Ionic contribution to the self-potential signals associated with a redox front

In contaminant plumes or in the case of ore bodies, a source current density is produced at depth in response to the presence of a gradient of the redox potential. Two charge carriers can exist in such a medium: electrons and ions. Two contributions to the source current density are associated with these charge carriers (i) the gradient of the chemical potential of the ionic species and (ii) the gradient of the chemical potential of the electrons (i.e., the gradient of the redox potential). We ran a set of experiments in which a geobattery is generated using electrolysis reactions of a pore water solution containing iron. A DC power supply is used to impose a difference of electrical potential of 3 V between a working platinum electrode (anode) and an auxiliary platinum electrode (cathode). Both electrodes inserted into a tank filled with a well-calibrated sand infiltrated by a (0.01 mol L^− 1 KCl + 0.0035 mol L^− 1 FeSO₄) solution. After the direct current is turned off, we follow the pH, the redox potential, and the self-potential at several time intervals. The self-potential anomalies amount to a few tens of millivolts after the current is turned off and decreases over time. After several days, all the redox-active compounds produced initially by the electrolysis reactions are consumed through chemical reactions and the self-potential anomalies fall to zero. The resulting self-potential anomalies are shown to be much weaker than the self-potential anomalies observed in the presence of an electronic conductor in the laboratory or in the field. In the presence of a biotic or an abiotic electronic conductor, the self-potential anomalies can amount to a few hundred millivolts. These observations point out indirectly the potential role of bacteria forming biofilms in the transfer of electrons through sharp redox potential gradient in contaminant plumes that are rich in organic matter.     

Self-potential monitoring of a crude oil-contaminated site (Trecate,Italy)

We present a multidisciplinary approach for characterization of a crude oil-contaminated site (Trecate, Italy), integrating geophysical data, such as subsoil electrical potential (in millivolts) and electrical resistivity (in ohm meters) distribution, with hydrogeological and bio-chemical data. Self-potential measurements have been evaluated together with active geoelectrical measurements and hydrological information, to provide spatial and temporal information about the self-potential sources and their possible correlations with the contamination state of the subsoil. Three self-potential surveys (March 2010, October 2010, and March 2011) were conducted at the site, both in the contaminated and uncontaminated regions. The obtained self-potential maps show large time-lapse differences in correspondence of the contaminated area, with positive electrical potential values (up to 50 mV) in spring surveys and an electrical potential dipolar distribution in October (2010) survey (amplitude from ?15 to 25 mV). To understand the origin of the measured self-potential signals, a model using vertical dipolar electrical sources was built, taking into account the electrical resistivity distribution deduced from electrical resistivity tomography. The self-potential source identification allows the Trecate contamination state to be better delineated. In particular, two self-potential contributions are superimposed: the electrokinetic mechanism is predominant in spring, while the redox mechanism represents the most important contribution in autumn.     

Assessing impacts of partial mass depletion in DNAPL source zones: II. Coupling source strength functions to plume evolution

Analytical solutions, describing the time-dependent DNAPL source-zone mass and contaminant discharge rate, derived previously in Part I [Falta, R.W., Rao, P.S., Basu, N., this issue. Assessing the impacts of partial mass depletion in DNAPL source zones: I. Analytical modeling of source strength functions and plume response. J. Contam. Hydrol.] are used as a flux-boundary condition in a semi-analytical contaminant transport model. These analytical solutions assume a power relationship between the flow-averaged source concentration, and the source DNAPL mass; the empirical exponent (gamma) is a function of the flow field heterogeneity, DNAPL architecture, and the correlation between them. The DNAPL source strength terms can account for partial source remediation, either at time zero, or at some later time after the DNAPL release. The transport model considers advection, retardation, three-dimensional dispersion, and sequential first-order decay/production of several species. A separate solution is used to compute the time-dependent mass of each contaminant in the plume. A series of examples using different values of gamma shows how the benefits of partial DNAPL source remediation can vary with site conditions. In general, when gamma>1, relatively large short-term reductions in the plume concentrations and mass occur, but the source longevity is not strongly affected. Conversely, when gamma<1, the short-term reductions in the plume concentrations and mass are smaller, but the source longevity can be greatly reduced. In either case, the source remediation effort is much more effective if it is undertaken at an early time, before much contaminant mass has entered the plume. If the remediation effort is significantly delayed, the leading parts of the plume are not affected by the source remediation, and additional control or remediation of the plume itself is required.     

Processes controlling the distribution and natural attenuation of dissolved phenolic compounds in a deep sandstone aquifer

Processes controlling the distribution and natural attenuation (NA) of phenol, cresols and xylenols released from a former coal-tar distillation plant in a deep Triassic sandstone aquifer are evaluated from vertical profiles along the plume centerline at 130 and 350 m from the site. Up to four groups of contaminants (phenols, mineral acids, NaOH, NaCl) form discrete and overlapping plumes in the aquifer. Their distribution reflects changing source history with releases of contaminants from different locations. Organic contaminant distribution in the aquifer is determined more by site source history than degradation. Contaminant degradation at total organic carbon (TOC) concentrations up to 6500 mg l(-1) (7500 mg l(-1) total phenolics) is occurring by aerobic respiration NO3-reduction, Mn(IV)-/Fe(III)-reduction, SO4-reduction, methanogenesis and fermentation, with the accumulation of inorganic carbon, organic metabolites (4-hydroxybenzaldehyde, 4-hydroxybenzoic acid), acetate, Mn(II), Fe(II), S(-II), CH4 and H2 in the plume. Aerobic and NO3-reducing processes are restricted to a 2-m-thick plume fringe but Mn(IV)-/Fe(II)-reduction, SO4-reduction, methanogenesis and fermentation occur concomitantly in the plume. Dissolved H2 concentrations in the plume vary from 0.7 to 110 nM and acetate concentrations reach 200 mg l(-1). The occurrence of a mixed redox system and concomitant terminal electron accepting processes (TEAPs) could be explained with a partial equilibrium model based on the potential in situ free energy (deltaGr) yield for oxidation of H2 by specific TEAPs. Respiratory processes rather than fermentation are rate limiting in determining the distribution of H2 and TEAPs and H2 dynamics in this system. Most (min. 90%) contaminant degradation has occurred by aerobic and NO3-reducing processes at the plume fringe. This potential is determined by the supply of aqueous O2 and NO3 from uncontaminated groundwater, as controlled by transverse mixing, which is limited in this aquifer by low dispersion. Consumption to date of mineral oxides and SO4 is, respectively, <0.15% and 0.4% of the available aquifer capacity, and degradation using these oxidants is <10%. Fermentation is a significant process in contaminant turnover, accounting for 21% of degradation products present in the plume, and indicating that microbial respiration rates are slow in comparison with fermentation. Under present conditions, the potential for degradation in the plume is very low due to inhibitory effects of the contaminant matrix. Degradation products correspond to <22% mass loss over the life of the plume, providing a first-order plume scale half-life >140 years. The phenolic compounds are biodegradable under the range of redox conditions in the aquifer and the aquifer is not oxidant limited, but the plume is likely to be long-lived and to expand. Degradation is likely to increase only after contaminant concentrations are reduced and aqueous oxidant inputs are increased by dispersion of the plume. The results imply that transport processes may exert a greater control on the natural attenuation of this plume than aquifer oxidant availability.     

Contaminant degradation in physically and chemically heterogeneous aquifers

This paper examines the importance of the correlation between hydraulic conductivity (K) and degradation rate constant (k) during the transport of reactive contaminants in heterogeneous aquifers. We simulated reactive transport in an ensemble of two-dimensional heterogeneous aquifers. Two sets of transport simulations were conducted: one in which a perfect positive correlation was assumed between ln(K) and ln(k), and one in which a perfect negative correlation was assumed. We found that the sign of the correlation has important consequences for the contaminant transport. Qualitatively, a negative correlation leads to significantly more pronounced "fingering" of the contaminant plume than does a positive correlation, with potentially important consequences for downgradient receptors. Quantitatively, the expected behavior (as quantified by the contaminant mass remaining in the aquifer) is statistically different between the positive and negative cases: on average, more contaminant mass persists when K and k are negatively correlated. Also, the negative correlation leads to more variability between realizations of the ensemble, whereas a positive correlation induces relatively little variability between realizations. We discuss the implications of these findings for the management of contaminated aquifers.     

Flux-based assessment at a manufacturing site contaminated with trichloroethylene

Groundwater and contaminant fluxes were measured, using the passive flux meter (PFM) technique, in wells along a longitudinal transect passing approximately through the centerline of a trichloroethylene (TCE) plume at a former manufacturing plant located in the Midwestern US. Two distinct zones of hydraulic conductivity were identified from the measured groundwater fluxes; a 6-m-thick upper zone ( approximately 7 m to 13 m below the ground surface or bgs) with a geometric mean Darcy flux (q(0)) of 2 cm/day, and a lower zone ( approximately 13 m to 16.5m bgs) with a q(0) approximately 15 cm/day; this important hydrogeologic feature significantly impacts any remediation technology used at the site. The flux-averaged TCE concentrations estimated from the PFM results compared well with existing groundwater monitoring data. It was estimated that at least 800 kg of TCE was present in the source zone. The TCE mass discharge across the source control plane (85 m x 38 m) was used to estimate the "source strength" ( approximately 365 g/day), while mass discharges across multiple down-gradient control planes were used to estimate the plume-averaged, TCE degradation rate constant (0.52 year(-1)). This is close to the rate estimated using the conventional centerline approach (0.78 year(-1)). The mass discharge approach provides a more robust and representative estimate than the centerline approach since the latter uses only data from wells along the plume centerline while the former uses all wells in the plume.     

Modelling of an enhanced PAH attenuation experiment and associated biogeochemical changes at a former gasworks site in southern Germany

Former manufactured gas plant sites often form a widespread contaminant source in the subsurface, leading to large plumes that contain a wide variety of tar-oil related compounds. Although most of these compounds eventually degrade naturally, the relevant processes tend to be slow and inefficient, often leaving active remediation as the only viable option to eliminate the risks of toxic substances to reach potential receptors such as surface waters or drinking water wells. In this study we use a reactive transport model to analyse the fate of a contaminant plume containing acenaphthene, methylbenzofurans and dimethylbenzofurans (i) prior to the installation of an active remediation scheme and (ii) for an enhanced remediation experiment during which O(2) and H(2)O(2) were added to the contaminated groundwater through a recirculation well. The numerical model developed for this study considers the primary contaminant degradation reactions (i.e., microbially mediated redox reactions) as well as secondary and competing mineral precipitation/dissolution reactions that affect the site's hydrochemistry and/or contaminant fate. The model was calibrated using a variety of constraints to test the uncertainty on model predictions resulting from the undocumented presence of reductants such as pyrite. The results highlight the important role of reactive transport modelling for the development of a comprehensive process understanding.     

Measurements of the streaming potential of clay soils from tropical and subtropical regions using self-made apparatus

The streaming potential has been wildly used in charged parallel plates, capillaries, and porous media. However, there have been few studies involving the ζ potential of clay soils based on streaming potential measurements. A laboratory apparatus was developed in this study to measure the streaming potential (ΔE) of bulk clay soils’ coupling coefficient (C) and cell resistance (R) of saturated granular soil samples. Excellent linearity of ΔE versus liquid pressure (ΔP) ensured the validity of measurements. The obtained parameters of C and R can be used to calculate the ζ potential of bulk soils. The results indicated that the ζ potentials measured by streaming potential method were significantly correlated with the ζ potentials of soil colloids determined by electrophoresis (r ²?=?0.960**). Therefore, the streaming potential method can be used to study the ζ potentials of bulk clay soils. The absolute values of the ζ potentials of four soils followed the order: Ultisol from Jiangxi?>?Ultisol from Anhui?>?Oxisol from Guangdong?>?Oxisol from Hainan, and this was consistent with the cation exchange capacities of these soils. The type and concentration of electrolytes affected soil ζ potentials. The ζ potential became less negative with increased electrolyte concentration. The ζ potentials were more negative in monovalent than in divalent cationic electrolyte solutions because more divalent cations were distributed in the shear plane of the diffuse layer as counter-cations on the soil surfaces than monovalent cations at the same electrolyte concentration.     

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.     

Assessing the natural attenuation of organic contaminants in aquifers using plume-scale electron and carbon balances: model development with analysis of uncertainty and parameter sensitivity

A quantitative methodology is described for the field-scale performance assessment of natural attenuation using plume-scale electron and carbon balances. This provides a practical framework for the calculation of global mass balances for contaminant plumes, using mass inputs from the plume source, background groundwater and plume residuals in a simplified box model. Biodegradation processes and reactions included in the analysis are identified from electron acceptors, electron donors and degradation products present in these inputs. Parameter values used in the model are obtained from data acquired during typical site investigation and groundwater monitoring studies for natural attenuation schemes. The approach is evaluated for a UK Permo-Triassic Sandstone aquifer contaminated with a plume of phenolic compounds. Uncertainty in the model predictions and sensitivity to parameter values was assessed by probabilistic modelling using Monte Carlo methods. Sensitivity analyses were compared for different input parameter probability distributions and a base case using fixed parameter values, using an identical conceptual model and data set. Results show that consumption of oxidants by biodegradation is approximately balanced by the production of CH4 and total dissolved inorganic carbon (TDIC) which is conserved in the plume. Under this condition, either the plume electron or carbon balance can be used to determine contaminant mass loss, which is equivalent to only 4% of the estimated source term. This corresponds to a first order, plume-averaged, half-life of > 800 years. The electron balance is particularly sensitive to uncertainty in the source term and dispersive inputs. Reliable historical information on contaminant spillages and detailed site investigation are necessary to accurately characterise the source term. The dispersive influx is sensitive to variability in the plume mixing zone width. Consumption of aqueous oxidants greatly exceeds that of mineral oxidants in the plume, but electron acceptor supply is insufficient to meet the electron donor demand and the plume will grow. The aquifer potential for degradation of these contaminants is limited by high contaminant concentrations and the supply of bioavailable electron acceptors. Natural attenuation will increase only after increased transport and dilution.     

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.     

Optimization-Based Atmospheric Plume Source Identification

This work applies optimization and an Eulerian inversion approach presented by Bagtzoglou and Baun in 2005 in order to reconstruct contaminant plume time histories and to identify the likely source of atmospheric contamination using data from a real test site for the first time. Present-day distribution of an atmospheric contaminant plume as well as data points reflecting the plume history allow the reconstruction and provide the plume velocity, distribution, and probable source. The method was tested to a hypothetical case and with data from the Forest Atmosphere Transfer and Storage (FACTS) experiment in the Duke experimental forest site. In the scenarios presented herein, as well as in numerous cases tested for verification purposes, the model conserved mass, successfully located the peak of the plume, and managed to capture the motion of the plume well but underestimated the contaminant peak.     

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.     

Microbiological analysis of multi-level borehole samples from a contaminated groundwater system

A range of bacteriological, geochemical process-related and molecular techniques have been used to assess the microbial biodegradative potential in groundwater contaminated with phenol and other tar acids. The contaminant plume has travelled 500 m from the pollutant source over several decades. Samples were obtained from the plume using a multi-level sampler (MLS) positioned in two boreholes (boreholes 59 and 60) which vertically transected two areas of the plume. Activity of the microbial community, as represented by phenol degradation potential and ability to utilise a range of substrates, was found to be influenced by the plume. Phenol degradation potential appeared to be influenced more by the concentration of the contaminants than the total bacterial cell numbers. However, in the areas of highest phenol concentration, the depression of cell numbers clearly had an effect. The types of bacteria present were assessed by culture and DNA amplification by polymerase chain reaction (PCR). Bacterial groups or processes associated with major geochemical processes, such as methanogenesis, sulphate reduction and denitrification, that have the potential to drive contaminant degradation, were detected at various borehole levels. A comparative molecular analysis of the microbial community between samples obtained from the MLS revealed the microbial community was diverse. The examination of microbial activity complemented those results obtained through chemical analysis, and when combined with hydrological data, showed that MLS samples provided a realistic profile of plume effects and could be related to the potential for natural attenuation of the site.     

Groundwater flow and contaminant transport modeling applications in urban area: scopes and limitations

Background

A three-dimensional groundwater flow model was used to evaluate the groundwater potential and assess the effects of groundwater withdrawal on the regional water level and flow direction in the central Beijing area. A program of groundwater modeling aimed at estimating current contaminant fluxes to the central area and site streams via groundwater was developed.

Results and discussion

The conceptual model developed for the site attempted to incorporate a complex stratigraphic profile in which groundwater flow and contaminant transport is strongly controlled by a shallow aquifer. Here, a conceptual model for groundwater flow and contaminant transport in central Beijing is presented.

Conclusion

Model simulations indicated that a sharp drop in the hydraulic head occurs at the center of the model area, which generates a cone of depression and a continuous decline of head with respect to time as a result of heavy groundwater abstraction.     

Monitoring groundwater contamination and delineating source zones at industrial sites: uncertainty analyses using integral pumping tests

Field-scale characterisations of contaminant plumes in groundwater, as well as source zone delineations, are associated with uncertainties that can be considerable. A major source of uncertainty in environmental datasets is due to variability of sampling results, as a direct consequence of the heterogeneity of environmental matrices. We develop a methodology for quantifying uncertainties in field-scale mass flow and average concentration estimations, using integral pumping tests (IPTs), where the contaminant concentration is measured as a function of time in a pumping well. This procedure increases the sampling volume and reduces the effect of small-scale variability that may bias point-scale measurements. In particular, using IPTs, the interpolation uncertainty of conventional point-scale measurements is transformed to a quantifiable uncertainty related to the (unknown) plume position relative to the pumping well. We show that this plume position uncertainty generally influenced the predicted mass flows and average concentrations (of acenapthene, benzene and CHCs) to a greater extent than a boundary condition uncertainty related to the local water balance, considering 19 control planes at a highly heterogeneous industrial site in southwest Germany. Furthermore, large (order of magnitude) uncertainties only occurred if the conditions were strongly heterogeneous in the nearest vicinity of the well. We also develop a consistent methodology for an assessment of the combined effect of uncertainty in hydraulic conditions and uncertainty in reactive transport parameters for delimiting of both contaminant source zones and zones absent of source, based on (downgradient) IPTs.     

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.     

Simultaneous optimization of dense non-aqueous phase liquid (DNAPL) source and contaminant plume remediation

A framework is developed for simultaneous, optimal design of groundwater contaminant source removal and plume remediation strategies. The framework allows for varying degrees of effort and cost to be dedicated to source removal versus plume remediation. We have accounted for the presence of physical heterogeneity in the DNAPL source, since source heterogeneity controls mass release into the plume and the efficiency of source removal efforts. We considered high and low estimates of capital and operating costs for chemical flushing removal of the source, since these are expected to vary form site to site. Using the lower chemical flushing cost estimates, it is found that the optimal allocation of funds to source removal or plume remediation is sensitive to the degree of heterogeneity in the source. When the time elapsed between the source release and the implementation of remediation was varied, it was found that, except for the longest elapsed time (50,000 days), a combination of partial source removal and plume remediation was most efficient. When first-order, dissolved contaminant degradation was allowed, source removal was found to be unnecessary for the cases where the degradation rate exceeded intermediate values of the first-order rate constant. Finally, it was found that source removal became more necessary as the degree of aquifer heterogeneity increased.     

An assessment of natural biotransformation of petroleum hydrocarbons and chlorinated solvents at an aquifer plume transect

Field biogeochemical characterization and laboratory microcosm studies were performed to assess the potential for future biotransformation of trichloroethylene (TCE) and toluene in a plume containing petroleum hydrocarbons and chlorinated solvents at the former Wurtsmith Air Force Base in Oscoda, MI. In situ terminal electron accepting processes (TEAPs), contaminant composition and microbial phylogeny were studied at a plume transect 100 m downgradient of the source. The presence of reduced electron acceptors, relevant microbial communities, and elevated dissolved methane and carbon dioxide concentrations at the transect, as well as downgradient accumulation of BTEX metabolites and dechlorination products, indicated that past or current reductive dechlorination at the transect was likely driven by BTEX biodegradation in the methanogenic zone. However, TCE and toluene mineralization in sediment-groundwater microcosms without added electron acceptors did not exceed 5% during 300 days of incubation and was nearly invariant with original sediment TEAP, even following amendments of nitrogen and phosphorus. Mineralization rates were on the order of 0.0015-0.03 mumol/g day. After 8 months, microcosms showed evidence of methanogenesis, but CH4 and CO2 production arose from the degradation of contaminants other than toluene. Cis-dichloroethylene was observed in only one methanogenic microcosm after more than 500 days. It appears likely that spatially and temporally dynamic redox zonation at the plume transect will prevent future sustained reductive dehalogenation of highly chlorinated solvents, for during the course of a year, the predominant TEAP at the highly contaminated water table shifted from methanogenesis to iron- and sulfate-reduction. It is recommended that biotransformation studies combine considerations of long-term, spatially relevant changes in redox zonation with laboratory-scale studies of electron donor utilization and cometabolic substrate transformation to yield a more accurate assessment of natural bioattenuation of specific pollutants in aquifers contaminated by undefined organic waste mixtures.     

Rebound of a coal tar creosote plume following partial source zone treatment with permanganate

The long-term management of dissolved plumes originating from a coal tar creosote source is a technical challenge. For some sites stabilization of the source may be the best practical solution to decrease the contaminant mass loading to the plume and associated off-site migration. At the bench-scale, the deposition of manganese oxides, a permanganate reaction byproduct, has been shown to cause pore plugging and the formation of a manganese oxide layer adjacent to the non-aqueous phase liquid creosote which reduces post-treatment mass transfer and hence mass loading from the source. The objective of this study was to investigate the potential of partial permanganate treatment to reduce the ability of a coal tar creosote source zone to generate a multi-component plume at the pilot-scale over both the short-term (weeks to months) and the long-term (years) at a site where there is >10 years of comprehensive synoptic plume baseline data available. A series of preliminary bench-scale experiments were conducted to support this pilot-scale investigation. The results from the bench-scale experiments indicated that if sufficient mass removal of the reactive compounds is achieved then the effective solubility, aqueous concentration and rate of mass removal of the more abundant non-reactive coal tar creosote compounds such as biphenyl and dibenzofuran can be increased. Manganese oxide formation and deposition caused an order-of-magnitude decrease in hydraulic conductivity. Approximately 125 kg of permanganate were delivered into the pilot-scale source zone over 35 days, and based on mass balance estimates <10% of the initial reactive coal tar creosote mass in the source zone was oxidized. Mass discharge estimated at a down-gradient fence line indicated >35% reduction for all monitored compounds except for biphenyl, dibenzofuran and fluoranthene 150 days after treatment, which is consistent with the bench-scale experimental results. Pre- and post-treatment soil core data indicated a highly variable and random spatial distribution of mass within the source zone and provided no insight into the mass removed of any of the monitored species. The down-gradient plume was monitored approximately 1, 2 and 4 years following treatment. The data collected at 1 and 2 years post-treatment showed a decrease in mass discharge (10 to 60%) and/or total plume mass (0 to 55%); however, by 4 years post-treatment there was a rebound in both mass discharge and total plume mass for all monitored compounds to pre-treatment values or higher. The variability of the data collected was too large to resolve subtle changes in plume morphology, particularly near the source zone, that would provide insight into the impact of the formation and deposition of manganese oxides that occurred during treatment on mass transfer and/or flow by-passing. Overall, the results from this pilot-scale investigation indicate that there was a significant but short-term (months) reduction of mass emanating from the source zone as a result of permanganate treatment but there was no long-term (years) impact on the ability of this coal tar creosote source zone to generate a multi-component plume.