A deep reaching solution

• Sep 01, 2000

Leaking underground storage tank (LUST) sites are very common throughout the United States and comprise a significant portion of known contaminated sites. LUSTs are a significant source of environmental contamination, and can pose considerable threats to health and safety. According to recent U.S. Environemntal Protection Agency (EPA) estimates, about 900,000 active, federally regulated USTs are buried at more than 300,000 sites nationwide. As of this year, more than 370,000 confirmed petroleum releases from LUSTs have been reported. Petroleum releases from LUSTs often impact both soil and groundwater matrices and can go undetected for many years. As a result, substantial volumes of soil and groundwater can be affected, complicating site cleanup.

Various remediation technologies are available for site cleanups depending on the type of petroleum released as well as other site specific issues such as geology, hydrology and location. Gasoline releases are typically remediated using soil vapor extraction, air sparging, in situ bioremediation or a combination of these technologies. Diesel- and oil-range petroleum releases can be more difficult to treat, and bioremediation treatment methods are becoming increasingly successful as a remedial application, especially as new products and systems are developed to better support the biological degradation process.

Bioremediation is the process of using bacteria and other biological enhancements under controlled conditions to convert organic compounds (in this case, petroleum) to carbon dioxide, water and energy for cell production. The main requirements critical to bioremediation success are an active, healthy petroleum-specific biological population and continuous sources of oxygen and nutrients for biological support.

A unique application of an in situ bioremediation technology took place at an Arizona LUST site impacted with diesel-range contaminants in the soil and groundwater. The process, enzyme-catalyzed Dissolved Oxygen In Situ Treatment (DO-IT), proved itself effective at in situ treatment of petroleum hydrocarbon contaminants. The site was a former truck stop with diesel LUSTs and diesel-contaminated vadose zone (unsaturated) soil, impacted to a depth of 90 feet below ground surface (bgs).

How the DO-IT technology works

The enzyme-catalyzed DO-IT process is an automated treatment system that supplies both biologial products and oxygen in abundance. The biological enhancements include a proprietary multi-enzyme complex solution in combination with a highly specialized TPH-specific bacterial consortium that facilitates rapid reduction of TPH compounds including benzene, toluene, ethylbenzene and xylenes (BTEX) and methyl tertiary butyl ether (MTBE) contaminants. The enzymes are complex three-dimensional proteins that are extracted directly from living TPH-degrading bacterial cultures. The application of these enzymes significantly increases the rate of contaminant degradation by catalyzing the conversion of aromatic and aliphatic hydrocarbons to fatty acid. The bacterial consortium then provides complete mineralization of the fatty acid complexes and remaining hydrocarbon constituents to carbon dioxide and water. As with most bioremediation efforts, the rate of contaminant degradation is usually limited by the amount of available oxygen (the electron acceptor in aerobic biological processes). This bioremediation technology 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 levels are approximately four times those of conventional systems, effectively quadrupling the rate of contaminant degradation. This treatment water is injected into the subsurface, providing continuous support (oxygenation) for biological degradation.

For treatment of soil contaminants in the vadose zone and the hydraulic zone of fluctuation ("smear zone"), this bioremediation process includes vapor-phase air/oxygen injection capability. This feature provides additional oxygenation to the subsurface to support biological degradation of adsorbed soil contaminants in the unsaturated and semi-saturated soil zone. The system's oxygen/air injection feature gives the process great flexibility in site application. A conceptual diagram of process is shown in Figure 1.

A challenging site

The site is a former truck stop/fueling station located in a mixed-use area, with industrial, commercial, and residential real estate surrounding the site. Remediation of contaminated soil and groundwater at the site is necessary to complete property transfer. The site currently contains a restaurant and other associated facilities. Diesel fuel releases occurred on the site as a result of LUSTs. Removal and decommissioning of the USTs in question was performed in the mid-1990s, and subsequent site characterization activities indicated significant diesel-range petroleum constituents in the soil and groundwater.

The contaminant plume exhibited an extreme vertical migration pathway, with limited horizonal migration. The contamination existed largely in vadose zone soils that extended in a 40-foot diameter cylinder from ground surface to the groundwater interface, approximately 95 feet below ground surface (bgs). Free product and impacted saturated zone soils existed from approximately 95 feet to 110 feet bgs. The total volume of impacted soil at this site was approximately 5,000 cubic yards. The soil was a silty, sandy, cemented soil (caliche) with intermittent sand lenses. The soils were impacted by aged #2-Diesel fuel ranging from 13,000 ppm to 83,000 ppm, and there was approximately 3 feet of free product floating on the groundwater in the center of the plume.

Treatment Goals

The groundwater aquifer beneath the site was not considered an aquifer that would be a potential source of drinking water. As a result of this classification, groundwater concentrations were not a primary regulatory driver. Diesel concentrations in the soil were the primary concern. As a result, total petroleum hydrocarbon (TPH) cleanup levels were set at 7,000 ppm for the unsaturated soil zone and free product removal was a secondary treatment goal.

The remedial solution

The extensive deep vadose zone contamination contributed to the already difficult remedial challenges posed by the site soil types (cemented sands and silts) and the free-phase product at the groundwater interface. Due to the presence of diesel-range petroleum hydrocarbons, soil vapor extraction and/or air sparging were not viable treatment options. Groundwater pump-and-treat technology was also not an attractive option since it would not have addressed the soil contamination. As a result, in situ bioremediation using the DO-IT system was chosen for active site remediation. The system is currently operational and in the final treatment stages. A picture of the DO-IT unit on-site is shown in Figure 2.

The system application

The critical application component for any in situ bioremediation project is adequate contact between the biological enhancements (and oxygenation) with the contaminants in the soil and/or groundwater. This bioremediation process uses a liquid extraction/enhancement/re-injection scenario to accomplish this contact. Ideally, up-gradient injection points and down-gradient extraction points are used to create a closed-loop groundwater recirculation system that recycles oxygenated, biologically-amended groundwater and air/oxygen vapor from the system throughout the plume zone until the contaminated media is fully treated. When groundwater extraction/re-use is not technically feasible, this bioremediation process also achieves successful site treatment using fresh water as the source for the injection water and augmenting oxygenation with air/oxygen injection components.

For this particular site, application of the bioremediation process involved installation of both horizontal injection points at the surface of the plume and vertical injection points throughout the depth of the soil zone for oxygenated water, nutrients and biological product contact. Due to the extreme depth of the impacted vadose soils, three separate rings of nested vertical injection wells were installed throughout the 95-foot soil zone (see Figure 3). The nested wells decreased the soil volume and thickness that each well was required to treat, allowing for a more even distribution of injected water, bacteria and nutrients. A conceptual diagram of this nested well layout is shown in Figure 4. Two groundwater extraction/recovery wells — one located in the middle of the plume and one located at the edge of the plume — created a close d- loop treatment system, effectively cycling treated groundwater (with dissolved oxygen, bacteria and nutrient amendments) throughout the contaminate d plume zone.

In addition to the oxygenated water, air vapor is continuously injected into the soil zone, providing more oxygen for biological degradation (a modified biovent scenario). Together, this treatment layout allows for both hydraulic control of the free-phase diesel and dissolved-phase contaminants as well as continuous movement of enhanced water and air vapor throughout the soil. Pre-determined soil sampling areas (designated RSB-1, RSB-6, and RSB-11) within the contaminated soil zone are used to monitor remedial progress.

System operation

An initial inoculation with the enzyme complexes and the specialized TPH-degrading bacterial consortium was performed. Since that time, the system has performed automatic oxygenated water and vapor-phase oxygen injection into the horizontal and vertical injection points on a near-continual basis. Additionally, enzymes and the TPH-degrading bacterial culture are continuously metered into the oxygenated water to maintain a healthy degrading biological population. Soil samples have been collected on a quarterly basis to monitor petroleum degradation as well as soil nutrient concentrations and bacterial plate count populations. When samples indicate deficient levels of nutrients, a specialized nutrient blend consisting of nitrogen, phosphorus, potassium, and micro-nutrients is dissolved into the injection water and applied to the subsurface.

Treatment results

The bioremediation process achieved complete removal of the free-phase diesel product in the subsurface. In addition, significant dissolved-phase contaminant reductions were achieved in the groundwater. In the vadose zone, a 28 percent average-reduction in diesel-range soil concentrations was achieved in the first 50 days of system operation. Current diesel-range soil concentrations have been reduced by an average of 72% from baseline levels. Figure 5 illustrates these reductions. Most sitewide soil concentrations are below the 7,000 ppm cleanup level, with only a few remaining hotspots left to treat. Currently, the average soil concentration is 4,500 ppm, down from the baseline average of 17,000 ppm.

e-sources

U.S. Environmentla Protection Agency - Bioremediation documents — www.epa.gov/ord/WebPubs/biorem
U.S. Department of Energy- Reading room - Natural and Accelerated Bioremediation Research — elibrary.unm.edu/doe/doelink.htm
U.S. Environmental Protection Agency - Office of Underground Storage Tank Compliance Assistance — www.epa.gov/swerust1/cmplastc

This article appeared in Environmental Protection, Volume 11, Number 9, September 2000, Page 46.

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This article originally appeared in the 09/01/2000 issue of Environmental Protection.