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Resolving natural losses of LNAPL using CO2 traps




McCoy, Kevin M., author
Sale, Thomas C., advisor
Zimbron, Julio A., advisor
Shackelford, Charles D., committee member
Ronayne, Michael, committee member

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Pools of light non-aqueous phase liquids (LNAPLs) are a legacy of past practices at petroleum facilities. Traditional LNAPL remedies (e.g. hydraulic LNAPL recovery) are often costly and have limited effectiveness. Recent studies have indicated that natural losses of LNAPL can help to stabilize and even shrink subsurface LNAPL bodies once the LNAPL source is removed. Developing an effective understanding of natural losses of LNAPL is an important step in establishing LNAPL management strategies. Estimated rates of natural losses of LNAPL can be used to demonstrate LNAPL stability, form a basis for initiating or discontinuing hydraulic recovery, estimate longevity of LNAPL bodies, and as a benchmark to compare relative effectiveness of different remedial alternatives. Additionally, an understanding of underlying processes gained through field studies can guide development of new, more sustainable LNAPL remediation technologies. A novel integral CO2 Trap was created to measure soil CO2 efflux at grade. This addresses a need for an efficient tool to quantify natural losses of LNAPL. The hypothesis of this thesis is that CO2 Traps can be used to quantify natural losses of LNAPL at field sites. Laboratory and field tests were performed to test the CO2 Traps and demonstrate their utility. First, laboratory experiments were undertaken to demonstrate the ability of the traps to quantitatively capture CO2 and effectively estimate CO2 fluxes. Closed system column testing showed that the selected sorbent media is capable of quantitatively recovering CO2. This testing also verified that the sorption capacity of the media (~30% CO2 by mass) was in the range indicated by the manufacturer. This information is useful when planning maximum field deployment times, and as a means of quality checking field sampling results. Next, an open system column test showed that the CO2 Traps are capable of quantitatively measuring CO2 flux through porous media. The traps were field tested. Results of a single round of CO2 Trap deployment at one field site showed that the traps could distinguish zones of elevated CO2 flux over the LNAPL body, relative to naturally occurring CO2 flux at background locations. Background subtracted LNAPL loss rates ranging from 800 to 12,000 gallons per acre per year (gal/acre/yr) were observed. Carbon isotope analysis was performed on one travel blank sample, two background samples, and one LNAPL area sample. Radiocarbon (14C) results provided an independent means to estimate naturally occurring CO2 flux. Results of the 14C correction agreed well with the background subtraction method for that location. CO2 traps have been deployed at a total of 117 locations at 6 field sties. Seasonal resampling of selected locations has yielded a total of 194 CO2 flux readings. Calculated background corrected LNAPL loss rates for ranged from 400 - 18,000 gal/acre/yr with a mean of 3,500 gal/acre/yr. A detailed analysis of the influence of site and LNAPL characteristics on calculated LNAPL loss rates was performed for one of the six sites. Results indicated that natural losses of LNAPL are largely independent of in-well LNAPL thickness, depth to smear zone, smear zone thickness, or LNAPL type. However, temperature related seasonal trends were observed. Furthermore, natural losses of LNAPL appear to result in self heating of LNAPL zones with a potential benefit of enhancing natural losses. Additional data analysis suggests a link between temperature and natural LNAPL loss rate that may be useful in developing new, more sustainable, LNAPL management technologies.


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natural attenuation
natural losses


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