are usually subject to some degree of consistent monitoring. As a result, many companies are looking for cost-effective, efficient solutions to reduce their long-term liability at these types of sites.

A new and effective remediation approach is the subsurface application of oxygenated, biologically active treatment water to support in-situ degradation of low levels of soil and groundwater contamination within a relatively short timeframe (usually 6 months to 18 months). This persistent, accelerated bio-oxidation approach does not use oxygen-releasing compounds, but instead utilizes a small, trailer-mounted equipment system that stabilizes very high concentrations, less than 45 parts per million (ppm), of dissolved oxygen, nutrients and microbial consortia in a stream of water -- groundwater or fresh water -- and subsequently injects this solution into the subsurface for ongoing, aerobic contaminant degradation.

Why is Low-Level Contamination a Problem?
Many large companies, especially those whose size, revenues and risk management strategies allow for a significant degree of regulatory latitude, often simply assign a very low priority to sites with low-level, persistent contamination, and accept the costs of long-term monitoring and the presence of these properties on their liability balance sheets. Smaller companies, however, often need to deal with these properties on a more proactive basis.

As potential solutions, risk-assessments and risk-based closure criteria have been developed by many regulatory groups to better deal with the varying degrees of contamination on impacted sites. By assessing potential risks and long-term environmental receptors, site-specific cleanup goals can be applied to a site to reduce or eliminate the need for additional cleanup or monitoring. In certain cases, the residual concentrations are below the calculated risk factors, and thus regulatory closure can be obtained for the site.

Another approach that has gained popularity in recent years is the "remediation" alternative of long-term natural attenuation (LNA) and monitored natural attenuation (MNA). LNA and MNA are processes by which the soil and groundwater characteristics of a site are evaluated to determine the ability of the property to naturally reduce its contaminant mass (via physical, chemical and biological processes like volatilization, dilution, hydrolysis and biodegradation). There is no doubt that LNA and MNA can be excellent remedial tools, but they are too often applied as a default strategy to satisfy internal or external regulatory pressures, and also to stall or delay the need to make real environmental decisions. Additionally, because of significant attenuation timeframes, LNA and MNA often represent significant remediation costs because of the 10 years to 20 years of long-term monitoring, reporting and management. As a result, accelerated, low-cost remediation alternatives can be very appealing for many sites.

What Is Persistent Bio-Oxidation?
Biological treatment, or bioremediation, is a well-documented remediation technology that harnesses the ability of microbial species to degrade various organic contaminants. For petroleum contaminants, aerobic bioremediation is the most effective approach. Aerobic bioremediation requires the dissolved oxygen (DO) supply (the electron acceptor in the biochemical oxidation-reduction process), essential nutrients (nitrogen, phosphorus, potassium and other micro-nutrients) and, in many cases, contaminant-specific microorganisms to support ongoing subsurface contaminant degradation.

Persistent bio-oxidation is the optimization of natural biological processes through the consistent supply of required components, including DO, nutrients and bacteria. Since a full-scale bioremediation system is not cost-effective for many sites with low-level contamination, the injection of biological components on an intermittent basis can be used to polish and degrade contaminants like BTEX and MTBE. Additionally, since the BTEX or MTBE concentrations are low, their total mass is not significant, meaning that their mass requirements of DO, nutrients and bacteria are also reduced, making a persistent injection approach feasible.

How Does Persistent Bio-Oxidation Work?
Persistent bio-oxidation is most effective when the contaminant concentrations in groundwater are in the parts per billion (ppb) range, or with low ppm concentrations in soil. If significant contaminant mass is present, then a full-scale, continuous biological treatment system is probably warranted.

When a site has been identified as having the appropriate low-level contaminant concentrations, the following information is then utilized to set up an effective bio-oxidation strategy.

  • Plume Extent. The areal distribution of the soil or groundwater contaminants is needed in order to properly distribute oxygenated water within the plume area.
  • Number and Placement of Monitoring Wells. The location of these wells will help determine oxygenated water distribution within the plume and can be used to effectively evaluate the ongoing success of the bio-oxidation process.
  • Soil and Groundwater Characteristics. The depth-to-groundwater, soil type and any other important site-specific hydrologic or geologic features must be known in order to effective apply bio-oxidation within the impacted soil or groundwater plume.

For a small plume area, the most effective application of bio-oxidation involves the use of several wells for injection and extraction. Essentially, groundwater from the downgradient edge of the localized plume is extracted, treated (if necessary), oxygenated using a mobile bio-oxidation system, and then re-injected into the subsurface via several injection points. This occurs over a 2-hour to 4-hour period, and essentially creates a small groundwater recirculation cell that maximizes contact and movement of oxygenated, nutrient-rich, biologically active treatment water throughout the impacted groundwater volume. These events usually occur 1 week to 3 weeks apart, so that a reasonable mass of dissolved oxygen, nutrients and amendments can be put into the subsurface to support in situ degradation until the next event. By performing these events on a consistent basis, effective contaminant degradation can occur within as little as 3 months to 6 months. Even with longer timeframes, however, the approach is extremely cost-effective and the effectiveness can be measured by monitoring specific field and laboratory parameters during quarterly monitoring events.

Achieving Results with a Persistent Bio-Oxidation Approach
Two separate sites, a Pennsylvania convenience store and a Pennsylvania garage station, are currently using bio-oxidation is currently being used. A proprietary oxygenation system manufactured by Enzyme Technologies Inc. is being used at both properties every two weeks to oxygenate and biologically amend the groundwater. These sites have low levels of BTEX and MTBE contamination, and the site owners are anxious to obtain regulatory site closure for the properties.

At the convenience store site, one monitoring well contained BTEX and MTBE concentrations that continued to exceed standards, so the mobile oxygenation unit extracts groundwater from the downgradient side of the well, and re-injects the water on the upgradient side. This approach recirculates dissolved oxygen, nutrients and bacteria through the immediate area occupied by the monitoring well. At the garage facility, two monitoring wells contained the bulk of contaminants. Again, the remediation approach involves extraction of groundwater downgradient and re-injection within the upgradient area of these wells. The approach has worked extremely well at both properties. Treatment at the each site has been ongoing for about 12 months, and it is anticipated that cleanup will be completed within 18 months.

Total costs for the remediation system at each site has been approximately $10,000, including the rental of the oxygenation system, the purchase of microbial enhancements (nutrient and bacteria, primarily), training and start-up, and ongoing data evaluation and support. From a remediation standpoint, this represents a significant bargain, especially when considering the costs of full-scale treatment or ongoing monitoring for a significant period of time.

When applied appropriately, a persistent, accelerated bio-oxidation process can be an effective soil and groundwater treatment solution at many sites. This technology is straightforward, based on basic microbiological principles, and can be applied with a minimum of site infrastructure. These systems have proven results, and can provide measurable treatment at a very reasonable cost. Most importantly, the actual site work can be performed by the environmental consultants themselves. Since they have the greatest degree of direct site knowledge, this provides an even greater opportunity for success.

This article originally appeared in the 05/01/2004 issue of Environmental Protection.

About the Author

David Laughlin is environmental services director for Enzyme Technologies Inc. in Portland, Ore.

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