Migration and fate of ethanol-enhanced gasoline in groundwater: a modelling analysis of a field experiment

Effect of Fe2+-activated persulfate combined with biodegradation in removing gasoline BTX from karst groundwater: A box-column experimental study

In-situ chemical oxidation with persulfate (PS-ISCO) is a preferred approach for the remediation of fuel-contaminated groundwater. Persulfate (PS) can be activated by various methods to produce stronger sulfate radicals for more efficient ISCO. Despite karst aquifers being widespread, there are few reports on PS-ISCO combined with Fe2+-activated PS. To better understand the effects of Fe2+-activated PS for the remediation of gasoline-contaminated aquifers in karst areas, a box-column experiment was conducted under flow conditions, using karst groundwater and limestone particles to simulate an aquifer. Gasoline was used as the source of hydrocarbon contaminants. Dissolved oxygen and nitrate were added to enhance bioremediation (EBR) and ferrous sulfate was used to activate PS. The effect of Fe2+-activated PS combined with biodegradation was compared during the periods of EBR + ISCO and ISCO alone, using the mass flow method for data analysis. The results showed that the initial dissolution of benzene, toluene, and xylene (BTX) from gasoline injection was rapid and variable, with a decaying trend at an average pseudo-first-order degradation rate constant of 0.032 d-1. Enhanced aerobic biodegradation and denitrification played a significant role in limestone-filled environments, with dissolved oxygen and nitrate utilization ratios of 59 ~ 72% and 12-70%, respectively. The efficiency of EBR + ISCO was the best method for BTX removal, compared with EBR or ISCO alone. The pseudo-first-order degradation rate constants of BTX reached 0.022-0.039, 0.034-0.070, and 0.027-0.036 d-1, during the periods of EBR alone, EBR + ISCO, and ISCO alone, respectively. The EBR + ISCO had a higher BTX removal ratio range of 71.0 ~ 84.3% than the ISCO alone with 30.1 ~ 45.1%. The presence of Fe2+-activated PS could increase the degradation rate of BTX with a range of 0.060 ~ 0.070 d-1, otherwise, with a range of 0.034-0.052 d-1. However, Fe2+-activated PS also consumed about 3 times the mass of PS, caused a further decrease in pH with a range of 6.8-7.6, increased 3-4 times the Ca2+ and 1.6-1.8 times the HCO3- levels, and decreased the BTX removal ratio of ISCO + EBR, compared to the case without Fe2+ activation. In addition, the accumulation of ferric hydroxides within a short distance indicated that the range of PS activated by Fe2+ may be limited. Based on this study, it is suggested that the effect of Fe2+-activated PS should be evaluated in the remediation of non-carbonate rock aquifers.

A novel concept for estimating the contaminant mass discharge of chlorinated ethenes emanating from clay till sites

Interest in using contaminant mass discharge (CMD) for risk assessment of contaminated sites has increased over the years, as it accounts for the contaminant mass that is moving and posing a risk to water resources and receptors. The most common investigation of CMD involves a transect of multilevel wells; however, this is an expensive undertaking, and it is difficult to place it in the right position in a plume. Additionally, infrastructure at the site needs to be considered. To derive an initial CMD estimate at a contaminated site and to allow for the prioritization of further investigations and remedial actions, the ProfileFlux method has been developed. It is targeted at former industrial sites with a source zone in a low conductivity layer with primarily vertical flow overlying an aquifer with primarily horizontal groundwater flow. The ProfileFlux method was developed for mature chlorinated solvent plumes, typically originating from more than 30 to 50-year-old spills, as the usage of chlorinated solvents is primarily historical. Thus, it is assumed that the contaminant had time to distribute in the low conductivity layer by mainly diffusive processes. Today the contamination is continuously released to the underlying aquifer, where advection and dispersive (other than diffusive) processes are of higher importance. The approach combines high-resolution, depth-discrete vertical concentration profiles and a simple 2D flow and transport model to estimate CMD by comparing measured and simulated concentration profiles. The study presented herein includes a global sensitivity analysis, in order to identify crucial field parameters, and of particular importance in this regard are source length, groundwater flux and infiltration. The ProfileFlux method was tested at a well-examined industrial site primarily contaminated with trichloroethylene, thereby allowing a comparison between CMD from the ProfileFlux method and the traditional transect method. CMD was estimated at 117-170 g/year, when using the ProfileFlux method, against 143 g/year with the transect method, thus validating ProfileFlux method's ability to estimate CMD. In addition, applying the method identified weak points in the conceptual site model. The method will be incorporated into a user-friendly online tool directed at environmental consultants and decision-makers working on the risk assessment and prioritization of contaminated sites with the specific hydrogeological conditions of an aquifer with an overlying low permeability layer.

The Potential of Oxygenates to Increase the Risk of Exposure to Polycyclic Aromatic Hydrocarbons through Groundwater Contamination

The Clean Air Act (42 U.S. Code § 7401) is one of the United States of America’s most influential environmental laws. Under the Clean Air Act Amendments of 1990, oxygen-containing organic compounds must be added to some fossil fuels with the goal of combating CO2 and particulate emissions. However, one major implication is the effect of co-solvency on the leaching potentials of polycyclic aromatic hydrocarbons (PAHs) into groundwater. Our research investigated this effect on three groups of recalcitrant PAHs that are present in diesel fuel. Our results reveal that ethanol addition enhances the leaching potentials of these otherwise hydrophobic contaminants, with 10% ethanol (E10) resulting in elution of all the PAHs studied. While 5% ethanol addition to diesel fuel resulted in the elution of an average of 2.5% of all the trimethylnaphthalenes and 6.0% of the C2 alkylphenanthrenes present in diesel fuel, 10% ethanol addition led to the elution of more than 80% of each of the studied trimethylnaphthalene peaks and more than 70% of each of the studied C2 alkylphenanthrene peaks present in diesel fuel. In view of the known mutagenic and carcinogenic risks associated with exposure to PAHs through groundwater contamination, our study highlights the need for energy scientists to carefully consider the environmental and health implications of ethanol-blended innovations holistically. It is not enough to save the atmosphere but ruin the hydrosphere and most importantly, human health.

Towards a digital twin for characterising natural source zone depletion: A feasibility study based on the Bemidji site

Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) may be a valid long-term management option at petroleum impacted sites. However, its future long-term reliability needs to be established. NSZD includes partitioning, biotic and abiotic degradation of LNAPL components plus multiphase fluid dynamics in the subsurface. Over time, LNAPL components are depleted and those partitioning to various phases change, as do those available for biodegradation. To accommodate these processes and predict trends and NSZD over decades to centuries, for the first time, we incorporated a multi-phase multi-component multi-microbe non-isothermal approach to representatively simulate NSZD at field scale. To validate the approach we successfully mimic data from the LNAPL release at the Bemidji site. We simulate the entire depth of saturated and unsaturated zones over the 27 years of post-release measurements. The study progresses the idea of creating a generic digital twin of NSZD processes and future trends. Outcomes show the feasibility and affordability of such detailed computational approaches to improve decision-making for site management and restoration strategies. The study provided a basis to progress a computational digital twin for complex subsurface systems.

Remediation of BTEX and Cr(VI) contamination in soil using bioelectrochemical system-an eco-friendly approach

Soil contamination with benzene, toluene, ethylbenzene and xylene isomers (BTEX) has raised increasing concern because of its high solubility in water and toxicity to biotic communities. This study aims at investigating the process and prospects of deploying bioelectrochemical system (BES) for the removal of BTEX from artificially contaminated soil using Pseudomonas putida YNS1, alongside the reduction of hexavalent chromium (Cr(VI)). The BES was setup with desired operating conditions: initial concentration of BTEX (50-400 mg/L in 100 mL of sterilized water), pH (4-10) and applied potential voltage (0.6-1.2 V) with 10 μL log-phase culture along with the addition of Cr(VI) (10 mg/L). Samples were collected at regular intervals and analysed for BTEX degradation using gas chromatography and Cr(VI) reduction using UV-Vis spectrophotometer. Under optimized conditions (initial BTEX concentration, 200 mg/L; pH 7; and applied voltage 0.8 V with Cr(VI) of 10 mg/L), 92% of BTEX was degraded and 90% Cr(VI) was reduced from the contaminated soil. The intermediates produced during degradation were analysed through gas chromatography-flame ionization detector (GC-FID), and the possible degradation pathway was elucidated. The results indicated that BES could be effective for simultaneous degradation of BTEX along with Cr(VI) reduction.

Natural source zone depletion of LNAPL: A critical review supporting modelling approaches

Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) includes partitioning, transport and degradation of LNAPL components. NSZD is being considered as a site closure option during later stages of active remediation of LNAPL contaminated sites, and where LNAPL mass removal is limiting. To ensure NSZD meets compliance criteria and to design enhanced NSZD actions if required, residual risks posed by LNAPL and its long term behaviour require estimation. Prediction of long-term NSZD trends requires linking physicochemical partitioning and transport processes with bioprocesses at multiple scales within a modelling framework. Here we expand and build on the knowledge base of a recent review of NSZD, to establish the key processes and understanding required to model NSZD long term. We describe key challenges to our understanding, inclusive of the dominance of methanogenic or aerobic biodegradation processes, the potentially changeability of rates due to the weathering profile of LNAPL product types and ages, and linkages to underlying bioprocesses. We critically discuss different scales in subsurface simulation and modelling of NSZD. Focusing on processes at Darcy scale, 36 models addressing processes of importance to NSZD are investigated. We investigate the capabilities of models to accommodate more than 20 subsurface transport and transformation phenomena and present comparisons in several tables. We discuss the applicability of each group of models for specific site conditions.

Flow field dynamics and high ethanol content in gasohol blends enhance BTEX migration and biodegradation in groundwater

Gasohol spills may easily descend through the soil column down and impact sensitive receptors as contaminants dissolve into the groundwater. Gasoline formulations are commonly blended with ethanol to alleviate environmental and economic issues associated with fossil fuels. However, the amount of ethanol added to gasoline and the groundwater hydraulic regime can significantly affect BTEX plume dynamics and lifespan. In this study, two long-term (5 and 10 years) field-scale gasohol releases with ethanol contents of 85% (E85) and 24% (E24), respectively, were assessed to discern the different dynamics undergone by gasohol blends. Statistical, geochemical, microbiological and trend approaches were employed to estimate the influence of groundwater flow variations on ethanol and dissolved BTEX transport, and the associated biodegradation rates of different gasohol blend spills. Ethanol and BTEX groundwater flow were quantified in terms of breakthrough curve characteristics, plume centroid positions and spreading, source depletion and mass degradation rates. In addition, bromide migration was evaluated to address the contribution of flow-driven dissolution. Results revealed that the high amount of ethanol along with a fast and dynamic flow exerted a flushing behavior that enhanced BTEX dissolution, migration (vertical and horizontal) and concentrations in groundwater. The higher amount of ethanol in E85 enhanced BTEX dissolution (and bioavailability) relative to E24 site and led to faster biodegradation rates, which can be explained by the cosolvency effect and metabolic flux dilution. Therefore, flow field dynamics and high ethanol content in gasohol blends enhance BTEX migration and biodegradation in gasohol-contaminated sites. The balance of these factors is crucial to determine fate and transport of contaminants in field sites. These findings suggest that hydraulic regime should be spatially and temporally characterized to support decisions on appropriate monitoring plan and remedial strategies for gasohol spills.

Evaluation of the Fenton process effectiveness in the remediation of soils contaminated by gasoline: Effect of soil physicochemical properties

The remediation of four different soils contaminated by gasoline was performed using Fenton processes. Herein, the effect of the main physicochemical characteristics of the soils in the Fenton performance is emphasized. Fenton processes were applied in a column system, with and without addition of soluble iron (II), using undisturbed soil samples collected in four regions of the Paraná State (Brazil). Two groups of contaminants were monitored during the remediation process: BTEX (benzene, toluene, ethylbenzene and xylenes) and TRHs (total recoverable hydrocarbons). Superior degradation efficiencies were observed in the soils with elevated mineral iron content (Red Argisol, Red-Yellow Argisol and Red Latosol), while the soils with low iron content (Spodosol) presented comparable degradation efficiencies only in the presence of soluble Fe²⁺. Although the presence of mineral iron enabled the Fenton processes, a good correlation between the iron content and the degradation efficiency was not observed, suggesting a dependence on the chemical nature of the native iron. BTEX leaching was observed in all systems, suggesting that the process should be applied with caution, especially in soils with high drainage.

Phytoremediation of contaminated soils containing gasoline using Ludwigia octovalvis (Jacq.) in greenhouse pots

Greenhouse experiments were carried out to determine the phytotoxic effects on the plant Ludwigia octovalvis in order to assess its applicability for phytoremediation gasoline-contaminated soils. Using plants to degrade hydrocarbons is a challenging task. In this study, different spiked concentrations of hydrocarbons in soil (1, 2, and 3 g/kg) were tested. The results showed that the mean efficiency of total petroleum hydrocarbon (TPH) removal over a 72-day culture period was rather high. The maximum removal of 79.8 % occurred for the 2 g/kg concentration, while the removal rate by the corresponding unplanted controls was only (48.6 %). The impact of gasoline on plants included visual symptoms of stress, yellowing, growth reduction, and perturbations in the developmental parameters. The dry weight and wet weight of the plant slightly increased upon exposure to gasoline until day 42. Scanning electron microscopy (SEM) indicated change to the root and stem structure in plant tissue due to the direct attachment with gasoline contaminated compared to the control sample. The population of living microorganisms in the contaminated soil was found to be able to adapt to different gasoline concentrations. The results showed that L. octovalvis and rhizobacteria in gasoline-contaminated soil have the potential to degrade organic pollutants.

Screening-level estimates of mass discharge uncertainty from point measurement methods

The uncertainty of mass discharge measurements associated with point-scale measurement techniques was investigated by deriving analytical solutions for the mass discharge coefficient of variation for two simplified, conceptual models. In the first case, a depth-averaged domain was assumed, consisting of one-dimensional groundwater flow perpendicular to a one-dimensional control plane of uniformly spaced sampling points. The contaminant flux along the control plane was assumed to be normally distributed. The second case consisted of one-dimensional groundwater flow perpendicular to a two-dimensional control plane of uniformly spaced sampling points. The contaminant flux in this case was assumed to be distributed according to a bivariate normal distribution. The center point for the flux distributions in both cases was allowed to vary in the domain of the control plane as a uniform random variable. Simplified equations for the uncertainty were investigated to facilitate screening-level evaluations of uncertainty as a function of sampling network design. Results were used to express uncertainty as a function of the length of the control plane and number of wells, or alternatively as a function of the sample spacing. Uncertainty was also expressed as a function of a new dimensionless parameter, Ω, defined as the ratio of the maximum local flux to the product of mass discharge and sample density. Expressing uncertainty as a function of Ω provided a convenient means to demonstrate the relationship between uncertainty, the magnitude of a local hot spot, magnitude of mass discharge, distribution of the contaminant across the control plane, and the sampling density.

Succession of microbial functional communities in response to a pilot-scale ethanol-blended fuel release throughout the plume life cycle

GeoChip, a comprehensive gene microarray, was used to examine changes in microbial functional gene structure throughout the 4-year life cycle of a pilot-scale ethanol blend plume, including 2-year continuous released followed by plume disappearance after source removal. Canonical correlation analysis (CCA) and Mantel tests showed that dissolved O2 (which was depleted within 5 days of initiating the release and rebounded 194 days after source removal) was the most influential environmental factor on community structure. Initially, the abundance of anaerobic BTEX degradation genes increased significantly while that of aerobic BTEX degradation genes decreased. Gene abundance for N fixation, nitrification, P utilization, sulfate reduction and S oxidation also increased, potentially changing associated biogeochemical cycle dynamics. After plume disappearance, most genes returned to pre-release abundance levels, but the final functional structure significantly differed from pre-release conditions. Overall, observed successions of functional structure reflected adaptive responses that were conducive to biodegradation of ethanol-blend releases.

Nitrate addition to groundwater impacted by ethanol-blended fuel accelerates ethanol removal and mitigates the associated metabolic flux dilution and inhibition of BTEX biodegradation

A comparison of two controlled ethanol-blended fuel releases under monitored natural attenuation (MNA) versus nitrate biostimulation (NB) illustrates the potential benefits of augmenting the electron acceptor pool with nitrate to accelerate ethanol removal and thus mitigate its inhibitory effects on BTEX biodegradation. Groundwater concentrations of ethanol and BTEX were measured 2 m downgradient of the source zones. In both field experiments, initial source-zone BTEX concentrations represented less than 5% of the dissolved total organic carbon (TOC) associated with the release, and measurable BTEX degradation occurred only after the ethanol fraction in the multicomponent substrate mixture decreased sharply. However, ethanol removal was faster in the nitrate amended plot (1.4 years) than under natural attenuation conditions (3.0 years), which led to faster BTEX degradation. This reflects, in part, that an abundant substrate (ethanol) can dilute the metabolic flux of target pollutants (BTEX) whose biodegradation rate eventually increases with its relative abundance after ethanol is preferentially consumed. The fate and transport of ethanol and benzene were accurately simulated in both releases using RT3D with our general substrate interaction module (GSIM) that considers metabolic flux dilution. Since source zone benzene concentrations are relatively low compared to those of ethanol (or its degradation byproduct, acetate), our simulations imply that the initial focus of cleanup efforts (after free-product recovery) should be to stimulate the degradation of ethanol (e.g., by nitrate addition) to decrease its fraction in the mixture and speed up BTEX biodegradation.

The uncertainty of mass discharge measurements using pumping methods under simplified conditions

Mass discharge measurements at contaminated sites have been used to assist with site management decisions, and can be divided into two broad categories: point-scale measurement techniques and pumping methods. Pumping methods can be sub-divided based on the pumping procedures used into sequential, concurrent, and tandem circulating well categories. Recent work has investigated the uncertainty of point measurement methods, and to a lesser extent, pumping methods. However, the focus of this study was a direct comparison of uncertainty between the various pumping method approaches that have been used, as well as a comparison of uncertainty between pumping and point measurement methods. Mass discharge measurement error was investigated using a Monte Carlo modeling analysis as a function of the contaminant plume position and width, and as a function of the pumping conditions used in the different pumping tests. Results indicated that for the conditions investigated, uncertainty in mass discharge estimates based on pumping methods was 1.3 to 16 times less than point measurement method uncertainty, and that a sequential pumping approach resulted in 5 to 12 times less uncertainty than the concurrent pumping or tandem circulating well approaches. Uncertainty was also investigated as a function of the plume width relative to well spacing. For a given well spacing, uncertainty decreased for all methods as the plume width increased, and comparable levels of uncertainty between point measurement and pumping methods were obtained when three wells were distributed across the plume. A hybrid pumping technique in which alternate wells were pumped concurrently in two separate campaigns yielded similar uncertainty to the sequential pumping approach. This suggests that the hybrid approach can be used to capitalize on the advantages of sequential pumping yet minimize the overall test duration.

Complex resistivity signatures of ethanol biodegradation in porous media

Numerous adverse effects are associated with the accidental release of ethanol (EtOH) and its persistence in the subsurface. Geophysical techniques may permit non-invasive, real time monitoring of microbial degradation of hydrocarbon. We performed complex resistivity (CR) measurements in conjunction with geochemical data analysis on three microbial-stimulated and two control columns to investigate changes in electrical properties during EtOH biodegradation processes in porous media. A Debye Decomposition approach was applied to determine the chargeability (m), normalized chargeability (m(n)) and time constant (τ) of the polarization magnitude and relaxation length scale as a function of time. The CR responses showed a clear distinction between the bioaugmented and control columns in terms of real (σ') and imaginary (σ″) conductivity, phase (ϕ) and apparent formation factor (F(app)). Unlike the control columns, a substantial decrease in σ' and increase in F(app) occurred at an early time (within 4 days) of the experiment for all three bioaugmented columns. The observed decrease in σ' is opposite to previous studies on hydrocarbon biodegradation. These columns also exhibited increases in ϕ (up to ~9 mrad) and σ″ (up to two order of magnitude higher) 5 weeks after microbial inoculation. Variations in m and m(n) were consistent with temporal changes in ϕ and σ″ responses, respectively. Temporal geochemical changes and high resolution scanning electron microscopy imaging corroborated the CR findings, thus indicating the sensitivity of CR measurements to EtOH biodegradation processes. Our results offer insight into the potential application of CR measurements for long-term monitoring of biogeochemical and mineralogical changes during intrinsic and induced EtOH biodegradation in the subsurface.

Impacts of an ethanol-blended fuel release on groundwater and fate of produced methane: Simulation of field observations

In a field experiment at Vandenberg Air Force Base (VAFB) designed to mimic the impact of a small-volume release of E10 (10% ethanol and 90% conventional gasoline), two plumes were created by injecting extracted groundwater spiked with benzene, toluene, and o-xylene, abbreviated BToX (No-Ethanol Lane) and BToX plus ethanol (With-Ethanol Lane) for 283 days. We developed a reactive transport model to understand processes controlling the fate of ethanol and BToX. The model was calibrated to the extensive field dataset and accounted for concentrations of sulfate, iron, acetate, and methane along with iron-reducing bacteria, sulfate-reducing bacteria, fermentative bacteria, and methanogenic archaea. The benzene plume was about 4.5 times longer in the With-Ethanol Lane than in the No-Ethanol Lane. Matching this different behavior in the two lanes required inhibiting benzene degradation in the presence of ethanol. Inclusion of iron reduction with negligible growth of iron-reducers was required to reproduce the observed constant degradation rate of benzene. Modeling suggested that vertical dispersion and diffusion of sulfate from an adjacent aquitard were important sources of sulfate in the aquifer. Matching of methane data required incorporating initial fermentation of ethanol to acetate, methane loss by outgassing, and methane oxidation coupled to sulfate and iron reduction. Simulation of microbial growth using dual Monod kinetics, and including inhibition by more favorable electron acceptors, generally resulted in reasonable yields for microbial growth of 0.01-0.05.

Persulfate injection into a gasoline source zone

One pore volume of unactivated sodium persulfate was delivered into an emplaced gasoline residual source zone at CFB Borden. Concentrations of inorganic species (S2O8(2-), SO4(2-), Na(+), dissolved inorganic carbon (DIC)) and selected gasoline compounds (benzene, toluene, ethylbenzene, xylenes, trimethylbenzenes and naphthalene) were monitored across a transect equipped with 90 multilevel sampling points for >10months post-injection. Mass loading (M˙) of compounds constructed from the transect data was used for assessment purposes. Breakthrough of inorganic species was observed when the injection slug crossed the monitoring transect. An increase in [Formula: see text] indicated persulfate consumption during oxidation of gasoline compounds or degradation due to the interaction with aquifer materials. M˙DIC increased by >100% suggesting some mineralization of gasoline compounds during treatment. Mass loading for all the monitored gasoline compounds reduced by 46 to 86% as the inorganic slug crossed the monitoring transect. The cumulative mass discharge across the monitoring transect was 19 to 58% lower than that expected without persulfate injection. After the inorganic injection slug was flushed from the source zone a partial rebound (40 to 80% of baseline levels) of mass discharge of the monitored gasoline compounds was observed. The ensemble of data collected provides insight into the fate and transport of the injected persulfate solution, and the accompanying treatment of a gasoline the source zone.

Complex resistivity signatures of ethanol in sand-clay mixtures

We performed complex resistivity (CR) measurements on laboratory columns to investigate changes in electrical properties as a result of varying ethanol (EtOH) concentration (0% to 30% v/v) in a sand-clay (bentonite) matrix. We applied Debye decomposition, a phenomenological model commonly used to fit CR data, to determine model parameters (time constant: τ, chargeability: m, and normalized chargeability: mn). The CR data showed a significant (P≤0.001) time-dependent variation in the clay driven polarization response (~12 mrad) for 0% EtOH concentration. This temporal variation probably results from the clay-water reaction kinetics trending towards equilibrium in the sand-clay-water system. The clay polarization is significantly suppressed (P≤0.001) for both measured phase (ϕ) and imaginary conductivity (σ″) with increasing EtOH concentration. Normalized chargeability consistently decreases (by up to a factor of ~2) as EtOH concentration increases from 0% to 10% and 10 to 20%, respectively. We propose that such suppression effects are associated with alterations in the electrical double layer (EDL) at the clay-fluid interface due to (a) strong EtOH adsorption on clay, and (b) complex intermolecular EtOH-water interactions and subsequent changes in ionic mobility on the surface in the EDL. Changes in the CR data following a change of the saturating fluid from EtOH 20% to plain water indicate strong hysteresis effects in the electrical response, which we attribute to persistent EtOH adsorption on clay. Our results demonstrate high sensitivity of CR measurements to clay-EtOH interactions in porous media, indicating the potential application of this technique for characterization and monitoring of ethanol contamination in sediments containing clays.

Denatured ethanol release into gasoline residuals, Part 2: fate and transport

When denatured ethanol (E95) is spilled in a site with previous gasoline contamination, it modifies the source distribution (Part 1). But it can also impact the transport and fate of hydrocarbons in the groundwater. Ethanol could cause an increase in dissolved concentrations and more persistent plumes due to cosolvency and decreased hydrocarbon biodegradation rates. To investigate these possibilities, two controlled releases were performed: first of E10 (gasoline with 10% ethanol) and one year later of E95 on top of the gasoline. Groundwater concentrations were monitored above and below the water table in multilevel wells. Soil cores and vapor samples were also collected over a period of approximately 400 days. Surprisingly, ethanol transport was very limited; at wells located 2.3m downgradient from the mid-point of the release trench, the maximum concentration measured was around 2400 mg/L. After 392 days, only 3% of the ethanol released migrated past 2.3 m, and no ethanol remained in the source. The processes that caused ethanol loss were likely volatilization, aerobic biodegradation in the unsaturated zone, and anaerobic biodegradation. Evidence that biodegradation was significant in the source zone includes increased CO2 concentrations in the vapor and the presence of biodegradation products (acetate concentrations up to 2300 mg/L). The position of the dissolved hydrocarbon plumes was slightly shifted, but the concentrations and mass flux remained within the same range as before the spill, indicating that cosolvency was not significant. Hydrocarbons in the groundwater were significantly biodegraded, with more than 63% of the mass being removed in 7.5m, even when ethanol was present in the groundwater. The impacts of ethanol on the hydrocarbon transport and fate were minimal, largely due to the separation of ethanol and hydrocarbons in the source (Part 1).

Electrical signatures of ethanol-liquid mixtures: implications for monitoring biofuels migration in the subsurface

Ethanol (EtOH), an emerging contaminant with potential direct and indirect environmental effects, poses threats to water supplies when spilled in large volumes. A series of experiments was directed at understanding the electrical geophysical signatures arising from groundwater contamination by ethanol. Conductivity measurements were performed at the laboratory scale on EtOH-water mixtures (0 to 0.97 v/v EtOH) and EtOH-salt solution mixtures (0 to 0.99 v/v EtOH) with and without a sand matrix using a conductivity probe and a four-electrode electrical measurement over the low frequency range (1-1000 Hz). A Lichtenecker-Rother (L-R) type mixing model was used to simulate electrical conductivity as a function of EtOH concentration in the mixture. For all three experimental treatments increasing EtOH concentration resulted in a decrease in measured conductivity magnitude (|σ|). The applied L-R model fitted the experimental data at concentration ≤0.4v/v EtOH, presumably due to predominant and symmetric intermolecular (EtOH-water) interaction in the mixture. The deviation of the experimental |σ| data from the model prediction at higher EtOH concentrations may be associated with hydrophobic effects of EtOH-EtOH interactions in the mixture. The |σ| data presumably reflected changes in relative strength of the three types of interactions (water-water, EtOH-water, and EtOH-EtOH) occurring simultaneously in EtOH-water mixtures as the ratio of EtOH to water changed. No evidence of measurable polarization effects at the EtOH-water and EtOH-water-mineral interfaces over the investigated frequency range was found. Our results indicate the potential for using electrical measurements to characterize and monitor EtOH spills in the subsurface.