IT
AWARD WINNER IN THE 2000 ENGINEERING EXCELLENCE AWARDS COMPETITION OFTHE CONSULTING ENGINEERS OF TENNESSEE
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Copyright© Geo/Environmental Associates, Inc. 1995 through 2008
IT
AWARD WINNER IN THE 2000 ENGINEERING EXCELLENCE AWARDS COMPETITION OF
| In September 1998, Geo/Environmental Associates, Inc. successfully completed the design and installation monitoring of a below-ground pit bioreactor which was excavated in an area previously occupied by five underground storage tanks (USTs). Kerosene, gasoline, diesel, and lubricating oil apparently leaked from the USTs through relict weaknesses in the residual soil overlying karst dolomite bedrock prior to the time when the tanks were removed in 1988. Monitoring wells installed in the early 1990's indicated the presence of free product floating on the groundwater table over an area exceeding two (2) acres. |
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Analysis of pumping test data revealed that the site could be modeled as a homogeneous confined aquifer with a barrier (impermeable) boundary. The barrier boundary delineated by the pumping test data coincides with the near-vertical contact between two bedrock formations. Additional groundwater modeling indicated that a capture zone for the free product plume could be created by withdrawing groundwater from beneath the free product plume and reinjecting the groundwater in the area of the previous tank pit. |
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The remediation scheme involves withdrawing groundwater from a recovery well and passing the groundwater through a mixing tank where nutrients are added. From the mixing tank, the water is discharged by gravity evenly across the top of a below-ground bioreactor. Aeration of the water is accomplished as it passes over a crushed stone zone in the bioreactor. Naturally occurring microorganisms introduced into the bioreactor destroy the hydrocarbons, converting them to carbon dioxide and water. The treated groundwater then flows from the crushed stone zone through a sand filter zone and is reinjected by gravity into the subsurface via groundwater injection wells. Water also seeps from the bottom of the bioreactor pit through the relict weaknesses in the residual soil along the pathways where free product originally leaked from the tank pit. The aerated water supplemented with nutrients supports the growth of additional naturally occurring microorganisms in the residual soil which feed on the hydrocarbons dissolved in the groundwater. Finally, free product is being removed from the recovery well using a separate top-loading pneumatic pump and properly disposed. |
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Since September 1998, the system has been operating at its design pumping rate of 6 gallons per minute. Sampling of water from the bottom of the bioreactor shows that regulatory limits for reinjection are being met. Approximately 1000 gallons of free product have been recovered from the site since remediation efforts began.
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The role of Geo/Environmental Associates, Inc. included:
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Conducted and analyzed data from pumping test and laboratory treatability testing; |
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Served as design engineer for the groundwater remediation system; |
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Developed plans and specifications for regulatory review/approval and construction; |
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Monitored construction; and |
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Performing on-going operations and maintenance of the treatment system. |
Before environmental remediation can proceed to the design phase, the site must be characterized with regard to groundwater and contaminant flow paths. Steeply dipping, karst bedrock with thick residual soil overburden hinders site characterization due to its complexity. Analysis of pumping test data showed that this site could be modeled as a homogeneous confined aquifer with an impermeable (barrier) boundary. The barrier boundary occurs at the change in two near-vertical strata, as confirmed by published geologic maps and site drilling. Application of standard groundwater modeling methods enabled completion of the site characterization phase so that remediation design could begin.
| Prior to start-up, microorganisms obtained from the site of a surface diesel spill in 1996 were mixed with groundwater and nutrients and aerated in drums. Gasoline and diesel fuel constituents dissolved in groundwater, obtained from onsite monitoring wells, were then added to the drums along with additional nutrients over a period of two (2) years. During start-up operations, the microorganisms in the water from the drums were applied to the nutrient mixing tank and to the surface of the bioreactor pit to expedite the treatment process. |
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One important reason for the success of the groundwater
remediation at this site is the identification of the relict weaknesses in the
residual soil as the path for contaminant migration from the original UST pit.
Locating the below-ground bioreactor at the location of the previous USTs allows
aerated water with nutrients to follow the same migration path followed by the
free product. The aerated water with nutrients can then serve to enhance the
growth of microorganisms which feed on the hydrocarbons present in the
subsurface along this route.
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The below-ground bioreactor was designed using an adaptation of the Eckenfelder analytical model developed to design trickling filters in municipal wastewater treatment plants. In the existing method, biochemical oxygen demand (BOD) reductions are measured at different flow application rates. Treatability constants are then calculated which can be used to design new trickling filters with different dimensions and flow rates. Reductions in organic contaminants at varying flow application rates were measured during laboratory testing to calculate treatability constants for the contaminants of concern at the site. The treatability constants were then used to design the required size of the below-ground bioreactor. |
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Some of the advantages of the bioreactor pit system are as follows:
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The system has only one operating part, a submersible pump in the recovery well; |
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Because the bioreactor is installed below ground, operating temperatures are constant through the year which makes it more efficient than above-ground biological treatment; |
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The use of analytical models, as developed for the proposed bioreactor, enables a designer to examine the potential applicability of this technique during the feasibility phase of the project; |
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The remediation system can be installed using currently available equipment; and |
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A capture zone is created during the operation of the system to reduce the potential for spreading of the contaminant plume during the recovery of the free product. |
As a major industry in East Tennessee, Alcoa must meet stringent
air quality emission standards due to its proximity to the Great Smoky
Mountains. As a result, air stripping was not a remedial option for this site.
The use of in-situ bioremediation allows Alcoa to remediate the groundwater
without impacting their air quality emission standards. Furthermore,
construction and operation of the system had to be performed without impacting
the active plant operations of the owner. The attached questionnaire prepared by
the owner/client provides additional comments regarding how we met and exceeded
their needs.
The observation that steeply dipping, karst bedrock with thick overlying residual soil can be modeled as a homogeneous confined aquifer with impermeable boundaries provides other engineers with a method to model groundwater flow in this complex geologic setting. This technique may enable similar sites to be characterized in a more cost-effective manner so that groundwater remediation design and implementation can begin. Geo/Environmental Associates, Inc. personnel published details of this method in the Proceedings of the Ohio River Valley Soils Seminar on Environmental and Geotechnical Site Investigations in October 1995 (
ASite Characterization Aided by Evaluation of Pumping Test Data on Environmental Remediation Projects@).The design method proposed for the below-ground bioreactor can be used by other designers with different organic contaminants. The observation that in-situ bioreactors should be located at the source of the original leakage of contamination will enable future designers to introduce aerated water with nutrients at the source of the original contamination with an improved chance of enhancing in-situ bioremediation. Geo/Environmental Associates, Inc. staff published these design methods and procedures in the Proceedings of the Environmental Engineering Journal of the American Society of Civil Engineers in December 1999 (
AIn-Situ Bioremediation Technique for Sites Underlain by Silt and Clay@).SOCIAL ECONOMIC CONSIDERATIONS
In most cases, environmental restoration is attempted by pumping the contaminated groundwater to an air stripper which forces hydrocarbons dissolved in the groundwater into the atmosphere. Air stripping does not destroy the hydrocarbons, it merely transfers the contaminants from the groundwater to the atmosphere. Due to constraints set by the Tennessee Division of Superfund, air stripping of the pumped groundwater prior to reinjection was not a treatment option.
Rather than using the common method of air stripping, groundwater clean-up at the Alcoa site is being accomplished with the aid of naturally occurring microorganisms which use the hydrocarbons as a source of food. By injecting the treated water into the area of the original source of the contamination, additional groundwater remediation is achieved by enhancing the growth of naturally occurring microorganisms within the impacted area. This method of groundwater remediation provides a cost-effective alternative that is more environmentally sound than the current method of air stripping.
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Projects] [Technical Publications]
[Technical
Awards & Events] [Staff Awards & Accomplishments]
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Post] [Map to our Office]
Copyright© Geo/Environmental Associates, Inc. 1995 through 2008
