More Results with Less Infrastructure

A low impact remediation strategy for mobile MTBE plumes

Groundwater impacted by methyl tertiary butyl ether (MTBE) continues to challenge remediation efforts around the country, and it remains a high priority for many companies due to the potential for contamination of potable groundwater supplies. MTBE is a flammable liquid that is used as an additive in unleaded gasoline. Drinking or breathing MTBE may cause nausea, nose and throat irritation, and nervous system effects. Although there is no current evidence that MTBE causes cancer in humans, laboratory studies indicate that it may cause kidney cancer in rats and liver cancer in mice. In addition to these potential risks comes associated health concerns and legal ramifications that can impact companies both financially and politically. Since MTBE plumes can migrate significant distances, treatment of these plumes on adjacent properties can be extremely difficult, both logistically and from a property access standpoint.

An innovative remediation technique is available that specifically focuses on treatment of low- to medium-level MTBE concentrations in groundwater. This approach involves the subsurface application of oxygenated, biologically-active treatment water on a consistent basis to accelerate biological attenuation of MTBE within a reasonable timeframe (usually 6 to 18 months). This comprehensive biological oxidation approach utilizes a small, mobile equipment system that stabilizes very high concentrations -- less than 45 parts per million (ppm) -- of dissolved oxygen (DO), nutrients, and microbial consortia in a stream of water (extracted groundwater or potable water). It then injects this solution into the subsurface to stimulate ongoing, aerobic MTBE degradation. The following paragraphs summarize this low-impact remediation strategy and its potential for controlling and/or mitigating mobile MTBE plumes.

Backgrounder on MTBE
The specific physical and chemical characteristics of MTBE that produce large and mobile plumes have been detailed in numerous technical publications, and is not the primary focus of this discussion. However, to briefly summarize, MTBE's high water solubility, low capacity for soil adsorption, and resistance to biological degradation can result in significant migration through a groundwater aquifer. This migration often results in groundwater impact to surrounding properties, which complicates remediation. Most remedial action plans involve treatment of the source area and natural attenuation of the migrated MTBE. However, in some situations natural attenuation is not an option, and this is where a comprehensive biological oxidation approach can accelerate MTBE degradation below regulatory criteria.

MTBE treatment can be difficult for a number of reasons. MTBE is a biologically-degradable compound that requires aerobic conditions (i.e. dissolved oxygen as the electron acceptor), so DO delivery is a requirement for in situ biological degradation at any MTBE-impacted site. However, oxygen alone will not support effective MTBE degradation in most cases, primarily because MTBE is not a favorable substrate for stimulating significant microbial growth, and also because many indigenous bacteria are either unable or unwilling to utilize MTBE as a food source. Because of this, biological augmentation (the addition of nutrients and active MTBE-degrading bacterial colonies that will scavenge any available food sources prior to expiring) in combination with an effective DO delivery system is critical to ensure real, measurable degradation of MTBE.

The other limitation with treating large MTBE plumes is infrastructure. Most active remediation systems require temporary wells, piping, and trenches. Unfortunately, installing extensive infrastructure on adjacent properties and homeowner sites is difficult, and can further aggravate an already delicate situation. As a result, the need for an effective MTBE treatment technology requiring limited infrastructure is obvious.

An Innovative Approach
Injection of oxygenated water (stable dissolved oxygen concentrations of 45 milligrams per liter mg/L) with appropriate biological enhancements (bacteria and nutrients) into the subsurface on a consistent basis can effectively deliver the components necessary to support ongoing MTBE degradation. This approach is feasible for MTBE due to its chemical properties, which ensure that very little MTBE mass is adsorbed to organic soil material, thus creating an opportunity for biological degradation that is focused solely on dissolved-phase groundwater treatment. This approach would not be appropriate for sites with significant adsorbed contaminant mass.

The water oxygenation and biological enhancement mixing is accomplished with a small, mobile equipment system that pumps this water into temporary or short-term injection points. These injection points are usually small-diameter PVC wells installed for use over a few months. However, the system can also be coupled with a direct-push system that drives new probe holes into the subsurface during each site visit, with subsequent injection of the oxygenated, biologically-enhanced water down these probe holes; the probe holes can be de-commissioned at the end of every site visit. These probe holes or the temporary PVC wells can be installed at strategic locations within an off-site plume to eliminate MTBE in groundwater over a reasonable timeframe.

This in situ biological oxidation approach can only be successful when the oxygenated water and enhancements are delivered on a regular basis. Most successful applications have used a minimum of 2 oxygenated water injection events per month. In this way, the in situ microbial process is "fed" and supported on a continuous basis until MTBE concentrations are below the desired levels. Unlike other approaches, this in situ biological oxidation technique applies all the primary components (i.e. DO, nutrients, and bacteria) required to optimize biological MTBE degradation.

This biological oxidation process is most effective when the dissolved MTBE concentrations are less than 5,000 parts per billion (ppb). MTBE concentrations greater than 5,000 ppb may warrant a larger, full-scale treatment system.

Achieving Results
A treatment site that utilized this comprehensive in situ biological oxidation process was adjacent to a gasoline station and two monitoring wells existed on the property. The MTBE plume impacted a significant portion of this adjacent site, at levels ranging from 1,000 ppb to 6,000 ppb. Regulators and the potentially-responsible party were anxious to reduce these concentrations and limit further MTBE migration. To complicate matters, the current site owner would not allow any additional semi-permanent infrastructure (i.e. wells) to be installed. As a result, the environmental consultant used the following treatment scenario:

  1. A small, mobile, water oxygenation unit delivered DO-saturated water and biological enhancements to direct-push probe holes. These probe holes were installed at the site with a small direct-push rig during each visit.
  2. A total of 3 injection events were performed over a two-month period, with a total volume of 3,000 gallons of oxygenated, enhanced treatment water delivered to the site during this period. This volume was distributed over a 60-foot by 150-foot area of the off-site MTBE plume.
  3. New direct-push probe holes were advanced at the site during every site visit, and were de-commissioned at the close of every visit using bentonite. For each event, up to 10 probe holes were installed across the 60-foot by 150-foot plume area.

MTBE reductions in two monitoring wells (approximately 120 feet apart) were significant. It is important to note that there were no injections into the monitoring wells, and that injection probe holes were placed no closer than 10 feet from either monitoring well. Additionally, no more than 150 gallons of water were injected in a single probe hole during any injection event.

Total costs for rental of the oxygenation system, purchase of microbial enhancements (nutrient and bacteria, primarily), training and start-up, and ongoing data evaluation and support was approximately $5,000.00. From a remediation standpoint, this represents a significant bargain, especially when compared with the costs of full-scale treatment or ongoing monitoring of an off-site plume covering a 60-foot by 150-foot area.

Conclusions
The key to the in situ technique described above can be summarized in one word -- comprehensive. This in situ biological oxidation approach is based on fundamental biological principles, and supplies all the primary components necessary to stimulate ongoing MTBE degradation in soil and groundwater. Using the oxygenation and delivery system to create contact and influence throughout a significant plume area can provide measurable results at a reasonable cost. Most importantly, application in sensitive locations is possible due to the limited permanent infrastructure required by this method.

This article originally appeared in the 05/01/2005 issue of Environmental Protection.

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