Changes in contaminant mass discharge from DNAPL source mass depletion: evaluation at two field sites

Changes in contaminant fluxes resulting from aggressive remediation of dense nonaqueous phase liquid (DNAPL) source zone were investigated at two sites, one at Hill Air Force Base (AFB), Utah, and the other at Ft. Lewis Military Reservation, Washington. Passive Flux Meters (PFM) and a variation of the Integral Pumping Test (IPT) were used to measure fluxes in ten wells installed along a transect down-gradient of the trichloroethylene (TCE) source zone, and perpendicular to the mean groundwater flow direction. At both sites, groundwater and contaminant fluxes were measured before and after the source-zone treatment. The measured contaminant fluxes (J; ML(-2)T(-1)) were integrated across the well transect to estimate contaminant mass discharge (M(D); MT(-1)) from the source zone. Estimated M(D) before source treatment, based on both PFM and IPT methods, were approximately 76 g/day for TCE at the Hill AFB site; and approximately 640 g/day for TCE, and approximately 206 g/day for cis-dichloroethylene (DCE) at the Ft. Lewis site. TCE flux measurements made 1 year after source treatment at the Hill AFB site decreased to approximately 5 g/day. On the other hand, increased fluxes of DCE, a degradation byproduct of TCE, in tests subsequent to remediation at the Hill AFB site suggest enhanced microbial degradation after surfactant flooding. At the Ft. Lewis site, TCE mass discharge rates subsequent to remediation decreased to approximately 3 g/day for TCE and approximately 3 g/day for DCE approximately 1.8 years after remediation. At both field sites, PFM and IPT approaches provided comparable results for contaminant mass discharge rates, and show significant reductions (>90%) in TCE mass discharge as a result of DNAPL mass depletion from the source zone.     

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 performance and closure for soil vapor extraction: integrating vapor discharge and impact to groundwater quality

Soil vapor extraction (SVE) is typically effective for removal of volatile contaminants from higher-permeability portions of the vadose zone. However, contamination in lower-permeability zones can persist due to mass transfer processes that limit the removal effectiveness. After SVE has been operated for a period of time and the remaining contamination is primarily located in lower-permeability zones, the remedy performance needs to be evaluated to determine whether the SVE system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. Numerical modeling of vapor-phase contaminant transport was used to investigate the correlation between measured vapor-phase mass discharge, MF(r), from a persistent, vadose-zone contaminant source and the resulting groundwater contaminant concentrations. This relationship was shown to be linear, and was used to directly assess SVE remediation progress over time and to determine the level of remediation in the vadose zone necessary to protect groundwater. Although site properties and source characteristics must be specified to establish a unique relation between MF(r) and the groundwater contaminant concentration, this correlation provides insight into SVE performance and support for decisions to optimize or terminate the SVE operation or to transition to another type of treatment.     

Model-based evaluation of controlled-release systems in the remediation of dissolved plumes in groundwater

Controlled-release, semi-passive reactive barrier systems have been recently developed as a long-term treatment option for controlling the spread of contaminant plumes in groundwater. This paper describes a new computer code, and applies it to study coupled processes of solute release, reaction, and mass transport in an in situ remediation scheme using the controlled release of potassium permanganate. Confidence with the modeling approach was developed by model verifications and simulating results of a pilot-scale test-cell experiment. Sensitivity analyses indicated the possibilities of treatment inefficiencies due to inability of transverse dispersion to mix the permanganate (MnO(4)(-)) within the zone of reaction, fluctuations in source strength due to variations in flow velocity, and the small length of treatment zone due to strong soil utilization of MnO(4)(-). Although problems associated with the fluctuating source strength and strong soil utilization can be addressed by optimizing the release rate, the inefficiency of transverse dispersion to create mixing could pose a serious limitation. Through a series of model simulations, a system of injection/withdrawal wells in a doublet arrangement was developed to facilitate lateral spreading and mixing of MnO(4)(-). A well-mixed, stable MnO(4)(-) zone with predetermined size (DxL=8m x 2m) and concentration ranges (1.5-20 mg l(-1)) was created by four 1-day injection/withdrawal pumping periods over 24 d. This type of mixing zone may persist for many years with periodic well mixing and replacements of exhausted controlled-release forms. Coupled use of the generalized code with field hydrologic data will help to optimize the design and operation of controlled-release systems in practice.     

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.     

Characterisation of microbial activity in the framework of natural attenuation without groundwater monitoring wells?: a new Direct-Push probe

At many contaminated field sites in Europe, monitored natural attenuation is a feasible site remediation option. Natural attenuation includes several processes but only the microbial degradation leads to real contaminant removal and very few methods are accepted by the authorities providing real evidence of microbial contaminant degradation activity. One of those methods is the recently developed in situ microcosm approach (BACTRAP®). These in situ microcosms consist of perforated stainless steel cages or PTFE tubes filled with an activated carbon matrix that is amended with ¹³C-labelled contaminants; the microcosms are then exposed within groundwater monitoring wells. Based on this approach, natural attenuation was accepted by authorities as a site remediation option for the BTEX-polluted site Zeitz in Germany. Currently, the in situ microcosms are restricted to the use inside groundwater monitoring wells at the level of the aquifer. The (classical) system therefore is only applicable on field sites with a network of monitoring wells, and only microbial activity inside the monitoring wells at the level of the aquifer can be assessed. In order to overcome these limitations, a new Direct-Push BACTRAP probe was developed on the basis of the Geoprobe® equipment. With respect to the mechanical boundary conditions of the DP technique, these new probes were constructed in a rugged and segmented manner and are adaptable to various sampling concepts. With this new probe, the approach can be extended to field sites without existing monitoring wells, and microbial activity was demonstrated to be measureable even under very dry conditions inside the vadose zone above the aquifer. In a field test, classical and Direct-Push BACTRAPs were applied in the BTEX-contaminated aquifer at the ModelPROBE reference site Zeitz (Germany). Both types of BACTRAPs were incubated in the centre and at the fringe of the BTEX plume. Analysis of phospholipid fatty acid (PLFA) patterns showed that the bacterial communities on DP-BACTRAPs were more similar to the soil than those found on classical BACTRAPs. During microbial degradation of the ¹³C-labelled substrate on the carrier material of the microcosms, the label was only slightly incorporated into bacterial biomass, as determined by PLFA analysis. This provides clear indication for decreased in situ natural attenuation potential in comparison to earlier sampling campaigns, which is presumably caused by a large-scale source remediation measure in the meantime. In conclusion, Direct-Push-based BACTRAPs offer a promising way to monitor natural attenuation or remediation success at field sites which are currently inaccessible by the technique due to the lack of monitoring wells or due to a main contamination present within the vadose zone.     

Petroleum mass removal from low permeability sediment using air sparging/soil vapor extraction: impact of continuous or pulsed operation

Air sparging and soil vapor extraction (AS/SVE) are innovative remediation techniques that utilize volatilization and microbial degradation to remediate petroleum spills from soils and groundwater. This in situ study investigated the use of AS/SVE to remediate a gasoline spill from a leaking underground storage tank (UST) in the low permeability, clayey soil of the Appalachian Piedmont. The objectives of this study were to evaluate AS/SVE in low permeability soils by quantifying petroleum mass removal rates, monitoring vadose zone contaminant levels, and comparing the mass extraction rates of continuous AS/SVE to 8 and 24 h pulsed operation. The objectives were met by collecting AS/SVE exhaust gas samples and vadose zone air from multi-depth soil vapor probes. Samples were analyzed for O₂, CO₂, BTEX (benzene, toluene, ethylbenzene, xylene), and total combustible hydrocarbon (TCH) concentrations using portable hand meters and gas chromatography. Continuous AS/SVE was effective in removing 608 kg of petroleum hydrocarbons from low permeability soil in 44 days (14.3 kg day⁻¹). Mass removal rates ranged from 2.6 times higher to 5.1 times lower than other AS/SVE studies performed in sandy sediments. BTEX levels in the vadose zone were reduced from about 5 ppm to 1 ppm. Ten pulsed AS/SVE tests removed 78 kg in 23 days and the mean mass removal rate (17.6 kg day⁻¹) was significantly higher than the last 15 days of continuous extraction. Pulsed operation may be preferable to continuous operation because of increased mass removal and decreased energy consumption.     

Plume persistence caused by back diffusion from thin clay layers in a sand aquifer following TCE source-zone hydraulic isolation

This paper concludes that back diffusion from one or a few thin clayey beds in a sand aquifer can cause contaminant persistence above MCLs in a sand aquifer long after the source zone initially causing the plume is isolated or removed. This conclusion is based on an intensive case study of a TCE contaminated site in Florida, with the processes evaluated using numerical modeling. At this site, the TCE DNAPL zone formed decades ago, and was hydraulically isolated by means of an innovative system performing groundwater extraction, treatment and re-injection. Treated water is re-injected in a row of injection wells situated a short distance downgradient of the extraction wells, creating a clean-water displacement front to efficiently flush the downgradient plume. This scheme avoids the creation of stagnation zones typical of most groundwater pump-and-treat systems, thereby minimizing the time for aquifer flushing and therefore downgradient cleanup. The system began operation in August 2002 and although the performance monitoring shows substantial declines in concentrations, detectable levels of TCE and degradation products persist downgradient of the re-injection wells, long after the TCE should have disappeared based on calculations assuming a nearly homogenous sand aquifer. Three hypotheses were assessed for this plume persistence: 1) incomplete source-zone capture, 2) DNAPL occurrence downgradient of the re-injection wells, and 3) back diffusion from one or more thin clay beds in the aquifer. After careful consideration, the first two hypotheses were eliminated, leaving back diffusion as the only plausible hypothesis, supported by detailed measurements of VOC concentrations within and near the clay beds and also by numerical model simulations that closely represent the field site hydrogeologic conditions. The model was also used to simulate a more generalized, hypothetical situation where more thin clayey beds occur in a sand aquifer with an underlying aquitard. While there is no doubt that DNAPL source mass reduction can eventually improve downgradient groundwater quality, the magnitude and time scale over which the improvement occurs is the major uncertainty given current characterization approaches. This study shows that even one thin clay bed, less than 0.2 m thick, can cause plume persistence due to back diffusion for several years or even decades after the flux from the source is completely isolated. Thin clay beds, which have a large storage capacity for dissolved and sorbed contaminant mass, are common in many types of sandy aquifers. However, without careful inspection of continuous cores and sampling, such thin clay beds, and their potential for causing long-term back-diffusion effects, can easily go unnoticed during site characterization.     

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.     

Predicting natural attenuation of xylene in groundwater using a numerical model.

The aquifer beneath an abandoned refinery in the Lower Rhine area, Germany, was contaminated with a number of different mineral oil products. Groundwater sampling in the area around the former xylene plant revealed that a xylene plume had developed in the underlying groundwater, and moreover, that there is strong evidence for in situ microbial xylene degradation with oxygen, nitrate, sulfate and ferric iron as electron acceptors. In order to prevent further xylene spreading, three pumping wells extracting contaminated water were installed downgradient of the spill zone. The numerical reactive transport code Transport Biochemisty Chemistry (TBC) was applied to this situation to quantify the relation of microbial degradation to xylene removal by the pumping wells. It could be shown that the unamended in situ degradation was an appreciable xylene removal process that contributed to about one-third to the total xylene removal (degradation plus extraction). A further objective of the model application was to predict xylene spreading under regional flow conditions, i.e. without operation of the three pumping wells, to consider the possible effects of natural xylene attenuation. To accomplish this, the model calibrated for the situation with operating wells was transferred to the hydraulic situation of regional flow while retaining the parameters of the biochemical model. It turned out that the xylene plume that is expected to develop downgradient of the source area will be limited to an extension of not more than 1000 m. An interesting feature of the simulations results was that xylene degradation under iron-reducing conditions, which was of minor importance for the situation with operating pumping wells, becomes the dominant degradation mechanism under regional flow conditions. Moreover, iron reduction will be the key process in controlling plume evolution. The model application illustrates that multi-species reactive transport models are needed to adequately transfer reactive processes from one hydraulic situation to another, while single species models are not suited for this predictive task.     

Modeling field-scale cosolvent flooding for DNAPL source zone remediation

A three-dimensional, compositional, multiphase flow simulator was used to model a field-scale test of DNAPL removal by cosolvent flooding. The DNAPL at this site was tetrachloroethylene (PCE), and the flooding solution was an ethanol/water mixture, with up to 95% ethanol. The numerical model, UTCHEM accounts for the equilibrium phase behavior and multiphase flow of a ternary ethanol-PCE-water system. Simulations of enhanced cosolvent flooding using a kinetic interphase mass transfer approach show that when a very high concentration of alcohol is injected, the DNAPL/water/alcohol mixture forms a single phase and local mass transfer limitations become irrelevant. The field simulations were carried out in three steps. At the first level, a simple uncalibrated layered model is developed. This model is capable of roughly reproducing the production well concentrations of alcohol, but not of PCE. A more refined (but uncalibrated) permeability model is able to accurately simulate the breakthrough concentrations of injected alcohol from the production wells, but is unable to accurately predict the PCE removal. The final model uses a calibration of the initial PCE distribution to get good matches with the PCE effluent curves from the extraction wells. It is evident that the effectiveness of DNAPL source zone remediation is mainly affected by characteristics of the spatial heterogeneity of porous media and the variable (and unknown) DNAPL distribution. The inherent uncertainty in the DNAPL distribution at real field sites means that some form of calibration of the initial contaminant distribution will almost always be required to match contaminant effluent breakthrough curves.     

Metal availability in a highly contaminated, dredged-sediment disposal site: Field measurements and geochemical modeling

Two complementary approaches were used to characterize arsenic and metal mobilizations from a dredged-sediment disposal site: a detailed field study combined with hydrogeochemical modeling. Contaminants in sediments were found to be mainly present as sulfides subject to oxidation. Secondary phases (carbonates, sulfates, (hydr)oxides) were also observed. Oxidative processes occurred at different rates depending on physicochemical conditions and contaminant contents in the sediment. Two distinct areas were identified on the site, each corresponding to a specific contaminant mobility behavior. In a reducing area, Fe and As were highly soluble and illustrated anoxic behavior. In well-oxygenated material, groundwater was highly contaminated in Zn, Cd and Pb. A third zone in which sediments and groundwater were less contaminated was also characterized. This study enabled us to prioritize remediation work, which should aim to limit infiltration and long-term environmental impact.     

Impact of injection system design on ISCO performance with permanganate--mathematical modeling results

In situ chemical oxidation (ISCO) using permanganate (MnO(4)(-)) can be a very effective technique for remediation of soil and groundwater contaminated with chlorinated solvents. However, many ISCO projects are less effective than desired because of poor delivery of the chemical reagents to the treatment zone. In this work, the numerical model RT3D was modified and applied to evaluate the effect of aquifer characteristics and injection system design on contact and treatment efficiency. MnO(4)(-) consumption was simulated assuming the natural oxidant demand (NOD) is composed of a fraction that reacts instantaneously and a fraction that slowly reacts following a 2nd order relationship where NOD consumption rate increases with increasing MnO(4)(-) concentration. MnO(4)(-) consumption by the contaminant was simulated as an instantaneous reaction. Simulation results indicate that the mass of permanganate and volume of water injected has the greatest impact on aquifer contact efficiency and contaminant treatment efficiency. Several small injection events are not expected to increase contact efficiency compared to a single large injection event, and can increase the amount of un-reacted MnO(4)(-) released down-gradient. High groundwater flow velocities can increase the fraction of aquifer contacted. Initial contaminant concentration and contaminant retardation factor have only a minor impact on volume contact efficiency. Aquifer heterogeneity can have both positive and negative impacts on remediation system performance, depending on the injection system design.     

A coupled simulation-optimization approach for groundwater remediation design under uncertainty: An application to a petroleum-contaminated site

This study provides a coupled simulation–optimization approach for optimal design of petroleum-contaminated groundwater remediation under uncertainty. Compared to the previous approaches, it has the advantages of: (1) addressing the stochasticity of the modeling parameters in simulating the flow and transport of NAPLs in groundwater, (2) providing a direct and response-rapid bridge between remediation strategies (pumping rates) and remediation performance (contaminant concentrations) through the created proxy models, (3) alleviating the computational cost in searching for optimal solutions, and (4) giving confidence levels for the obtained optimal remediation strategies. The approach is applied to a practical site in Canada for demonstrating its performance. The results show that mitigating the effects of uncertainty on optimal remediation strategies (through enhancing the confidence level) would lead to the rise of remediation cost due to the increase in the total pumping rate.     

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.     

Assessing microbial degradation of o-xylene at field-scale from the reduction in mass flow rate combined with compound-specific isotope analyses

In recent years, natural attenuation (NA) has evolved into a possible remediation alternative, especially in the case of BTEX spills. In order to be approved by the regulators, biodegradation needs to be demonstrated which requires efficient site investigation and monitoring tools. Three methods--the Integral Groundwater Investigation method, the compound-specific isotope analysis (CSIA) and a newly developed combination of both--were used in this work to quantify at field scale the biodegradation of o-xylene at a former gasworks site which is heavily contaminated with BTEX and PAHs. First, the Integral Groundwater Investigation method [Schwarz, R., Ptak, T., Holder, T., Teutsch, G., 1998. Groundwater risk assessment at contaminated sites: a new investigation approach. In: Herbert, M. and Kovar, K. (Editors), GQ'98 Groundwater Quality: Remediation and Protection. IAHS Publication 250, pp. 68-71; COH 4 (2000) 170] was applied, which allows the determination of mass flow rates of o-xylene by integral pumping tests. Concentration time series obtained during pumping at two wells were used to calculate inversely contaminant mass flow rates at the two control planes that are defined by the diameter of the maximum isochrone. A reactive transport model was used within a Monte Carlo approach to identify biodegradation as the dominant process for reduction in the contaminant mass flow rate between the two consecutive control planes. Secondly, compound-specific carbon isotope analyses of o-xylene were performed on the basis of point-scale samples from the same two wells. The Rayleigh equation was used to quantify the degree of biodegradation that occurred between the wells. Thirdly, a combination of the Integral Groundwater Investigation method and the compound-specific isotope analysis was developed and applied. It comprises isotope measurements during the integral pumping tests and the evaluation of delta13C time series by an inversion algorithm to obtain spatially integrated mean isotope values at the control planes. It was shown that the Rayleigh equation is applicable to spatially integrated mean isotope values in order to obtain the mean biodegradation between the consecutive control planes. All three approaches yielded consistently a 98-99% degradation of o-xylene.     

Attenuation reactions in a multiple contaminated aquifer in Bitterfeld (Germany)

Large-scale contaminated sites with multiple contaminants in the groundwater present a challenge to risk assessment and remediation. Attenuation reactions take place in the subsurface and act to contain contaminants, but must be thoroughly investigated on a site-specific basis. Field data from monitoring wells at a contaminated industrial site in Bitterfeld, Germany, are presented and analyzed for evidence of the prevalent biodegradation reactions. The groundwater in the Tertiary aquifer is contaminated with large quantities of chlorinated aliphatic compounds, in addition to chlorobenzenes and BTEX. In this strictly anaerobic environment, geochemical indications for several microbial processes were found, including methanogenesis, sulfate and iron reduction as well as reductive dechlorination of the chlorinated hydrocarbons. Direct evidence for the latter degradation reaction was observed along the flowpath due to the appearance of intermediates and an increase in the degree of dechlorination.     

A pragmatic approach for estimation of source-zone emissions at LNAPL contaminated sites

When considering natural attenuation as a remediation strategy at a site contaminated by a light non-aqueous phase liquid (LNAPL), it is important to consider the emission of contaminants from the source zone. A quantification of source-zone emissions is essential both for comparison with down-gradient mass fluxes to provide an estimate of fractional mass flux reduction, as well as for estimating the source lifetime. Because the spatial distribution of LNAPL at a field site is strongly dependent on both the spill circumstances and the heterogeneity of the geologic materials, which can be problematic for in-situ determination, alternative methods for estimating source-zone emissions are needed. In this work, a three-dimensional multiphase flow and transport modelling approach is used to investigate the relationship between the lateral extent of an LNAPL body and the emission of contaminants to groundwater at a contaminated site. For simulations involving an LNAPL release in an aquifer comprised of heterogeneous porosity and permeability distributions that were generated geostatistically, it is shown that a simple linear relationship exists between the lateral extent of the LNAPL body in the capillary fringe and the emission to the aqueous phase. The parameters describing the relationship are found to be linear functions of the groundwater flow velocity and the vertical infiltration rate. This site-specific relationship provides a simple method to estimate contaminant emissions to groundwater at LNAPL contaminated sites.     

Use of principal component analysis to profile temporal and spatial variations of chlorinated solvent concentration in groundwater

This work aims at evaluating spatial distribution patterns of concentration variations for chlorinated solvents in groundwater, based on principal component analysis and geographic information system (GIS) tools. The study investigates long-time series of chlorinated solvent concentrations in groundwater measured for 18 contaminated industrial sites. The characterization of contaminant plumes and delineation of pollutant sources are essential for choosing appropriate monitoring and remediation strategies, as contaminated groundwaters are characterized by complex patterns of spatial and temporal concentration variability, with wide unpredictable fluctuations over time. The present work describes the results of a new exploratory statistical method called the Variability Index Method (VIM) applied to environmental data to assess the performance of using concentration variations as molecular tracers to reveal aquifer dynamics, industrial impacts, and point sources for contamination plumes. The application of this method provides a useful assessment of controls over contaminant concentration variations as well as support for remediation techniques.