Combined application of conservative transport modelling and compound-specific carbon isotope analyses to assess in situ attenuation of benzene, toluene, and o-xylene

In recent years, compound specific isotope analyses (CSIA) have developed into one of the most powerful tools for the quantification of in situ biodegradation of organic contaminants. In this approach, the calculation of the extent of biodegradation of organic contaminants in aquifers is usually based on the Rayleigh equation, and thus neglects physical transport processes such as dispersion that contribute to contaminant dilution in aquifers. Here we combine compound specific isotope analyses with a conservative transport model to study the attenuation of aromatic hydrocarbons at a former gasworks site. The conservative transport model was first used to simulate concentration reductions caused by dilution at wells downgradient of a BTEX source. In a second step, the diluted concentrations, together with the available stable carbon isotope ratios and carbon fractionation factors for benzene, toluene and o-xylene were applied in the Rayleigh equation to quantify the degree of biodegradation at each of those wells. At the investigated site, where other attenuation processes such as sorption and volatilisation were proven to be negligible, the combined approach is recommended for benzene, which represents a compound for which the effect of biodegradation is comparable to or less than the effect of dilution. As demonstrated for toluene and o-xylene, the application of the Rayleigh equation alone is sufficient if dilution can be proved to be insignificant in comparison to biodegradation. The analysis also suggests that the source width and the position of the observation wells relative to the plume center line are significantly related to the degree of dilution.     

Microbial in situ degradation of aromatic hydrocarbons in a contaminated aquifer monitored by carbon isotope fractionation

We present an approach for characterizing in situ microbial degradation using the 13C/12C isotope fractionation of contaminants as an indicator of biodegradation. The 13C/12C isotope fractionation of aromatic hydrocarbons was studied in anoxic laboratory soil percolation columns with toluene or o-xylene as the sole carbon and electron source, and sulfate as electron acceptor. After approximately 2 months' of incubation, the soil microbial community degraded 32 mg toluene l(-1) and 44 mg o-xylene l(-1) to less than 0.05 mg l(-1), generating a stable concentration gradient in the column. The 13C/12C isotope ratio in the residual non-degraded fraction of toluene and o-xylene increased significantly, corresponding to isotope fractionation factors (alphaC) of 1.0015 and 1.0011, respectively. When the extent of biodegradation in the soil column was calculated based on the measured isotope ratios (R(t)) and an isotope fractionation factor (alphaC=1.0017) obtained from a sulfate-reducing batch culture the theoretical residual substrate concentrations (C(t)) matched the measured toluene concentrations in the column. This indicated that a calculation of biodegradation based on isotope fractionation could work in systems like soil columns. In a field study, a polluted, anoxic aquifer was analyzed for BTEX and PAH contaminants. These compounds were found to exhibit a significant concentration gradient along an 800-m groundwater flow path downstream of the source of contamination. A distinct increase in the carbon isotope ratio (delta13C) was observed for the residual non-degraded toluene (7.2 per thousand ), o-xylene (8.1 per thousand ) and naphthalene fractions (1.2 per thousand ). Based on the isotope values and the laboratory-derived isotope fractionation factors for toluene and o-xylene, the extent to which the residual substrate fraction in the monitoring wells had been degraded by microorganisms was calculated. The results revealed significant biodegradation along the groundwater flow path. In the wells at the end of the plume, the bioavailable toluene and o-xylene fractions had been almost completely reduced by in situ microbial degradation. Although indane and indene showed decreasing concentrations downstream of the groundwater flow path, suggesting microbial degradation, their carbon isotope ratios remained constant. As the physical properties of these compounds are similar to those of BTEX compounds, the constant isotope values of indane and indene indicated that microbial degradation did not lead to isotope fractionation of all aromatic hydrocarbons. In addition, physical interaction with the aquifer material during the groundwater passage did not significantly alter the carbon isotope composition of aromatic hydrocarbons.     

Quantification of mass fluxes and natural attenuation rates at an industrial site with a limited monitoring network: a case study

At many "real world" field sites, the number of available monitoring wells is limited due to economic or geological reasons. Under such restricted conditions, it is difficult to perform a reliable field investigation and to quantify primary lines of evidence for natural attenuation (NA), like the documentation of a decrease of contaminant mass flux in flow direction. This study reports the results of a groundwater investigation at a former manufactured gas plant situated in a Quaternary river valley in southwest Germany. The location, infrastructure and aquifer setting are typical of many industrial sites in Germany. Due to difficult drilling conditions (coarse glaciofluvial gravel deposits and an anthropogenic fill above the aquifer), only 12 monitoring wells were available for the investigation and localisation of the contaminant plume. These wells were situated along three control planes (CP) downgradient from the contaminant source, with four wells along each plane. Based on the sparse set of monitoring wells, field scale mass fluxes and first-order natural attenuation rate constants of benzene, toluene, ethylbenzene, and o-xylene and p-xylene (BTEX) and low molecular weight polycyclic aromatic hydrocarbons (PAH) were estimated utilizing different point scale and also a new integral investigation method. The results show that even at a heterogeneous site with a sparse monitoring network point scale investigation methods can provide reliable information on field scale natural attenuation rates, if a dependable flow model or tracer test data is available. If this information is not available, only the new integral investigation method presented can yield adequate results for the quantification of contaminant mass fluxes under sparse monitoring conditions.     

Quantification of groundwater contamination in an urban area using integral pumping tests

In this paper, the integral groundwater investigation method is used for the quantification of PCE and TCE mass flow rates at an industrialized urban area in Linz, Austria. In this approach, pumping wells positioned along control planes perpendicular to the groundwater flow direction are operated for a time period on the order of days and sampled for contaminants. The concentration time series of the contaminants measured during operation of the pumping wells are then used to determine contaminant mass flow rates, mean concentrations and the plume shapes and positions at the control planes. The three control planes used in Linz were positioned downstream of a number of potential source zones, which are distributed over the field site. By use of the integral investigation method, it was possible to identify active contaminant sources, quantify the individual source strength in terms of mass flow rates at the control planes and estimate the contaminant plume position relative to the control planes. The source zones emitting the highest PCE and TCE mass flow rates could be determined, representing the areas where additional investigation and remediation activities will be needed. Additionally, large parts of the area investigated could be excluded from further investigation and remediation activities.     

An analytical quantification of mass fluxes and natural attenuation rate constants at a former gasworks site

A new integral groundwater investigation approach was used for the first time to quantify natural attenuation rates at field scale. In this approach, pumping wells positioned along two control planes were operated at distances of 140 and 280 m downstream of a contaminant source zone at a former gasworks site polluted with BTEX- (benzene, toluene, ethyl-benzene, o-, p-xylene) and PAH- (polycyclic aromatic hydrocarbons) compounds. Based on the quantified changes in total contaminant mass fluxes between the control planes, first-order natural attenuation rate constants could be estimated. For BTEX-compounds, these ranged from 1.4e-02 to 1.3e-01 day(-1) whereas for PAH-compounds natural attenuation rate constants of 3.7e-04 to 3.1e-02 day(-1) were observed. Microbial degradation activity at the site was indicated by an increase in dissolved iron mass flux and a reduction in sulphate mass flux between the two investigated control planes. In addition to information about total contaminant mass fluxes and average concentrations, an analysis of the concentration-time series measured at the control planes also allowed to semi-quantitatively delineate the aquifer regions most likely contaminated by the BTEX- and PAH-compounds.     

Mechanisms of electron acceptor utilization: implications for simulating anaerobic biodegradation

Simulation of biodegradation reactions within a reactive transport framework requires information on mechanisms of terminal electron acceptor processes (TEAPs). In initial modeling efforts, TEAPs were approximated as occurring sequentially, with the highest energy-yielding electron acceptors (e.g. oxygen) consumed before those that yield less energy (e.g., sulfate). Within this framework in a steady state plume, sequential electron acceptor utilization would theoretically produce methane at an organic-rich source and Fe(II) further downgradient, resulting in a limited zone of Fe(II) and methane overlap. However, contaminant plumes often display much more extensive zones of overlapping Fe(II) and methane. The extensive overlap could be caused by several abiotic and biotic processes including vertical mixing of byproducts in long-screened monitoring wells, adsorption of Fe(II) onto aquifer solids, or microscale heterogeneity in Fe(III) concentrations. Alternatively, the overlap could be due to simultaneous utilization of terminal electron acceptors. Because biodegradation rates are controlled by TEAPs, evaluating the mechanisms of electron acceptor utilization is critical for improving prediction of contaminant mass losses due to biodegradation. Using BioRedox-MT3DMS, a three-dimensional, multi-species reactive transport code, we simulated the current configurations of a BTEX plume and TEAP zones at a petroleum-contaminated field site in Wisconsin. Simulation results suggest that BTEX mass loss due to biodegradation is greatest under oxygen-reducing conditions, with smaller but similar contributions to mass loss from biodegradation under Fe(III)-reducing, sulfate-reducing, and methanogenic conditions. Results of sensitivity calculations document that BTEX losses due to biodegradation are most sensitive to the age of the plume, while the shape of the BTEX plume is most sensitive to effective porosity and rate constants for biodegradation under Fe(III)-reducing and methanogenic conditions. Using this transport model, we had limited success in simulating overlap of redox products using reasonable ranges of parameters within a strictly sequential electron acceptor utilization framework. Simulation results indicate that overlap of redox products cannot be accurately simulated using the constructed model, suggesting either that Fe(III) reduction and methanogenesis are occurring simultaneously in the source area, or that heterogeneities in Fe(III) concentration and/or mineral type cause the observed overlap. Additional field, experimental, and modeling studies will be needed to address these questions.     

Inverse modeling of BTEX dissolution and biodegradation at the Bemidji, MN crude-oil spill site

The U.S. Geological Survey (USGS) solute transport and biodegradation code BIOMOC was used in conjunction with the USGS universal inverse modeling code UCODE to quantify field-scale hydrocarbon dissolution and biodegradation at the USGS Toxic Substances Hydrology Program crude-oil spill research site located near Bemidji, MN. This inverse modeling effort used the extensive historical data compiled at the Bemidji site from 1986 to 1997 and incorporated a multicomponent transport and biodegradation model. Inverse modeling was successful when coupled transport and degradation processes were incorporated into the model and a single dissolution rate coefficient was used for all BTEX components. Assuming a stationary oil body, we simulated benzene, toluene, ethylbenzene, m,p-xylene, and o-xylene (BTEX) concentrations in the oil and ground water, respectively, as well as dissolved oxygen. Dissolution from the oil phase and aerobic and anaerobic degradation processes were represented. The parameters estimated were the recharge rate, hydraulic conductivity, dissolution rate coefficient, individual first-order BTEX anaerobic degradation rates, and transverse dispersivity. Results were similar for simulations obtained using several alternative conceptual models of the hydrologic system and biodegradation processes. The dissolved BTEX concentration data were not sufficient to discriminate between these conceptual models. The calibrated simulations reproduced the general large-scale evolution of the plume, but did not reproduce the observed small-scale spatial and temporal variability in concentrations. The estimated anaerobic biodegradation rates for toluene and o-xylene were greater than the dissolution rate coefficient. However, the estimated anaerobic biodegradation rates for benzene, ethylbenzene, and m,p-xylene were less than the dissolution rate coefficient. The calibrated model was used to determine the BTEX mass balance in the oil body and groundwater plume. Dissolution from the oil body was greatest for compounds with large effective solubilities (benzene) and with large degradation rates (toluene and o-xylene). Anaerobic degradation removed 77% of the BTEX that dissolved into the water phase and aerobic degradation removed 17%. Although goodness-of-fit measures for the alternative conceptual models were not significantly different, predictions made with the models were quite variable.     

New methodology to investigate potential contaminant mass fluxes at the stream-aquifer interface by combining integral pumping tests and streambed temperatures

The spatial pattern and magnitude of mass fluxes at the stream-aquifer interface have important implications for the fate and transport of contaminants in river basins. Integral pumping tests were performed to quantify average concentrations of chlorinated benzenes in an unconfined aquifer partially penetrated by a stream. Four pumping wells were operated simultaneously for a time period of 5 days and sampled for contaminant concentrations. Streambed temperatures were mapped at multiple depths along a 60m long stream reach to identify the spatial patterns of groundwater discharge and to quantify water fluxes at the stream-aquifer interface. The combined interpretation of the results showed average potential contaminant mass fluxes from the aquifer to the stream of 272microgm(-2)d(-1) MCB and 71microgm(-2)d(-1) DCB, respectively. This methodology combines a large-scale assessment of aquifer contamination with a high-resolution survey of groundwater discharge zones to estimate contaminant mass fluxes between aquifer and stream.     

Influence of temporally variable groundwater flow conditions on point measurements and contaminant mass flux estimations

Monitoring of contaminant concentrations, e.g., for the estimation of mass discharge or contaminant degradation rates, often is based on point measurements at observation wells. In addition to the problem, that point measurements may not be spatially representative, a further complication may arise due to the temporal dynamics of groundwater flow, which may cause a concentration measurement to be not temporally representative. This paper presents results from a numerical modeling study focusing on temporal variations of the groundwater flow direction. “Measurements” are obtained from point information representing observation wells installed along control planes using different well frequencies and configurations. Results of the scenario simulations show that temporally variable flow conditions can lead to significant temporal fluctuations of the concentration and thus are a substantial source of uncertainty for point measurements. Temporal variation of point concentration measurements may be as high as the average concentration determined, especially near the plume fringe, even when assuming a homogeneous distribution of the hydraulic conductivity. If a heterogeneous hydraulic conductivity field is present, the concentration variability due to a fluctuating groundwater flow direction varies significantly within the control plane and between the different realizations. Determination of contaminant mass fluxes is also influenced by the temporal variability of the concentration measurement, especially for large spacings of the observation wells. Passive dosimeter sampling is found to be appropriate for evaluating the stationarity of contaminant plumes as well as for estimating average concentrations over time when the plume has fully developed. Representative sampling has to be performed over several periods of groundwater flow fluctuation. For the determination of mass fluxes at heterogeneous sites, however, local fluxes, which may vary considerably along a control plane, have to be accounted for. Here, dosimeter sampling in combination with time integrated local water flux measurements can improve mass flux estimates under dynamic flow conditions.     

Biodegradation of benzene, toluene, ethylbenzene and xylenes in gas-condensate-contaminated ground-water

The rate and extent of biodegradation of benzene, toluene, ethylbenzene and xylenes (BTEX) in ground-water was studied in samples from a contaminated site which contained total BTEX concentrations of up to 20 000 microg litre(-1). All compounds were rapidly degraded under natural aerobic conditions. Elevation of incubation temperature, supply of organic nutrients or addition of inorganic fertiliser did not increase the rate or extent of biodegradation and it appeared that oxygen supply was the factor limiting BTEX degradation at this site. Attempts to increase the dissolved oxygen concentration in the ground-water by the addition of hydrogen peroxide to give a final concentration of 200 mg litre(-1) resulted in the complete inhibition of biodegradation. No biodegradation occurred under anaerobic conditions except when nitrate was provided as a terminal electron acceptor for microbial respiration. Under denitrifying conditions there was apparent biodegradation of benzene, toluene, ethyl-benzene, m-xylene and p-xylene but o-xylene was not degraded. Degradation under denitrifying conditions occurred at a much slower rate than under oxygenated conditions.     

Quantifying MTBE biodegradation in the Vandenberg Air Force Base ethanol release study using stable carbon isotopes

Compound-specific isotope analysis (CSIA) was used to assess biodegradation of MTBE and TBA during an ethanol release study at Vandenberg Air Force Base. Two continuous side-by-side field releases were conducted within a preexisting MTBE plume to form two lanes. The first involved the continuous injection of site groundwater amended with benzene, toluene and o-xylene ("No ethanol lane"), while the other involved the continuous injection of site groundwater amended with benzene, toluene and o-xylene and ethanol ("With ethanol lane"). The delta(13)C of MTBE for all wells in the "No ethanol lane" remained constant during the experiment with a mean value of -31.3 +/- 0.5 per thousand (n=40), suggesting the absence of any substantial MTBE biodegradation in this lane. In contrast, substantial enrichment in (13)C of MTBE by 40.6 per thousand, was measured in the "With ethanol lane", consistent with the effects of biodegradation. A substantial amount of TBA (up to 1200 microg/L) was produced by the biodegradation of MTBE in the "With ethanol lane". The mean value of delta(13)C for TBA in groundwater samples in the "With ethanol lane" was -26.0 +/- 1.0 per thousand (n=32). Uniform delta(13)C TBA values through space and time in this lane suggest that substantial anaerobic biodegradation of TBA did not occur during the experiment. Using the reported range in isotopic enrichment factors for MTBE of -9.2 per thousand to -15.6 per thousand, and values of delta(13)C of MTBE in groundwater samples, MTBE first-order biodegradation rates in the "With ethanol lane" were 12.0 to 20.3 year(-1) (n=18). The isotope-derived rate constants are in good agreement with the previously published rate constant of 16.8 year(-1) calculated using contaminant mass-discharge for the "With ethanol lane".     

A strategy for aromatic hydrocarbon bioremediation under anaerobic conditions and the impacts of ethanol: a microcosm study

Increased use of ethanol-blended gasoline (gasohol) and its potential release into the subsurface have spurred interest in studying the biodegradation of and interactions between ethanol and gasoline components such as benzene, toluene, ethylbenzene and xylene isomers (BTEX) in groundwater plumes. The preferred substrate status and the high biological oxygen demand (BOD) posed by ethanol and its biodegradation products suggests that anaerobic electron acceptors (EAs) will be required to support in situ bioremediation of BTEX. To develop a strategy for aromatic hydrocarbon bioremediation and to understand the impacts of ethanol on BTEX biodegradation under strictly anaerobic conditions, a microcosm experiment was conducted using pristine aquifer sand and groundwater obtained from Canadian Forces Base Borden, Canada. The initial electron accepter pool included nitrate, sulfate and/or ferric iron. The microcosms typically contained 400 g of sediment, 600 approximately 800 ml of groundwater, and with differing EAs added, and were run under anaerobic conditions. Ethanol was added to some at concentrations of 500 and 5000 mg/L. Trends for biodegradation of aromatic hydrocarbons for the Borden aquifer material were first developed in the absence of ethanol, The results showed that indigenous microorganisms could degrade all aromatic hydrocarbons (BTEX and trimethylbenzene isomers-TMB) under nitrate- and ferric iron-combined conditions, but not under sulfate-reducing conditions. Toluene, ethylbenzene and m/p-xylene were biodegraded under denitrifying conditions. However, the persistence of benzene indicated that enhancing denitrification alone was insufficient. Both benzene and o-xylene biodegraded significantly under iron-reducing conditions, but only after denitrification had removed other aromatics. For the trimethylbenzene isomers, 1,3,5-TMB biodegradation was found under denitrifying and then iron-reducing conditions. Biodegradation of 1,2,3-TMB or 1,2,4-TMB was slower under iron-reducing conditions. This study suggests that addition of excess ferric iron combined with limited nitrate has promise for in situ bioremediation of BTEX and TMB in the Borden aquifer and possibly for other sites contaminated by hydrocarbons. This study is the first to report 1,2,3-TMB biodegradation under strictly anaerobic condition. With the addition of 500 mg/L ethanol but without EA addition, ethanol and its main intermediate, acetate, were quickly biodegraded within 41 d with methane as a major product. Ethanol initially present at 5000 mg/L without EA addition declined slowly with the persistence of unidentified volatile fatty acids, likely propionate and butyrate, but less methane. In contrast, all ethanol disappeared with repeated additions of either nitrate or ferric iron, but acetate and unidentified intermediates persisted under iron-enhanced conditions. With the addition of 500 mg/L ethanol and nitrate, only minor toluene biodegradation was observed under denitrifying conditions and only after ethanol and acetate were utilized. The higher ethanol concentration (5000 mg/L) essentially shut down BTEX biodegradation likely due to high EA demand provided by ethanol and its intermediates. The negative findings for anaerobic BTEX biodegradation in the presence of ethanol and/or its biodegradation products are in contrast to recent research reported by Da Silva et al. [Da Silva, M.L.B., Ruiz-Aguilar, G.M.L., Alvarez, P.J.J., 2005. Enhanced anaerobic biodegradation of BTEX-ethanol mixtures in aquifer columns amended with sulfate, chelated ferric iron or nitrate. Biodegradation. 16, 105-114]. Our results suggest that the apparent conservation of high residual labile carbon as biodegradation products such as acetate makes natural attenuation of aromatics less effective, and makes subsequent addition of EAs to promote in situ BTEX biodegradation problematic.     

In situ biodegradation determined by carbon isotope fractionation of aromatic hydrocarbons in an anaerobic landfill leachate plume (Vejen,Denmark)

Concentrations and isotopic compositions (13C/12C) of aromatic hydrocarbons were determined in eight samples obtained from the strongly anoxic part of the leachate plume downgradient from the Vejen Landfill (Denmark), where methanogenic, sulfate-reducing and iron-reducing conditions were observed. Despite the heterogeneous distribution of the compounds in the plume, the isotope fractionation proved that ethylbenzene and m/p-xylene were subject to significant biodegradation within the strongly anoxic plume. The isotope fractionation factors (alphaC) for the degradation of the m/p-xylene (1.0015) and ethylbenzene (1.0021) obtained from the field observations were similar to factors previously determined for the anaerobic degradation of toluene and o-xylene in laboratory experiments, and suggest that in situ biodegradation is one major process controlling the fate of these contaminants in this aquifer. The isotope fractionation determined for 1,2,4-trimethylbenzene and 2-ethyltoluene suggested in situ biodegradation; however, the isotopic composition did not correlate well with the respective concentration as expressed by the Rayleigh equation. Some other compounds (1,2,3-trimethylbenzene, o-xylene, naphthalene and fenchone) did not show significant enrichments in delta13C values along the flow path. The compound concentrations were too low for accurate isotope analyses of benzene, toluene, 1- and 2-methylnaphthalene, while interferences in the chromatography made it impossible to evaluate the isotopic composition for 4-ethyltoluene, 1,3,5-trimethylbenzene and camphor.In addition to demonstrating the potential of assessing isotopic fractionation as a means for documenting the in situ biodegradation of complex mixtures of aromatic hydrocarbons in leachate plumes, this study also illustrates the difficulties for data interpretation in complex plumes and high analytical uncertainties for isotope analysis of organic compounds in low concentration ranges.     

In situ sequential treatment of a mixed contaminant plume

Groundwater plumes often contain a mixture of contaminants that cannot easily be remediated in situ using a single technology. The purpose of this research was to evaluate an in situ treatment sequence for the control of a mixed organic plume (chlorinated ethenes and petroleum hydrocarbons) within a Funnel-and-Gate. A shallow plume located in the unconfined aquifer at Alameda Point, CA, was found to contain up to 218,000 μg/l of cis-1,2 dichloroethene (cDCE), 16,000 μg/l of vinyl chloride (VC) and <1000 μg/l of 1,1 dichloroethene (1,1 DCE), trans-1,2 dichloroethene (trans-1,2 DCE) and trichloroethene (TCE). Total benzene, toluene, ethylbenzene and xylenes (BTEX) concentrations were <10,000 μg/l. Contaminated groundwater was funneled into a gate, 3.0 m wide, 4.5 m long and 6.0 m deep (keyed into the underlying aquitard) where treatment occurred. The initial gate segment consisted of granular iron, for the reductive dechlorination of the higher chlorinated ethenes. The second segment, the biosparge zone, promoted aerobic biodegradation of petroleum hydrocarbons and any remaining lesser-chlorinated compounds, stimulated by dissolved oxygen (DO) and carbon dioxide (CO₂) additions via an in situ sparge system (CO₂ was used to neutralize the high pH produced from reactions in the iron wall). Groundwater was drawn through the gate by pumping two wells located at the sealed, downgradient, end. Over a 4-month period an estimated 1350 g of cDCE flowed into the treatment gate and the iron wall removed 1230 g, or 91% of the mass. The influent mass of VC was 572 g and the iron wall removed 535 g, corresponding to 94% mass removal. The other chlorinated ethenes had significantly lower influent masses (3 to 108 g) and the iron wall removed the majority of the mass resulting in >96% mass removal for any of the compounds. In spite of these high removal percentages, laboratory column tests indicated that at these levels of chlorinated contaminants, surface saturation of the iron grains likely contributed to lower than expected reaction rates. In the biosparge zone, mass removal of cDCE appeared to occur predominantly by biodegradation (65%) with volatilization (35%) being an important secondary process. The dominant removal process for VC was volatilization (70%) although significant biodegradation was also indicated (30%). Laboratory microcosm results confirmed the potential for aerobic biodegradation of cDCE and VC. When average influent field concentrations for cDCE and VC were 220,000 and 46,000 μg/l, respectively, the sequential treatment unit removed 99.6% of the total mass and when the influent concentrations decreased to 26,000 and 19,000 μg/l for cDCE and VC, respectively, >99.9% removal within the treatment gate was attained. BTEX compounds were found to be significantly retarded in the iron treatment zone. Although they did eventually break through the granular iron, and into the gravel transition zone, none of these compounds was detected in the biosparge zone. No noticeable interferences between the anaerobic (reductive) and aerobic parts of the system occurred during testing. The results of this experiment show that in situ treatment sequences are viable, although further work is needed to optimize performance.     

Optimal design of pump-and-treat systems under uncertain hydraulic conductivity and plume distribution

In this work, we present a stochastic optimal control framework for assisting the management of the cleanup by pump-and-treat of polluted shallow aquifers. In the problem being investigated, hydraulic conductivity distribution and dissolved contaminant plume location are considered as the uncertain variables. The framework considers the subdivision of the cleanup horizon in a number of stress periods over which the pumping policy implemented until that stage is dynamically adjusted based upon new information that has become available in the previous stages. In particular, by following a geostatistical approach, we study the idea of monitoring the cumulative contaminant mass extracted from the installed recovery wells, and using these measurements to generate conditional realizations of the hydraulic conductivity field. These realizations are thus used to obtain a more accurate evaluation of the initial plume distribution, and modify accordingly the design of the pump-and-treat system for the remainder of the remedial process. The study indicates that measurements of contaminant mass extracted from pumping wells retain valuable information about the plume location and the spatial heterogeneity characterizing the hydraulic conductivity field. However, such an information may prove quite soft, particularly in the instances where recovery wells are installed in regions where contaminant concentration is low or zero. On the other hand, integrated solute mass measurements may effectively allow for reducing parameter uncertainty and identifying the plume distribution if more recovery wells are available, in particular in the early stages of the cleanup process.     

Analysis of groundwater contamination using concentration-time series recorded during an integral pumping test: bias introduced by strong concentration gradients within the plume

When only few monitoring wells are available to assess the extent and level of groundwater contamination, inversion of concentration breakthrough curves acquired during an integral pumping test can be used as an alternative quantification method. The idea is to use concentration-time series recorded during integral pumping tests through an inversion technique to estimate contaminant mass fluxes crossing a control plane. In this paper, we examine how a longitudinal concentration gradient along a contaminant plume length scale affects the estimated inversed-concentration distribution and its associated mass flux. The analytically inversed-concentration distribution at the imaginary control plane (ICP) is compared to a numerically generated concentration distribution, treating the latter one as a "real contaminant plume" characterized by the presence of a longitudinal concentration gradient. It is found that the analytically inversed-concentration can lead to overestimation or underestimation of concentration distribution values depending on the transport time period and dispersivity values. At lower dispersivity values, with shorter transport time periods, the analytically inversed-concentration distribution overestimates the "real" concentration distribution. A better fit of the estimated concentration distribution to the "real" one is observed when the transport time period increases, i.e. when the advective front has already crossed the ICP. However, for higher dispersivity values, underestimation of the real concentration distribution is observed. Deviation of the inversed-concentration distribution from the "real" one is assessed for a site-specific concentration gradient term. A concentration gradient adjusted contaminant mass flux is thus formulated to evaluate groundwater contamination levels at a given time period through an ICP. This concentration gradient ratio can indicate whether the ICP is well positioned to evaluate accurately contaminant mass fluxes which are representative of groundwater contamination levels.     

Analysis of groundwater contamination using concentration–time series recorded during an integral pumping test: Bias introduced by strong concentration gradients within the plume

When only few monitoring wells are available to assess the extent and level of groundwater contamination, inversion of concentration breakthrough curves acquired during an integral pumping test can be used as an alternative quantification method. The idea is to use concentration–time series recorded during integral pumping tests through an inversion technique to estimate contaminant mass fluxes crossing a control plane. In this paper, we examine how a longitudinal concentration gradient along a contaminant plume length scale affects the estimated inversed-concentration distribution and its associated mass flux. The analytically inversed-concentration distribution at the imaginary control plane (ICP) is compared to a numerically generated concentration distribution, treating the latter one as a “real contaminant plume” characterized by the presence of a longitudinal concentration gradient. It is found that the analytically inversed-concentration can lead to overestimation or underestimation of concentration distribution values depending on the transport time period and dispersivity values. At lower dispersivity values, with shorter transport time periods, the analytically inversed-concentration distribution overestimates the “real” concentration distribution.A better fit of the estimated concentration distribution to the “real” one is observed when the transport time period increases, i.e. when the advective front has already crossed the ICP. However, for higher dispersivity values, underestimation of the real concentration distribution is observed. Deviation of the inversed-concentration distribution from the “real” one is assessed for a site-specific concentration gradient term. A concentration gradient adjusted contaminant mass flux is thus formulated to evaluate groundwater contamination levels at a given time period through an ICP. This concentration gradient ratio can indicate whether the ICP is well positioned to evaluate accurately contaminant mass fluxes which are representative of groundwater contamination levels.     

Removal of BTEX vapor from waste gases by a trickle bed biofilter

The system performance of a trickle bed biofilter for treating single and mixed benzene, toluene, ethylbenzene, and o-xylene (BTEX) vapors from waste gases was investigated under different gas flow rates and influent BTEX concentrations. When a single substrate was fed, removal efficiencies of greater than 90% could be achieved for the loads below 64 g benzene/m3/hr, 110 g toluene/m3/hr, 53 g ethylbenzene/m3/hr, and 55 g o-xylene/m3/hr. When a mixed substrate was fed, removal efficiencies of each compound could be above 90% at BTEX loads below 96 g/m3/hr. The trickle bed biofilter appears to be an effective treatment process for removing both single and mixed BTEX vapors with low to high loads. Under similar substrate loads, BTEX vapors were preferentially biodegraded in the order of toluene, benzene, o-xylene, and ethylbenzene. The volumetric removal rates (elimination capacities) of BTEX vapors for a single-substrate feed were higher than those for a mixed-substrate feed under similar substrate loads; these differences were enhanced at higher substrate loads and less significant for a preferred substrate.     

Importance of heterocylic aromatic compounds in monitored natural attenuation for coal tar contaminated aquifers: A review

NSO heterocycles (HET) are typical constituents of coal tars. However, HET are not yet routinely monitored, although HET are relatively toxic coal tar constituents. The main objectives of the study is therefore to review previous studies and to analyse HET at coal tar polluted sites in order to assess the relevance of HET as part of monitored natural attenuation (MNA) or any other long-term monitoring programme. Hence, natural attenuation of typical HET (indole, quinoline, carbazole, acridine, methylquinolines, thiophene, benzothiophene, dibenzothiophene, benzofuran, dibenzofuran, methylbenzofurans, dimethylbenzofurans and xanthene) were studied at three different field sites in Germany. Compound-specific plume lengths were determined for all main contaminant groups (BTEX, PAH and HET). The results show that the observed plume lengths are site-specific and are above 250m, but less than 1000m. The latter, i.e. the upper limit, however mainly depends on the level of investigation, the considered compound, the lowest measured concentration and/or the achieved compound-specific detection limit and therefore cannot be unequivocally defined. All downstream contaminant plumes exhibited HET concentrations above typical PAH concentrations indicating that some HET are generally persistent towards biodegradation compared to other coal tar constituents, which results in comparatively increased field-derived half-lives of HET. Additionally, this study provides a review on physicochemical and toxicological parameters of HET. For three well investigated sites in Germany, the biodegradation of HET is quantified using the centre line method (CLM) for the evaluation of bulk attenuation rate constants. The results of the present and previous studies suggest that implementation of a comprehensive monitoring programme for heterocyclic aromatic compounds is relevant at sites, if MNA is considered in risk assessment and for remediation.     

Hydrochemical and isotopic effects associated with petroleum fuel biodegradation pathways in a chalk aquifer

Hydrochemical data, compound specific carbon isotope analysis and isotopic enrichment trends in dissolved hydrocarbons and residual electron acceptors have been used to deduce BTEX and MTBE degradation pathways in a fractured chalk aquifer. BTEX compounds are mineralised sequentially within specific redox environments, with changes in electron acceptor utilisation being defined by the exhaustion of specific BTEX components. A zone of oxygen and nitrate exhaustion extends approximately 100 m downstream from the plume source, with residual sulphate, toluene, ethylbenzene and xylene. Within this zone complete removal of the TEX components occurs by bacterial sulphate reduction, with sulphur and oxygen isotopic enrichment of residual sulphate (epsilon(s) = -14.4 per thousand to -16.0 per thousand). Towards the plume margins and at greater distance along the plume flow path nitrate concentrations increase with delta15N values of up to +40 per thousand indicating extensive denitrification. Benzene and MTBE persist into the denitrification zone, with carbon isotope enrichment of benzene indicating biodegradation along the flow path. A Rayleigh kinetic isotope enrichment model for 13C-enrichment of residual benzene gives an apparent epsilon value of -0.66 per thousand. MTBE shows no significant isotopic enrichment (delta13C = -29.3 per thousand to -30.7 per thousand) and is isotopically similar to a refinery sample (delta13C = -30.1 per thousand). No significant isotopic variation in dissolved MTBE implies that either the magnitude of any biodegradation-induced isotopic fractionation is small, or that relatively little degradation has taken place in the presence of BTEX hydrocarbons. It is possible, however, that MTBE degradation occurs under aerobic conditions in the absence of BTEX since no groundwater samples were taken with co-existing MTBE and oxygen. Low benzene delta13C values are correlated with high sulphate delta34S, indicating that little benzene degradation has occurred in the sulphate reduction zone. Benzene degradation may be associated with denitrification since increased benzene delta13C is associated with increased delta15N in residual nitrate. Re-supply of electron acceptors by diffusion from the matrix into fractures and dispersive mixing is an important constraint on degradation rates and natural attenuation capacity in this dual-porosity aquifer.