Alkaline Degradation of Dissolved Organic Matter

The degradation of dissolved organic matter (DOM) was studied in alkaline solution. The products were characterised using UV/vis spectroscopy, size‐exclusion chromatography (SEC), and by the analysis of low‐molecular‐weight organic acids (LMWOA). The degradation experiments were performed with water from a brown water lake or its isolated fulvic acid fraction and sodium hydroxide at different reaction times and temperatures. Depending on the wavelength and the reaction time, the UV/vis absorbance between 230 nm and 600 nm increased or decreased. The behaviour of model compounds during reactions in alkaline media was compared to the UV/vis spectroscopic behaviour of DOM. The release of LMWOA was described by kinetic data and compared to the data of model reactions. Evidence was given for the carboxylic esters playing a significant role in the release of LMWOA only during the beginning of the alkaline degradation. The results gained by SEC with on‐line UV and DOC detection showed that the average size of DOM was decreasing, and that a major part of the degradation products consisted of low‐molecular‐weight mono‐ and dicarboxylic acids.     

Effect of Biofuels on Biodegradation of Benzene and Toluene at Gasoline Spill Sites

The risk that benzene and toluene from spills of gasoline will impact drinking water wells is largely controlled by the natural anaerobic biodegradation of benzene and toluene. Benzene and toluene, as well as ethanol and other biofuels, are degraded under anaerobic conditions to the same pool of degradation products. Biodegradation of biofuels may produce concentrations of degradation products that make the thermodynamics for degradation of benzene and toluene infeasible under methanogenic conditions and produce larger plumes of benzene and toluene. This study evaluated the concentrations of fuel alcohols that are necessary to inhibit the anaerobic degradation of benzene and toluene under methanogenic conditions. At two ethanol spill sites, concentrations of ethanol greater ≥42 mg/L inhibited the anaerobic degradation of toluene. The pH and concentrations of acetate, dissolved inorganic carbon, and molecular hydrogen were used to calculate the Gibbs free energy for the biodegradation of toluene. In general, the anaerobic biodegradation of toluene was not thermodynamically feasible in water with ≥42 mg/L ethanol. In a microcosm study, when the concentrations of ethanol were ≥14 mg/L or the concentrations of n‐butanol were ≥16 mg/L, the biodegradation of the alcohols consistently produced concentrations of hydrogen, dissolved inorganic carbon, and acetate that would preclude natural anaerobic biodegradation of benzene and toluene by syntrophic organisms. In contrast, iso‐butanol and n‐propanol only occasionally produced conditions that would preclude the biodegradation of benzene and toluene.     

Evaluating TCE Abiotic and Biotic Degradation Pathways in a Permeable Reactive Barrier Using Compound Specific Isotope Analysis

A pilot‐scale zero valent iron (ZVI) Permeable Reactive Barrier (PRB) was installed using an azimuth‐controlled ‐vertical hydrofracturing at an industrial facility to treat a chlorinated Volatile Organic Compound (VOC) plume. Following ZVI injection, no significant reduction in concentration was observed to occur with the exception of some multilevel monitoring wells, which also showed high levels of total organic carbon (TOC). These patterns suggested that the zero valent iron was not well distributed in the PRB creating leaky conditions. The geochemical data indicated reducing conditions in these areas where VOC reduction was observed, suggesting that biotic processes, associated to the guar used in the injection of the iron, could be a major mechanism of VOC degradation. Compound‐Specific Isotope Analysis (CSIA) using both carbon and chlorine stable isotopes were used as a complementary tool for evaluating the contribution of abiotic and biotic processes to VOC trends in the vicinity of the PRB. The isotopic data showed enriched isotope values around the PRB compared to the isotope composition of the VOC source confirming that VOC degradation is occurring along the PRB. A batch experiment using site groundwater collected near the VOC source and the ZVI used in the PRB was performed to evaluate the site‐specific abiotic isotopic fractionation patterns. Field isotopic trends, typical of biodegradations were observed at the site and were different from those obtained during the batch abiotic experiment. These differences in isotopic trends combined with changes in VOC concentrations and redox parameters suggested that biotic processes are the predominant pathways involved in the degradation of VOCs in the vicinity of the PRB.     

A Comparison of Benzene and Toluene Plume Lengths for Sites Contaminated with Regular vs. Ethanol-Amended Gasoline

This article describes various statistical analyses of plume-length data to evaluate the hypothesis that the presence of ethanol in gasoline may hinder the natural attenuation of hydrocarbon releases. Plume dimensions were determined for gasoline-contaminated sites to evaluate the effect of ethanol on benzene and toluene plume lengths. Data from 217 sites in Iowa (without ethanol; set 1) were compared to data from 29 sites in Kansas that were contaminated by ethanol-amended gasoline (10% ethanol by volume; set 2). The data were log-normally distributed, with mean benzene plume lengths (± standard deviation) of 193 ± 135 feet for set 1 and 263 ± 103 feet for set 2 (36% longer). The median lengths were 156 feet and 263 feet (69% longer), respectively. Mean toluene plume lengths were 185± 131 feet for set 1 and 211 ±99 feet for set 2 (14% longer), and the median lengths were 158 feet and 219 feet (39% longer), respectively. Thus, ethanol-containing BTEX plumes were significantly longer for benzene (p < 0.05), but not for toluene. A Wilcoxon signed rank test showed that toluene plumes were generally shorter than benzene plumes, which suggests that toluene was attenuated to a greater extent than benzene. This trend was more pronounced for set 2 (with ethanol), which may reflect that benzene attenuation is more sensitive to the depletion of electron acceptors caused by ethanol degradation. These results support the hypothesis that the presence of ethanol in gasoline can lead to longer benzene plumes. The importance of this effect, however, is probably site-specific, largely depending on the release scenario and the available electron acceptor pool.     

Control of BTEX Migration Using a Biologically Enhanced Permeable Barrier

A permeable barrier system. consisting of a line of closely spaced wclls. was installed perpendicular to ground water flow to control the migration of a dissolved hydrocarhon plume. The wells were charged wiih concrete briquets that release oxygen and nitrate at a controlled rate. enhancing aerobic bio-degradation in the downgradient aquifer.
Laboratory batch reactor experiments were conducted to identify concrete mixtures that slowly released oxygcn over an extended time period. Concretes prepared with urea hydrogen peroxide were unsatisfactory, while concretes prepared with calcium peroxide and a proprietary formalation of magnesium peroxide (ORC®) gradually released oxygen at a steadily declining rate. The 21 percent MgO₂ conerete cylinders and briquets released oxygen at measurable rates for up to 300 days, while the 14 percent CaO₂ briquets were exhausted by 100 days.
A full-scale permeable barrier system using ORC was constructed at a gasoline-spill site. During the first 242 days of operation. total BTFX decreased from 17 to 3.4 mg/L. and dissolved oxygen increased from 0.4 to 1.8 mg/L. during transport through the barrier. Over time, BTEX treatment efficiencies declined. indicating the barrier system had becomc less effective in releasing oxygen and nutrients to the highly contaminated portion of the aquifer. Point dilution tests and sediment analyses performed at the conclusion of the project indicated that ihc aquifer in the vicinity of the remediation wells had been clogged by precipitation with iron minerals. This clogging is believed to result from high pH from the concrete and oxygen released by ihc ORC. Oxygen-releasing permeable barriers and other aerobic bioremediation processes should be used with caution in aquifers with high levels of dissolved iron.     

Dissolved Organic Carbon Fractions Formed during Composting of Municipal Solid Waste: Properties and Significance

The properties and transformation of dissolved organic matter (DOM) extracted (10 L water per kilogram compost) from municipal solid waste (MSW) compost at five stages (days 47, 77, 105, 126, and 187) of composting were investigated. The DOM was fractionated into hydrophobic or hydrophilic neutrals, acids, and bases. The unfractionated DOM, the hydrophobic acids and neutrals (HoA and HoN, respectively), and the hydrophilic neutrals (HiN) fractions were studied using solid-state ¹³C-NMR, FTIR, and DRIFT spectroscopy. The HoA fraction was found to be the dominant (percentage of total DOM) hydrophobic fraction, exhibiting a moderate increase during composting. The HoN fraction increased sharply from less than 1% to 18% of the total DOM during 187 days of composting, while the hydrophobic bases (HoB) exhibited the opposite trend. The HiN represented the major fraction of the hydrophiles up to 120 days of composting, decreasing thereafter by 38%. The relative concentration of the hydrophilic acids and bases (HiA and HiB, respectively) exhibited no consistent trend during composting. DRIFT spectra of the unfractionated DOM taken from the composting MSW revealed a decreasing level of polysaccharide structures with time. The ¹³C-NMR and FTIR spectra of the HoA fraction exhibited a polyphenol-humic structure, whereas the HoN spectra exhibited strong aliphatic features. The spectra of the HiN fraction confirmed its polysaccharide nature. During the final stage of composting, the DOM concentration was steady, while a relative decrease of HiN concomitant with an increase of HoA and HoN fractions was observed. These indicate that the DOM contained a low concentration of biodegradable organic matter and a higher content of macromolecules related to humic substances. The biological significance and heavy metal binding of these fractions are being studied based on earlier observations showing enhanced plant growth in the presence of DOM extracted from mature as opposed to immature compost.     

Parallel 3-D Simulation of a Fault Gouge Using the Lattice Solid Model

Despite the insight gained from 2-D particle models, and given that the dynamics of crustal faults occur in 3-D space, the question remains, how do the 3-D fault gouge dynamics differ from those in 2-D? Traditionally, 2-D modeling has been preferred over 3-D simulations because of the computational cost of solving 3-D problems. However, modern high performance computing architectures, combined with a parallel implementation of the Lattice Solid Model (LSM), provide the opportunity to explore 3-D fault micro-mechanics and to advance understanding of effective constitutive relations of fault gouge layers. In this paper, macroscopic friction values from 2-D and 3-D LSM simulations, performed on an SGI Altix 3700 super-cluster, are compared. Two rectangular elastic blocks of bonded particles, with a rough fault plane and separated by a region of randomly sized non-bonded gouge particles, are sheared in opposite directions by normally-loaded driving plates. The results demonstrate that the gouge particles in the 3-D models undergo significant out-of-plane motion during shear. The 3-D models also exhibit a higher mean macroscopic friction than the 2-D models for varying values of interparticle friction. 2-D LSM gouge models have previously been shown to exhibit accelerating energy release in simulated earthquake cycles, supporting the Critical Point hypothesis. The 3-D models are shown to also display accelerating energy release, and good fits of power law time-to-failure functions to the cumulative energy release are obtained.     

Land Application of Sulfate Salts for Enhanced Natural Attenuation of Benzene in Groundwater: A Case Study

Sulfate reducing conditions are widely observed in groundwater plumes associated with petroleum hydrocarbon releases. This leads to sulfate depletion in groundwater which can limit biodegradation of hydrocarbons (usually benzene, toluene, ethylbenzene, xylenes [BTEX] compounds) and can therefore result in extended timeframes to achieve groundwater cleanup objectives by monitored natural attenuation. Under these conditions, sulfate addition to the subsurface can potentially enhance BTEX biodegradation and facilitate enhanced natural attenuation. However, a delivery approach that enables effective contact with the hydrocarbons and is able to sustain elevated and uniform sulfate concentrations in groundwater remains a key challenge. In this case study, sulfate addition to a groundwater plume containing predominantly benzene by land application of agricultural gypsum and Epsom salt is described. Over 4 years of groundwater monitoring data from key wells subjected to pilot‐scale and site‐wide land application events are presented. These are compared to data from pilot testing employing liquid Epsom salt injections as an alternate sulfate delivery approach. Sulfate land application, sulfate retention within the vadose zone, and periodic infiltration following ongoing precipitation events resulted in elevated sulfate concentrations (>150 mg/L) in groundwater that were sustained over 12 months between application events and stimulated benzene biodegradation as indicated by declines in dissolved benzene concentration, and compound‐specific isotope analysis data for carbon in benzene. Long‐term groundwater benzene concentration reductions were achieved in spite of periodic rebounds resulting from water table fluctuations across the smear zone. Land application of gypsum is a potentially cost‐effective sulfate delivery approach at sites with open, unpaved surfaces, relatively permeable geology, and shallow hydrocarbon impacts. However, more research is needed to understand the fate and persistence of sulfate and to improve the likelihood of success and effectiveness of this delivery approach.     

Predicting Diel,Diurnal and Nocturnal Dynamics of Dissolved Oxygen and Chlorophyll‐a Using Regression Models and Neural Networks

Human‐induced and natural interruptions with continuous streams of observational data necessitate the development of gap‐filling and prediction strategies towards better understanding, monitoring and management of aquatic systems. This study quantified the efficacy of multiple non‐linear regression (MNLR) versus artificial neural network (ANN) models as well as the temporal partitioning of diurnal versus nocturnal data for the predictions of chlorophyll‐a (chl‐a) and dissolved oxygen (DO) dynamics. The temporal partitioning increased the predictive performances of the best MNLR models of diurnal DO by 45% and nocturnal DO by 4%, relative to the best diel MNLR model of diel DO ($r_{{\rm adj}}^{2} = 68.8\%$ ). The ANN‐based predictions had a higher predictive power than the MNLR‐based predictions for both chl‐a and DO except for diurnal DO dynamics. The best ANNs based on independent validations were multilayer perceptron (MLP) for diel chl‐a, generalized feedforward (GFF) for diurnal and nocturnal chl‐a, MLP for diel DO, GFF for diurnal DO, and MLP for nocturnal DO.     

Field Test of Enhanced Remedial Amendment Delivery Using a Shear‐Thinning Fluid

Heterogeneity of hydraulic properties in aquifers may lead to contaminants residing in lower‐permeability zones where it is difficult to deliver remediation amendments using conventional injection processes. The focus of this study is to examine use of a shear‐thinning fluid (STF) to improve the uniformity of remedial amendment distribution within a heterogeneous aquifer. Previous studies have demonstrated the significant potential of STFs for improving remedial amendment delivery in heterogeneous aquifers, but quantitative evaluation of these improvements from field applications is lacking. A field‐scale test was conducted that compares data from successive injection of a tracer in water followed by injection of a tracer in an STF to evaluate the impact of the STF on tracer distribution uniformity in the presence of permeability contrasts within the targeted injection zone. Data from tracer breakthrough at multiple depth‐discrete monitoring intervals and electrical resistivity tomography (ERT) showed that inclusion of STF in the injection solution improved the distribution of the injected fluid within the targeted treatment zone. One improvement was a reduction in the movement of injected fluids through high‐permeability pathways, as evidenced by slower breakthrough of tracer at monitoring locations where breakthrough in baseline tracer‐only injection data was faster. In addition, STF‐amended injection solutions arrived faster and to a greater extent in monitoring locations within low‐permeability zones. ERT data showed that the STF injection covered a higher percentage of a two‐dimensional cross section within the injection interval between the injection well and a monitoring well about 3 m away.