Grantee Research Project Results
Final Report: Reformulated Gasoline Transport and Clean-up of Spills to the Subsurface
EPA Grant Number: R821114Title: Reformulated Gasoline Transport and Clean-up of Spills to the Subsurface
Investigators: Powers, Susan E.
Institution: Clarkson University
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1995 through September 30, 1998 (Extended to September 30, 1999)
Project Amount: $240,479
RFA: Exploratory Research - Chemistry and Physics of Water (1995) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Safer Chemicals
Objective:
The overall goal of this project was to provide a better understanding of the effects of gasoline additives on the fate, transport, and remediation of reformulated gasolines in subsurface environments. Two specific types of oxygenates in reformulated gasoline are of concern: ethers, such as methyl tert-butyl ether (MTBE), and alcohols, such as ethanol or methanol. These oxygenates are important if released to an aqueous environment because of their impact on the solubility of carcinogenic and potentially carcinogenic species from the gasoline, and because they lower gasoline-water interfacial tensions. Changes in the interfacial tension will impact the capillary nature of the multiphase fluid flow processes, and the increased solubility could cause greater exposure and human health risks associated with drinking water contaminated by reformulated gasoline.Two primary tasks were completed to explore the overall impact of reformulated gasolines spilled to the subsurface: (1) an evaluation of the dissolution of ethanol-containing gasolines and the subsequent fate of gasoline constituents in groundwater systems, and (2) the identification of better adsorbents for removing MTBE from gasoline-contaminated groundwater. Each of these tasks involved both experimental and modeling components. For all of the experimental work conducted, suitable surrogate gasolines were developed for the well-characterized experiments, and additional experiments with commercial gasolines were conducted to confirm the conclusions drawn with the simplified systems.
The environmental fate of oxygenated gasolines has been a significant concern since the discovery of widespread MTBE contamination in groundwaters and surface waters in the mid-1990s. The use of oxygenated gasolines in the United States is required under the Clean Air Act Amendments of 1990. The implementation of MTBE as the most commonly used oxygenate to meet these regulatory requirements is now seen as an environmental disaster; the remedy for air pollution problems has created water pollution problems. This has led to the recent phase-out of MTBE use in the State of California, and the passage of a U.S. Senate resolution expressing support for a nationwide phase-out of MTBE. The Senate supports the use of ethanol as its replacement.
The policy requiring fuel oxygenates in gasoline was implemented largely without consideration for the potential consequences of this policy on aquatic environments. Present deliberations, such as those occurring in the State of California, recognize the need for a lifecycle approach to understand potential implications of new policies related to gasoline formulation. The problems associated with past use of reformulated gasoline and the need for a sound basis for future policies set the stage for the research conducted in this grant supported by the U.S. Environmental Protection Agency. The two issues that were addressed in this research are directly related to these policy decisions. The drivers for this research were the facts that: (1) it is difficult to remove MTBE from contaminated water because of its hydrophobicity; and (2) there is limited understanding of the fate and transport of other fuel oxygenates in aquatic systems.
The research completed through this grant has had immediate and significant impacts on the oxygenated gasoline problem. The results of the MTBE treatment phase of the research was used to build a pilot plant for treating MTBE-contaminated groundwater in Santa Monica, CA. At the time of this report, the pilot study is waiting for regulatory approval prior to beginning the study. Results from research on the fate of gasohol in the subsurface were incorporated into an extensive review of current knowledge of the impacts of gasohol on the environment. This report will be used by the State of California in their deliberations on suitable gasoline formulations for their state. The technical findings associated with these two phases of the research are summarized below.
Summary/Accomplishments (Outputs/Outcomes):
Task 1: Dissolution and Fate of Gasohol in the Subsurface. This study was performed to assess the degree to which the presence of ethanol in gasoline can enhance the subsurface migration of other gasoline compounds, especially the BTEX (benzene, toluene, ethylbenzene, and xylenes) series of aromatic compounds. This assessment necessarily includes a study of the physiochemical processes governing the mass transfer of ethanol and the BTEX compounds from a gasoline to a groundwater, as well as the processes controlling the subsequent fate and transport of these compounds within a groundwater body. Computer modeling analysis was considered to be the most appropriate means of employing known physiochemical relationships to predict mass transport behavior at the field scale. Laboratory studies, in turn, were performed to: (1) observe mass transfer behavior between a gasoline and groundwater, (2) establish equilibrium partitioning and interphase mass-transfer relationships for use in the computer model, and (3) assess the validity of some of the assumptions incorporated into the computer model.
The fate of BTEX compounds in groundwater when ethanol is present involves a number of highly interdependent processes. Five such processes were incorporated into the model: the mass transfer of ethanol from the gasoline to the groundwater, advective and dispersive transport within the groundwater, sorption, biodegradation, and the formation of nonaqueous phase liquid (NAPL) droplets below the water table.
Cosolvency. An extensive experimental effort has been completed to measure concentrations of BTEX species and ethanol in both gasoline and water following equilibration. The results of these experiments, expressed as partition coefficients as a function of the ethanol volume fraction in the aqueous phase, displayed an approximate linear relationship when plotted on semi-log scale at ethanol volume fractions greater than approximately 0.2. At lower concentrations, however, there was a distinctly different trend. The cosolvency effects at these low ethanol concentrations were minimal (Heermann and Powers, 1998).
Three mathematical models were considered for quantifying the relationship between aqueous phase concentration and ethanol content in the groundwater. The log-linear and UNIFAC models were capable of representing the overall increase in partition coefficients as a function of increasing ethanol content in the aqueous phase; however, neither of them mimicked 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 superior to the UNIFAC predictions, especially at the low ethanol concentrations expected when gasolines presently sold are spilled in the environment.
Mass Transfer Rate Limitations. The rate of mass transfer of ethanol and BTEX through ethanol-bearing gasolines was investigated via column and modeling studies. The experimental apparatus consisted of a column of sand saturated with gasoline, below which water was pumped through a completely mixed chamber. The aqueous phase was sampled over time, and the gasoline composition was sampled as a function of height above the water at two different times. A mathematical model was employed to analyze the mechanisms controlling the net mass transfer rate.
Results of the experiments clearly show that diffusion of ethanol and BTEX through the gasoline is not the controlling phenomenon. The mechanism of free convection results in a greater extent of mixing in the gasoline than expected for diffusion-controlled mass transfer. Free convection occurs when density gradients arise in an unstable manner; that is, a higher density fluid resides above a lower density fluid. As a result, a convective flow is established within the fluid, typically as "fingers," thereby blending the high and low density. The presence of ethanol ( = 0.79 g/mL) increases the density of a standard gasoline ( ~ 0.74 g/mL). Thus, when mass transfer results in a depletion of the ethanol at the gasoline-water interface, unstable density gradients arise. Control experiments with toluene, which has a density greater than ethanol ( = 0.87 g/mL), confirmed that diffusion is the controlling mechanism in situations when the unstable density gradients do not arise. Mathematically modeling the transport processes within the gasoline illustrated that an assumption that the gasoline is well mixed results in a reasonable representation of the overall mass transfer rate.
The net result of the free convection process is a rapid depletion of ethanol from the gasoline source. Thus, a "slug" of ethanol and the associated elevated BTEX concentrations is introduced into the subsurface rather than a continuous source of these contaminants. With the rapid depletion of ethanol, dissolution of BTEX from the remaining gasoline pool is expected to behave in a manner similar to that of a standard formulation gasoline.
Computer Simulations. A computer model was developed to simultaneously solve transport equations in both the gasoline and aqueous phases. The model incorporates either free convection or molecular diffusion mechanisms in the gasoline, and advection, dispersion, and adsorption in the aqueous phase. The two transport equations were coupled at the gasoline-water interface by setting the flux of ethanol and BTEX out of the gasoline equal to that entering the groundwater. Thermodynamic equilibrium of each species also was maintained at this interface. Biodegradation has not yet been incorporated into this model.
For the simulations conducted to date, it appears that the ethanol is rapidly depleted from the gasoline, resulting in a slug of ethanol entering the groundwater. This result is associated with the free convection mechanism that dominates transport in the gasoline phase. The use of 6 percent ethanol by mass in gasoline does not result in any appreciable increase in toluene concentrations or plume lengths in the aqueous phase. The net impact of ethanol on groundwater contamination is increased with 20 percent ethanol in the gasoline, with the plume lengths increased by 5?10 percent in the simulations presented here.
The modeling effort presented cannot yet be used to predict the overall impact of ethanol on groundwater quality following a gasohol spill. The incorporation of free convection in the model is an accurate presentation of mass transfer of most of the ethanol into the groundwater, but does not incorporate the expected longer term tailing of ethanol concentrations. Thus, this will slightly overpredict the initial BTEX concentrations in the aqueous phase. In contrast, the model underpredicts the net effect of biodegradation on the length of BTEX plumes. The rapid degradation of the ethanol is expected to deplete the aquifer of electron acceptors, thereby decreasing the natural attenuation of BTEX species. The net effect of this will be a greater increase in the BTEX plume lengths than predicted by the current model. Efforts are ongoing to include this mechanism in the computer model.
Task 2: Sorbents for Groundwater Treatment. In a recent report prepared for the Interagency Oxygenated Fuel Assessment under the Office of Science and Technical Policy (Zogorski, et al., 1996), the difficulty of treating groundwater contaminated by oxygenated fuels was identified as a major concern. The difficulty stems from the need to remove both hydrophillic and hydrophobic contaminants from the groundwater to achieve drinking water quality standards. Although the standard treatment approaches of air stripping and carbon adsorption are currently used, they are not very efficient when the water contains oxygenates. The result is the need for very high airflow rates for air stripping and rapid breakthrough times when activated carbon is used.
Work completed under this research grant sought to identify alternative treatment strategies that will be more effective than air stripping or activated carbon. More specifically, sorbents were tested that are more appropriate than activated carbon for the removal of oxygenates from groundwater. Bottle point isotherms show that porous graphitic carbon and two synthetic carbonaceous resins have a greater capacity for MTBE than activated carbon (Davis and Powers, 2000). The required addition of a wetting agent, such as methanol, to suspend and activate the porous graphitic carbon makes this sorbent unsuitable. The synthetic carbonaceous resins tested were found to have capacities 3-5 times greater than activated carbon at an MTBE concentration of 1 mg/L. The application of the Dubinin-Astakov isotherm to data for the synthetic carbonaceous resins suggests that the mechanism of adsorption is micropore filling. Additional bisolute experiments with m-xylene as a representative gasoline contaminant indicate that the m-xylene is preferentially sorbed, depleting the micropore volume available for MTBE sorption.
Although the carbonaceous resins are technically feasible for the removal of MTBE from contaminated groundwater, they are very expensive. A comparison of the cost of employing virgin activated carbon ($0.88/g MTBE sorbed) versus the carbonaceous resin ($4.95/g MTBE sorbed) indicates that activated carbon is still much cheaper; however, the carbonaceous resins have a much higher capacity for onsite regeneration. Thermal desorption is proposed as a method for regenerating these resins that could then lead to their economical use in treating gasoline-contaminated groundwater. Although funds were not available in this present project to complete regeneration studies, proposals are being submitted to other funding agencies to continue this line of research.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 19 publications | 4 publications in selected types | All 3 journal articles |
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Davis SW, Powers SE. Alternative sorbents for removing MTBE from gasoline-contaminated ground water. Journal of Environmental Engineering 2000;126(4):354-360. |
R821114 (1998) R821114 (Final) |
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Heermann SE, Powers SE. Modeling the partitioning of BTEX in water-reformulated gasoline systems containing ethanol. Journal of Contaminant Hydrology 1998;34(4):315-341. |
R821114 (1998) R821114 (Final) |
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Powers SE, Hunt CS, Heermann SE, Corseuil HX, Rice D, Alvarez PJJ. The transport and fate of ethanol and BTEX in groundwater contaminated by gasohol. Critical Reviews in Environmental Science and Technology 2001;31(1):79-123. |
R821114 (Final) |
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Supplemental Keywords:
gasoline, oxygenates, MTBE, gasohol, ethanol, groundwater contamination, synthetic adsorbents, carbonaceous resins, NAPL dissolution, cosolvency, free convection, interphase mass transfer, UNIFAC, UNIQUAC., RFA, Scientific Discipline, Industry Sectors, Toxics, Water, Waste, TREATMENT/CONTROL, Ecosystem Protection/Environmental Exposure & Risk, Remediation, Treatment Technologies, Physics, Chemistry, Contaminant Candidate List, Mining - NAIC 21, Fate & Transport, chemical mixtures, Hazardous Waste, Transportation and Warehousing - NAIC 48-49, Drinking Water, Hazardous, Engineering, Chemistry, & Physics, Groundwater remediation, EPCRA, Watersheds, fate and transport, gasoline, cosolvency, Methyl tert butyl ether, contaminant transport, gasoline additives, solubility, contaminated waters, cleanup, MTBE, oxygenates, BTEX, UNIQUAC, spills, treatment, oil spills, aquifer remediation design, analytical chemistry, chemical kinetics, sorbents, transport models, environmental transport and fate, environmental chemistry, groundwater contamination, NAPLs, UNIFAC, ground water, other - risk management, groundwaterRelevant Websites:
http://www-erd.llnl.gov/ethanol/Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.