Method comparison for analysis of LNAPL natural source zone depletion using CO₂ fluxes
Date
2015
Authors
Tracy, Melissa Kay, author
Sale, Tom, advisor
Blotevogel, Jens, committee member
Butters, Greg, committee member
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Abstract
Accidental releases of subsurface petroleum hydrocarbons, widely referred to as Light Non-Aqueous Phase Liquids (LNAPLs), are a common occurrence in the industrial world. Given potential risks to human health and the environment, effective remediation approaches are needed to address impacts. Natural source zone depletion (NSZD) is a remedial approach gaining wide acceptance, wherein natural mechanisms in the subsurface act to deplete LNAPL in the source zone. Recent research indicates biodegradation of contaminant-related carbon results in a predominantly upward flux of carbon through the vadose zone. Building on this concept, three methods have recently emerged to quantify rates of NSZD using soil gas fluxes; these include the gradient, chamber, and trap methods. Unfortunately, side-by-side field applications of the methods have shown differing estimates of NSZD, leaving concerns about method comparability. The primary objective of this thesis was to conduct a laboratory comparison of the gradient, chamber, and trap methods using uniform porous media, constant environmental conditions, and a known CO₂ flux (i.e., ideal conditions). Given these experimental conditions, challenges associated with field comparisons could be minimized and the fundamental accuracy of the methods could be resolved. Preliminary efforts were also made to understand the effect of surface wind on the accuracy of the methods. A large-scale column (1.52 m high x 0.67 m ID) was filled with dry, homogenous, well-sorted fine sand. Known CO₂ fluxes were imposed through the bottom of the column spanning a range typical of contaminant-related CO₂ fluxes observed at field sites (3.3-15.2 μmol/m²/s). Results under ideal experimental conditions indicated that on average, the chamber and trap methods accurately captured the imposed flux to within ± 7% of the true value, and the gradient method underestimated the imposed flux to within 38% of the true value. Accuracy of the gradient method was largely dependent on estimates of effective diffusion coefficients. Consistent underestimation of the true flux using the gradient method was attributed to the method only quantifying diffusive gas transport. Considering the accuracy of measurements for other subsurface processes (e.g., hydraulic conductivity), the range of accuracy observed among all methods is not surprising. Surface winds were simulated by placing a fan on top of the column; achieved wind speeds ranged from 2.2-5.4 m/s. Laboratory studies identified that all methods were adversely affected by wind; however, the magnitude of laboratory results may have been exaggerated relative to what would be expected at field sites due to the laboratory sand being dry. Wind speeds within the tested range caused the gradient method to further underestimate the true flux to within 44% of the true value. The chamber method underestimated the true flux by 45-47% and 78% for wind speeds ranging from 2.2-3.6 m/s and 4.5-5.4 m/s, respectively. Wind had the opposite effect on the trap method, causing overestimations of the true flux by 60% and 122% for wind speeds ranging from 2.2-3.6 m/s and 4.5-5.4 m/s, respectively. Given similar results under ideal experimental conditions, wind and other environmental factors common to field conditions are suspected to be the primary cause of disagreement observed in side-by-side comparisons of the methods at field sites. Each method has advantages and limitations for field application. Method selection should be predominately driven by site-specific attributes, including environmental factors that may make one method more applicable over another for a given field site. Further consideration of all methods under environmental conditions may provide greater insight into potential biases and support additional recommendations for method selection. Secondary objectives included efforts to test design features specific to the trap method to support continued method development and to advance a model to describe steady-state advective and diffusive transport of a compressible gas through porous media. Results from trap modification studies suggested certain design features of the trap method may have affected the accuracy of measurements. Additional research and method development for the trap method could be undertaken to resolve issues raised in this thesis. Results from modeling efforts suggested gas transport was primarily diffusion driven, accounting for approximately 58-79% of transport, depending on estimates of the effective diffusion coefficient. Analytical modeling did not indicate an appreciable difference in advective and diffusive contributions to gas transport as the imposed flux was varied; however, measured concentration gradients counterintuitively indicated the advective contribution to transport increased as the imposed flux decreased.
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Subject
CO2 flux
methanogenesis
petroleum hydrocarbons
LNAPL
biodegradation
natural source zone depletion