James F. Barker

Biotransformation of BTEX under anaerobic, denitrifying conditions: Field and laboratory observations

Abstract Three natural-gradient injection experiments in the Borden aquifer (Ontario, Canada) (∼ 100–300 days in duration) and a 452-day laboratory microcosm experiment were performed to evaluate the biotransformation of BTEX (benzene, toluene, ethylbenzene and o -, m -, p -xylenes) derived from gasoline under anaerobic, denitrifying conditions. Both NO 3 − - amended and unamended control (i.e. no NO 3 − added) experiments were performed. In the unamended control injection experiment, toluene biotransformed between 1 and 5 m from the injection well. All other aromatic compounds were recalcitrant in this field experiment and all aromatic compounds were recalcitrant in unamended control microcosms. After an acclimatization period, toluene biotransformed relatively rapidly in the presence of NO 3 − in both the laboratory and field to a residual level of ∼ 100 μg L −1 . In the presence of NO 3 − the xylene isomers and ethylbenzene biotransformed to a lesser degree. Benzene was recalcitrant in all experiments. The acetylene blockage technique was used to demonstrate that denitrifying bacteria were active in the presence of NO 3 − . In the NO 3 − -amended injection experiments, little BTEX mass loss occurred beyond the 1-m multilevel-piezometer fence. However, NO 3 − continued to decline downgradient, suggesting that other sources of carbon were being utilized by denitrifying bacteria in preference to residual BTEX. In addition to observations on mass loss, these experiments provided evidence of inhibition of BTEX biotransformation in the presence of acetylene, and competitive utilization between toluene, ethylbenzene and the xylene isomers. Given the recalcitrance of benzene and high thresholds of the compounds that did biotransform, the addition of NO 3 − as an alternate electron acceptor would not be successful in this aquifer as a remedial measure.

In situ biodegradation potential of aromatic hydrocarbons in anaerobic groundwaters

Three types of experiments were conducted to assess the potential for enhancing the in situ biodegradation of nine aromatic hydrocarbons in anaerobic, leachate-impacted aquifers at North Bay, Ontario, and at Canada Forces Base Borden. Laboratory micrososms containing authentic aquifer material and groundwater from the North Bay site were amended with nitrate and glucose. No significant losses of aromatic hydrocarbons were observed compared to unamended controls, over a period of 187 days. A total of eight in situ biodegradation columns were installed in the North Bay and Borden aquifers. Remedial additions included electron acceptors (nitrate and sulphate) and primary substrates (acetate, lactate and yeast extract). Six aromatic hydrocarbons [toluene, ethylbenzene, m-xylene, o-xylene, cumene and 1,2,4-trimethylbenzene (1,2,4-TMB)] were completely degraded in at least one in situ column at the North Bay site. Only toluene was degraded in the Borden aquifer. In all cases, aromatic hydrocarbon attenuation was attributed to biodegradation by methanogenic and fermentative bacteria. No evidence of aromatic hydrocarbon degradation was observed in columns remediated with nitrate or primary substrates. A continuous forced gradient injection experiment with sulphate addition was conducted at the North Bay site over a period of 51 days. The concentration of six aromatic hydrocarbons was monitored over time in the injection wells and at piezometer fences located 2, 5 and 10 m downgradient. All compounds except toluene reached injection concentration between 14 and 26 days after pumping began, and showed some evidence of selective retardation. Toluene broke through at a subdued concentration (∼ 50% of injection levels), and eventually declined to undetectable levels on day 43. This attenuation was attributed to adaptation and biodegradation by anaerobic bacteria. The results from these experiments indicate that considerable anaerobic biodegradation of aromatic hydrocarbons in landfill leachate plumes does occur. The acclimatized population rapidly biodegraded toluene, much more rapidly than reported in other laboratory studies. This biodegradation is selective, at least in the time frame of our experiments, with benzene and chlorobenzene remaining recalcitrant.

Biodegradation modelling of a dissolved gasoline plume applying independent laboratory and field parameters

Abstract Biodegradation of organic contaminants in groundwater is a microscale process which is often observed on scales of 100s of metres or larger. Unfortunately, there are no known equivalent parameters for characterizing the biodegradation process at the macroscale as there are, for example, in the case of hydrodynamic dispersion. Zero- and first-order degradation rates estimated at the laboratory scale by model fitting generally overpredict the rate of biodegradation when applied to the field scale because limited electron acceptor availability and microbial growth are not considered. On the other hand, field-estimated zero- and first-order rates are often not suitable for predicting plume development because they may oversimplify or neglect several key field scale processes, phenomena and characteristics. This study uses the numerical model BIO3D to link the laboratory and field scales by applying laboratory-derived Monod kinetic degradation parameters to simulate a dissolved gasoline field experiment at the Canadian Forces Base (CFB) Borden. All input parameters were derived from independent laboratory and field measurements or taken from the literature a priori to the simulations. The simulated results match the experimental results reasonably well without model calibration. A sensitivity analysis on the most uncertain input parameters showed only a minor influence on the simulation results. Furthermore, it is shown that the flow field, the amount of electron acceptor (oxygen) available, and the Monod kinetic parameters have a significant influence on the simulated results. It is concluded that laboratory-derived Monod kinetic parameters can adequately describe field scale degradation, provided all controlling factors are incorporated in the field scale model. These factors include advective–dispersive transport of multiple contaminants and electron acceptors and large-scale spatial heterogeneities.

Migration and natural fate of a coal tar creosote plume: 1. Overview and plume development

A volume of sand containing coal tar creosote was emplaced below the water table at CFB Borden to investigate natural attenuation processes for complex biodegradable mixtures. Coal tar creosote is a mixture of more than 200 polycyclic aromatic hydrocarbons, heterocyclic compounds and phenolic compounds. A representative group of seven compounds was selected for detailed study: phenol, m-xylene, naphthalene, phenanthrene, 1-methylnaphthalene, dibenzofuran and carbazole. Movement of groundwater through the source led to the development of a dissolved organic plume, which was studied over a 4-year period. Qualitative plume observations and mass balance calculations indicated two key conclusions: (1) compounds from the same source can display distinctly different patterns of plume development and (2) mass transformation was a major influence on plume behaviour for all observed compounds.

The organic geochemistry of a sanitary landfill leachate plume

Abstract Leachate from the North Bay municipal landfill has contaminated an unconfined, sandy aquifer throughout the 700 m flow system from the site to a discharge zone at a creek. The major organic contaminants identified are aromatic hydrocarbons, especially substituted benzenes. The high groundwater velocity of about 75 m yr −1 and the low organic sorption properties of the sand have permitted non-transformed contaminants to spread throughout the total flow system. There is considerable temporal and spatial variability in groundwater chemistry. Most of the aqueous organic carbon has a nominal molecular weight of + and to a far lesser extent by toxic metals such as Pb, Cd or Zn. Dispersion is clearly responsible for considerable decrease in contaminant concentration along the flow system. Biotransformation under strictly anaerobic conditions has probably caused 1,1,1-trichloroethane and trichloroethylene to be restricted to the immediate vicinity of the landfill. A simple method of comparing the concentrations of pairs of organics at points along the flow system provides relative transformation rates for pairs of organics even with variable inputs from the landfill and dispersive dilution. Relative to ethylbenzene, o-xylene is rapidly lost from this system. O-xylene may be less persistent than m- or p-xylene; a result unexpected from previous studies of these dimethylbenzenes. In the initial, strictly anarobic segment of the flow system 1,2,4-trimethylbenzene and 1,4-dichlorobenzene are equally persistent, but in the final, less anaerobic segment, the former appears to be degraded more rapidly than the latter. Contaminant distributions in aquifers reflect the results of a number of processes integrated in a complex manner and so are difficult to interpret in terms of specific processes. However, they do provide evidence for what processes are most significant in real groundwater systems and they will also provide critical tests of how well laboratory-derived information relates to real groundwater contamination situations.

Migration and natural fate of a coal tar creosote plume. 2. Mass balance and biodegradation indicators

Abstract A source of coal tar creosote was emplaced below the water table at CFB Borden to investigate natural attenuation processes for complex biodegradable mixtures. A mass balance indicated that ongoing transformation occurred for seven study compounds. Phenol migrated as a discrete slug plume and almost completely disappeared after 2 years, after being completely leached from the source early in the study. The m -xylene plume migrated outward to a maximum distance at approximately 2 years, and then receded back towards the source as the rate of mass flux out of the source decreased to below the overall rate of plume transformation. Carbazole showed similar behaviour, although the reversal in plume development occurred more slowly. The dibenzofuran plume remained relatively constant in extent and mass over the last 2 years of monitoring, despite constant source input over this period, providing evidence that the dibenzofuran plume was at steady state. Meanwhile, the naphthalene and 1-methylnaphthalene plumes continued to advance and increase in mass over the observation period, although at a decreasing rate. The phenanthrene plume was also subject to transformation, although measurement of the rate was less conclusive due to the higher proportion of sorbed mass for this compound. Three lines of evidence are presented to evaluate whether the observed plume mass loss was due to microbial biodegradation. Measurement of redox-sensitive parameters in the vicinity of the plume showed the types of changes that would be expected to occur due to plume biodegradation: dissolved oxygen and SO 4 2− decreased in groundwater within the plume while significant increases were noted for Fe 2+ , Mn 2+ and methane. Further evidence that plume mass loss was microbially-mediated was provided by the accumulation of aromatic acids within the plume. Measurements of phospholipid fatty acids (PLFA) in aquifer material indicated that microbial biomass and turnover rate were greater within the plume than outside: also consistent with biodegradation. Study results highlight the potential for utilizing natural attenuation as a site cleanup approach for dissolved phase plumes from complex organic mixture like coal tar creosote.

Origin of methane in the Elk Valley coalfield, southeastern British Columbia, Canada

Coalbead methane is part of the non-conventional gas reservoirs and makes a significant contribution to gas production in some parts of the world. Initially, it was assumed that coalbed methane was of thermogenic origin, but most recent studies based on isotope and chemical data and taking into account the hydrogeology of the basin have demonstrated that secondary biogenic gases are formed in many coal-bearing basins. This study, using a similar approach, evaluated the origin of the gas in the Elk Valley coalfield located in British Columbia, Canada. Isotope data in methane samples collected from testhole wells and piezometers show a range that varies from −51.8‰ to −65.4‰ and −303‰ to −415‰ for δ13C and δ2H, respectively. The δ2H data, which are among the most depleted data reported for coalbed methane, have to be related to the very depleted δ2H values of the groundwater (−148‰ to −163‰). Isotope and chemical data collected from DIC show a trend of increasing δ13C values (−11.9‰ to +34.9‰) associated with an increase in DIC concentration (216 to 1650 mg/l). The most 13C depleted DIC and low DIC waters are found in the shallow groundwater flow system representing conditions close to recharge areas, while the most 13C enriched DIC and high concentration DIC waters are found in the discharge areas associated with a deep groundwater flow system. The DIC pattern, which is typical for methanogenic aquifers, and the isotope data obtained in methane samples clearly indicate that the gas found in the Elk Valley coalfield is mainly biogenic in origin. This study reaffirms that an approach that combines an evaluation of the groundwater flow system, the isotopic characterization and concentration pattern of the main carbon pools (CH4 and DIC), and the isotopic characterization of the groundwater is needed to fully evaluate the origin of gases in coal basins.

Laboratory evidence of MTBE biodegradation in Borden aquifer material.

Mainly due to intrinsic biodegradation, monitored natural attenuation can be an effective and inexpensive remediation strategy at petroleum release sites. However, gasoline additives such as methyl tert-butyl ether (MTBE) can jeopardize this strategy because these compounds often degrade, if at all, at a slower rate than the collectively benzene, toluene, ethylbenzene and the xylene (BTEX) compounds. Investigation of whether a compound degrades under certain conditions, and at what rate, is therefore important to the assessment of the intrinsic remediation potential of aquifers. A natural gradient experiment with dissolved MTBE-containing gasoline in the shallow, aerobic sand aquifer at Canadian Forces Base (CFB) Borden (Ontario, Canada) from 1988 to 1996 suggested that biodegradation was the main cause of attenuation for MTBE within the aquifer. This laboratory study demonstrates biologically catalyzed MTBE degradation in Borden aquifer-like environments, and so supports the idea that attenuation due to biodegradation may have occurred in the natural gradient experiment. In an experiment with batch microcosms of aquifer material, three of the microcosms ultimately degraded MTBE to below detection, although this required more than 189 days (or >300 days in one case). Failure to detect the daughter product tert-butyl alcohol (TBA) in the field and the batch experiments could be because TBA was more readily degradable than MTBE under Borden conditions.

Persistence of persulfate in uncontaminated aquifer materials.

Batch and stop-flow column experiments were performed to estimate persulfate decomposition kinetic parameters in the presence of seven well-characterized aquifer materials. Push-pull tests were conducted in a sandy aquifer to represent persulfate decomposition under in situ conditions. The decomposition of persulfate followed a first-order rate law for all aquifer materials investigated. Reaction rate coefficients (k(obs)) increased by an order of magnitude when persulfate concentration was reduced from 20 g/L to 1 g/L, due to ionic strength effects. The column experiments yielded higher k(obs) than batch experiments due to the lower oxidant to solids mass ratio. The kinetic model developed from the batch test data was able to reproduce the observed persulfate temporal profiles from the push-pull tests. The estimated k(obs) indicate that unactivated persulfate is a persistent oxidant for the range of aquifer materials explored with half-lives ranging from 2 to 600 d.

Isotopic composition (13C, 14C, 2H) and geochemistry of aquatic humic substances from groundwater

Abstract Aquatic humic substances from eight confined and unconfined aquifer/aquitard groundwater systems were analyzed for their stable carbon, radiocarbon, deuterium, and elemental contents. Fractionation of groundwater dissolved organic carbon (DOC) showed that humic substances are an important part of the organic solute load in all groundwaters. Groundwater humates are distinct from other terrestrial humates, and are characterized by low oxygen (36%) and high carbon (53%) contents. Elemental data from water table wells suggests this characteristic oxygen depletion is mainly a result of biochemical processes that occur in the vadose zone. The stable carbon isotopic composition of groundwater humates range between −31 and −24%. (PDB) with ann average of −26%., and reflect their terrestrial origins. The deuterium values of groundwater humate vary widely, and are mainly a reflection of 2 H/H variability in the continental water cycle. Radiocarbon analyses suggest a predominant soil zone origin for most groundwater humates, although some groundwater systems are influenced to varying degrees by buried peat or coal. Soluble humate 14 C mean residence times of up to several hundred years in the vadose zone before recharging to groundwater are due to the mixing of young and old organic carbon sources, rather than advective residence times.