Susan E. Powers

An experimental investigation of nonaqueous phase liquid dissolution in saturated subsurface systems: Transient mass transfer rates

This work focuses on the experimental measurement and mathematical modeling of processes affecting the dissolution of nonaqueous phase liquids (NAPLs) entrapped in sandy porous media. Results of a series of laboratory-scale one-dimensional column dissolution experiments indicate that the length of time required to dissolve NAPLs and substantially reduce aqueous phase effluent concentrations is many times greater than predicted by equilibrium calculations. Experimental measurements clearly show an influence of both grain size and grain size distribution on the evolution of effluent concentrations. The longer cleaning times associated with coarse or graded media are attributed to the larger and more amorphous NAPL blobs associated with these media. A general correlation for transient dissolution rates is proposed which incorporates porous medium properties, Reynolds number, and volumetric fraction of NAPL. The model is calibrated with results from styrene dissolution experiments and is shown to adequately predict trichloroethylene dissolution rates in the same sandy media over the period of time required to dissolve the NAPL.

The Transport and Fate of Ethanol and BTEX in Groundwater Contaminated by Gasohol

Ethanol is used a component in all gasoline in Brazil, and its use could increase significantly in the U.S. to meet the requirements of the Clean Air Act Amendments. Recent problems with ground water contamination by methyl tert-butyl ether (MTBE) have made policymakers more cognizant of the need to consider the overall impact of gasoline oxygenates in the environment. Therefore, a thorough understanding of the impact of incorporating ethanol as a gasoline component on the fate and transport of gasoline releases is required. This article provides a comprehensive review of the transport of ethanol and monoaromatic hydrocarbons (BTEX) in the subsurface following a gasohol spill. Two mechanisms related to the presence of ethanol are generally considered to impact BTEX transport. Ethanol can increase the aqueous concentration of BTEX compounds due to a cosolvent effect, and it can inhibit BTEX biodegradation by preferentially consuming electron acceptors and nutrients. Our review illustrates that cosolvent effects should be minor at the ethanol concentrations expected from gasohol spills. Nevertheless, the inhibition of BTEX biodegradation and the possible decrease in sorption-related retardation suggests that ethanol is likely to increase BTEX plume lengths. The net effect of ethanol on natural attenuation of BTEX is likely to be system specific, depending largely on the release scenario and the assimilative capacity of the aquifer.

Wettability of porous media after exposure to synthetic gasolines

The wettability of a porous medium plays a critical role in the capillary phenomena governing the migration of a nonaqueous-phase liquid (NAPL) and subsequent efforts to recover this type of pollutant source from the subsurface. Although it is usually assumed that water-wetting conditions occur, limited field evidence at NAPL sites suggests that wettability characteristics can change to intermediate or organic-phase wetting, especially for complex NAPLs containing polar or surfactant molecules. The focus of this work was an assessment of potential wettability conditions for quartz mineral surfaces after exposure to synthetic gasolines. Many of the chemicals added to gasoline to increase engine performance have polar or surfactant characteristics. It is hypothesized that these additives could sorb to the quartz, causing the surface to become less hydrophilic. Four gasoline additives were added to isooctane, the base chemical for the synthetic gasoline. Following the exposure of the mineral surfaces to the organic phases, wettability was measured by three different techniques: oil-water contact angles, air-water imbibition rates and oil-water capillary pressure curves coupled with calculation of the USBM wettability index. Results show a change to intermediate-wetting conditions for two of the additives considered. Both concentration and the molecular structure of the additives affect the extent of these alterations.

Non–aqueous phase liquid dissolution in heterogeneous systems: Mechanisms and a local equilibrium modeling approach

Experiments quantifying rates of non–aqueous phase liquid (NAPL) dissolution from heterogeneous media are presented and compared with model simulations. This work specifically addresses the overall dissolution of NAPL entrapped in a coarse sand lens at a high saturation. To explore the mechanisms governing dissolution rates, mathematical models describing the hydrodynamics of flow through the heterogeneous system were developed and coupled with a mass balance equation and a local equilibrium assumption (LEA) to quantify interphase mass transfer processes. Variations in the effective permeabilities as a function of NAPL saturation and the intrinsic permeabilities of the sands were employed to characterize the hydrodynamic aspects of flow through the heterogeneous system. Relative to errors generated by the ill-defined aqueous phase relative permeabilities at high NAPL saturations, the model incorporating the system hydrodynamics as the sole rate-limiting process provided a reasonable first estimate of effluent concentrations. With the representative elemental volume defined here, rate-limited dissolution becomes important for low-NAPL saturations (Sn < ∼0.05–0.15) causing tailing in the observed dissolution data and deviations between these data and the LEA model.

Modeling the partitioning of BTEX in water-reformulated gasoline systems containing ethanol

Abstract The objectives of this research were to quantify the extent of cosolvency for water–gasoline mixtures containing ethanol and to identify appropriate modeling tools for predicting the equilibrium partitioning of BTEX compounds and ethanol between an ethanol-bearing gasoline and water. Batch-equilibrium experiments were performed to measure ethanol and BTEX partitioning between a gasoline and aqueous phase. The experiments incorporated simple binary and multicomponent organic mixtures comprised of as many as eight compounds as well as highly complex commercial gasolines where the composition of the organic phase was not completely defined. At high ethanol volume fractions, the measured partition coefficients displayed an approximate linear relationship when plotted on semi-log scale as a function of ethanol volume fraction. At lower concentrations, however, there was a distinctly different trend which is attributed to a change in solubilization mechanisms at these concentrations. Three mathematical models were compared with or fit to the experimental results. Log-linear and UNIFAC-based models were used in a predictive capacity and were capable of representing the overall increase in partition coefficients as a function of increasing ethanol content in the aqueous phase. However, neither of these predicted the observed two-part curve. A piecewise model comprised of a linear relationship for low ethanol volume fractions and a log-linear model for higher concentrations was fit to data for a surrogate gasoline comprised of eight compounds and was then used to predict BTEX concentrations in the aqueous phase equilibrated with three different commercial gasolines. This model was superior to the UNIFAC predictions, especially at the low aqueous ethanol concentrations.

NAPL dissolution in heterogeneous systems: an experimental investigation in a simple heterogeneous system

Abstract Dissolution experiments were conducted in a two-dimensional cell with simple heterogeneous packing consisting of well-defined coarse lenses contaminated by NAPL surrounded by a clean fine sand matrix. Experiments were conducted for a range of conditions by varying the grain size of sand, initial NAPL saturations, and size and number of the coarse lenses. Experimental results show that aqueous phase concentrations at the effluent port were always well below equilibrium concentrations, while concentrations at the local sampling port were at equilibrium for S n >0.3. Sufficient experimental conditions were tested to suggest three important mechanisms affect the overall mass transfer behavior in this heterogeneous system. (1) Variability in the effective permeabilities affect the relative volume of water flowing through the NAPL source zone and therefore, the extent of dilution of contaminated water with clean water flowing around the source zone. (2) The perimeter surface area of the NAPL source zone affects the total interfacial area for mass transfer. This is an especially important contribution to the overall mass transfer rate during early phases of the experiment when there is little flow of water through this zone. (3) Within the NAPL source zone, decreased mass transfer rates due to limited interfacial area and/or increased aqueous phase flow rates becomes the overall rate-limiting factor in this system for S n

Establishing Measurement Criteria for an Energy Literacy Questionnaire

Energy literacy is a broad term encompassing content knowledge as well as a citizenship understanding of energy that includes affective and behavioral aspects. This article presents explicit criteria that will serve as a foundation for developing measurable objectives for energy literacy in three dimensions: cognitive (knowledge, cognitive skills), affective (attitude, values, personal responsibility); and behavioral. The outcome of this research is a framework from which a quantitative survey of energy literacy for secondary students in New York State, United States, can be created. Efforts supported by this research may help assess the broader impacts of educational programs in terms of their effectiveness for improving students’ energy literacy.

Energy literacy among middle and high school youth

An energy literacy survey for middle and high school students has been developed according to established psychometric principles and methodologies. The survey measures energy-related knowledge, attitudes and behaviors and is correlated to established benchmarks that define energy literacy as determined by a panel of energy- and energy-education specialists. Results from a pilot of the survey among 955 New York State students indicate low levels of energy-related knowledge, with fewer than 1% of the students scoring above 80%. Attitude and behavior scores are slightly better, suggesting that while students may recognize the existence of an energy problem, they generally lack the knowledge and capabilities to effectively contribute toward a solution. Results support the need for development and implementation of energy education programs as part of the regular school curriculum.

Estimating unique soil hydraulic parameters for sandy media from multi-step outflow experiments

Abstract Estimating unique soil hydraulic parameters is required to provide input for numerical models simulating transient water flow in the vadose zone. In this paper, we analyze the capability of six soil hydraulic functions to provide unique parameter sets for sandy soils from multi-step outflow data. Initial parameter estimates and experimental boundary conditions were explored to determine their affect on the uniqueness of soil hydraulic functions. Of the hydraulic functions tested, the lognormal distribution–Mualem (LDM) function provided the best performance and a unique solution for error-free numerically generated multi-step outflow data. For experimental multi-step outflow data with inherent measurement errors, the LDM function again showed better performance and uniqueness than the van Genuchten–Mualem and Gardner–Mualem functions. In experiments with different boundary conditions, the LDM function provided the best fitting ability, resulting in unique parameter sets when the intrinsic permeability ( k ) was fixed at its measured value. The experiment that had a greater number of pneumatic pressure steps, thereby causing a lower flow rate, provided better fitting ability and more unique solutions than faster experiments.

Organic bases in NAPLs and their impact on wettability

Abstract Organic bases in crude oil have been linked to complex interfacial phenomena that could affect the multiphase flow of these fluids in subsurface systems. Little information is available, however, to correlate the strength and concentration of organic bases in nonaqueous phase liquids (NAPLs) to interfacial properties. The specific objectives of this paper were: (1) to evaluate and apply titration techniques employing nonaqueous solvents to quantify the acid neutralizing capacity and characterize the strength of the organic base constituents in complex NAPL mixtures; and, (2) to relate these characteristics to the wetting properties of quartz minerals exposed to environmentally significant NAPLs. The selection of a suitable organic solvent is critical for nonaqueous titration. Mixtures of known organic bases were used to verify the capability of three different solvent systems for quantifying the concentration and strength of organic bases. Methyl iso-butyl ketone was identified as the best solvent for the nonaqueous titration—providing both accurate measurement of the total concentration of bases as well as providing a means of differentiating between strong and weak base constituents. The NAPLs tested showed a wide range of base concentrations and were found to contain only weak organic bases. All of the NAPLs with a base number greater than two significantly altered the system wettability at pH=4.6. Only a few, however affected the wettability at pH=7.2. These observations confirm the characterization of the organic bases as weak bases. The titration techniques, however, cannot characterize the strength of the bases precisely enough to predict the aqueous phase pH at which a NAPL–water–quartz system would become oil wetting.