A mass balance approach to resolving the stability of LNAPL bodies
Date
2010
Authors
Mahler, Nicholas T., author
Sale, Thomas C., advisor
Bau, Domenico A., committee member
McWhorter, David B., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Light non-aqueous phase liquids (LNAPLs) are commonly present in soils and groundwater beneath petroleum facilities. When sufficient amounts of LNAPL have been released continuous bodies of LNAPL form. These bodies can have detrimental impacts to soil gas and groundwater. Furthermore, with time they can expand or translate laterally. Measurements of LNAPL flux within continuous bodies typically indicate that LNAPL is moving, albeit slowly. Commonly, these fluxes have been used to infer (by continuity) that the bodies as a whole are expanding and/or translating laterally. In conflict with this, dissolved plumes downgradient of LNAPL bodies are widely thought to be stable or shrinking due to natural attenuation. The hypothesis of this research is that natural losses of LNAPL in contiguous bodies can play an important role in limiting expansion and/or lateral translation of LNAPL bodies. Much like dissolved phase plumes, LNAPL bodies can be stable when internal fluxes are balanced by natural losses. As a first step, 50 measurements of LNAPL fluxes through wells from seven field sites are reviewed. All the values were acquired using tracer dilution techniques. The mean and median of the LNAPL flux measurements are 0.15 and 0.064 m/year, respectively. The measured LNAPL fluxes are three to five orders of magnitude less than typical groundwater fluxes. The primary significance of the small magnitude of the LNAPL fluxes relative to groundwater fluxes is that LNAPL discharge to the downgradient body could easily be equal to or less than the natural downgradient LNAPL losses that occur through dissolution into groundwater or evaporation into soil gas. In general no clear correlations are seen between measured LNAPL fluxes and LNAPL thicknesses in wells, lengths to downgradient edges of LNAPL, or the specific gravities (density of LNAPL/ density of water) of the LNAPL. Secondly, a proof-of-concept sand tank experiment is presented. The objective was to resolve if natural LNAPL losses can limit expansion of an LNAPL body given a constant source. An open top glass and stainless steel tank (1 m by 0.5 m by 0.025 m) was filled with uniform coarse sand and water. Water was pumped through the tank producing a water seepage velocity of 0.25 m/day. Methyl tert-butyl ether (MTBE) was added to the tank at constant rates that were step-wise increased five times through a 120 day experiment. In all cases the MTBE body initially expanded followed by subsequent stabilization at a finite length. The key observation was that steady LNAPL pool lengths were achieved with a constant inflow of LNAPL into the system. Lastly, analytical models are developed. The models describe the size of LNAPL bodies and spatial variations in LNAPL fluxes as a function of influent loading, rates of natural losses, and time. Three idealized geometries of LNAPL bodies are considered. These include one dimensional, circular, and oblong. Results indicate LNAPL fluxes decline progressing from the interior to the edges of an LNAPL body. Per the laboratory studies, the solutions show that LNAPL bodies with a constant source reach finite dimensions at large times. Building on this research it seems that a pragmatic goal for management of contiguous LNAPL bodies is attaining a condition where the LNAPL bodies as a whole are stable or shrinking.
Description
Department Head: Luis A. Garcia.