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.     

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.     

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.     

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.     

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.     

Estimation of Methyl tert -Butyl Ether Plume Length Using the Domenico Analytical Model

Methyl tert -butyl ether (MTBE) plume is controlled by many factors, primarily by groundwater flow velocity, dispersion, natural attenuation. This study employed an analytical model introduced by Domemico (1987, J. Hydrol 91 , 49-58.) to describe the MTBE concentration distribution horizontal pattern and estimated the MTBE plume length. The model was applied to 90 leaking underground storage tank cases in Los Angeles, CA, U.S.A. The analytical model was calibrated with field data for each ease using a Microsoft Excel spreadsheet program. Methyl tert -butyl ether concentrations in one source monitoring well and one to two downgradient centerline monitoring wells were used for each case study. When the centerline well is not available, the closest off-centerline wells were projected to the centerline using an ellipse trigonometry method. The model parameter values for longitudinal dispersivity, groundwater velocity, and degradation rate constant were calibrated using the field data and then used to estimate the maximum distance between source well and the plume edge. This study demonstrates that the Domenico model can be applied to MTBE plume investigation when adequate field data are available. The correlation coefficients calculated based on the results of the 90 case studies indicate that MTBE plume length has a poor correlation with MTBE concentration at the source well, and a moderate negative correlation with the degradation rate constant ( m 0.65) and u / v ratio (0.64). Furthermore, MTBE plume length has a poor correlation with the longitudinal dispersivity ( m 0.4), hydraulic gradient ( m 0.1), and groundwater velocity (0.17).     

Field demonstration and evaluation of the Passive Flux Meter on a CAH groundwater plume

This study comprises the first application of the Passive Flux Meter (PFM) for the measurement of chlorinated aliphatic hydrocarbon (CAH) mass fluxes and Darcy water fluxes in groundwater at a European field site. The PFM was originally developed and applied to measurements near source zones. The focus of the PFM is extended from near source to plume zones. For this purpose, 48 PFMs of 1.4 m length were constructed and installed in eight different monitoring wells in the source and plume zone of a CAH-contaminated field site located in France. The PFMs were retrieved, sampled, and analyzed after 3 to 11 weeks of exposure time, depending on the expected contaminant flux. PFM evaluation criteria include analytical, technical, and practical aspects as well as conditions and applicability. PFM flux data were compared with so-called traditional soil and groundwater concentration data obtained using active sampling methods. The PFMs deliver reasonable results for source as well as plume zones. The limiting factor in the PFM applicability is the exposure time together with the groundwater flux. Measured groundwater velocities at the field site range from 2 to 41 cm/day. Measured contaminant flux data raise up to 13 g/m²/day for perchloroethylene in the plume zone. Calculated PFM flux averaged concentration data and traditional concentration data were of similar magnitude for most wells. However, both datasets need to be compared with reservation because of the different sampling nature and time. Two important issues are the PFM tracer loss during installation/extraction and the deviation of the groundwater flow field when passing the monitoring well and PFM. The demonstration of the PFM at a CAH-contaminated field site in Europe confirmed the efficiency of the flux measurement technique for source as well as plume zones. The PFM can be applied without concerns in monitoring wells with European standards. The acquired flux data are of great value for the purpose of site characterization and mass discharge modeling, and can be used in combination with traditional soil and groundwater sampling methods.     

Pulsed pumping process optimization using a potential flow model

A computational model is applied to the optimization of pulsed pumping systems for efficient in situ remediation of groundwater contaminants. In the pulsed pumping mode of operation, periodic rather than continuous pumping is used. During the pump-off or trapping phase, natural gradient flow transports contaminated groundwater into a treatment zone surrounding a line of injection and extraction wells that transect the contaminant plume. Prior to breakthrough of the contaminated water from the treatment zone, the wells are activated and the pump-on or treatment phase ensues, wherein extracted water is augmented to stimulate pollutant degradation and recirculated for a sufficient period of time to achieve mandated levels of contaminant removal. An important design consideration in pulsed pumping groundwater remediation systems is the pumping schedule adopted to best minimize operational costs for the well grid while still satisfying treatment requirements. Using an analytic two-dimensional potential flow model, optimal pumping frequencies and pumping event durations have been investigated for a set of model aquifer-well systems with different well spacings and well-line lengths, and varying aquifer physical properties. The results for homogeneous systems with greater than five wells and moderate to high pumping rates are reduced to a single, dimensionless correlation. Results for heterogeneous systems are presented graphically in terms of dimensionless parameters to serve as an efficient tool for initial design and selection of the pumping regimen best suited for pulsed pumping operation for a particular well configuration and extraction rate. In the absence of significant retardation or degradation during the pump-off phase, average pumping rates for pulsed operation were found to be greater than the continuous pumping rate required to prevent contaminant breakthrough.     

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.     

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.     

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.     

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.     

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.     

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.     

Field study of TCE diffusion profiles below DNAPL to assess aquitard integrity

An area where a free-product accumulation of trichloroethylene (TCE) dense non-aqueous phase liquid (DNAPL) occurs at the bottom of a 10-m-thick surficial sand aquifer was studied to determine the integrity of the underlying, 20-m-thick, clayey silt aquitard formed of glaciolacustrine sediment. TCE concentration-versus-depth profiles determined from aquitard cores collected at five locations indicated penetration of detectable TCE 2.5 to 3.0 m into the aquitard. Two of the profiles show persistent DNAPL at the aquitard interface, while two others indicate that DNAPL, present initially, was completely dissolved away producing concentration declines at the aquitard interface. The fifth profile suggests shallow DNAPL penetration (<0.5 m) into the aquitard, however, this penetration, which was likely caused by cross-contamination during core collection or cone penetrometry (CPT) of the aquitard interface, did not increase the maximum depth of TCE penetration. Combining the field profiles with one-dimensional model simulations, downward migration of the aqueous TCE front, defined as the EPA MCL of 5 microg/l, which was below the analytical detection limit, was projected to a distance between 4 and 5 m below the top of the aquitard. Using a single set of estimated aquitard parameter values, simulations of aqueous TCE migration into the aquitard provided a good fit to four of the field profiles with a migration time of 35 to 45 years, consistent with the history of TCE use at the site. These simulations indicate aqueous TCE migration is diffusion-dominated with only small advective influence by the downward groundwater velocity of 2 to 3 cm/year or less in the aquitard due to pumping of the underlying aquifer to supply water to the facility in the past 50 years. The applicability of the parameter values was confirmed by in situ diffusion experiments of 1-year duration, in which stainless steel cylinders containing DNAPL were inserted into the aquitard. The diffusion-dominated nature of the profiles indicates that the aquitard provides long-term protection of the underlying aquifer from contamination from this DNAPL zone. Simulations of long-term migration of the TCE solute front indicate breakthrough to the lower aquifer at 1200 years for the no advection scenario and at 500 years if the strong downward hydraulic gradient persists. However, even after breakthrough, the mass flux through the aquitard to the underlying aquifer remains relatively low, and when considered in terms of potential impacts to pumping wells, concentrations are not expected to increase significantly above present-day MCLs. The use of contaminant profiles of different time and distance scales, in addition to hydraulic data, dramatically improves the ability to assess aquitard integrity, and provides improved transport parameter values for estimating contaminant arrival times and fluxes. The apparent lack of deep preferential pathways for TCE migration, such as open fractures, is probably due to the softness of the silty aquitard deposit and minimal physical or chemical weathering of the aquitard provides long-term protection of the underlying aquifer from contamination from this DNAPL zone. Simulations of long-term migration of the TCE solute front indicate breakthrough to the lower aquifer at 1200 years for the no advection scenario and at 500 years if the strong downward hydraulic gradient persists. However, even after the breakthrough, the mass flux through the aquitard to the underlying aquifer remains relatively low, and when considered in terms of potential impacts to pumping wells , concentrations are not expected to increase significantly above present-day MCLs. The use of contaminant profiles of different time and distance scales, in addition to hydraulic data, dramatically improves the ability to assess aquitard integrity, and provides improved transport parameter values for estimating contaminant arrival times and fluxes. The apparent lack of deep preferential pathways for TCE migration, such as open fractures, is probably due to the softness of the silty aquitard deposit and minimal physical or chemical weathering of the aquitard.     

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.     

Assessing the impacts of partial mass depletion in DNAPL source zones I. Analytical modeling of source strength functions and plume response

Analytical solutions are developed for approximating the time-dependent contaminant discharge from DNAPL source zones undergoing dissolution and other decay processes. The source functions assume a power relationship between source mass and chemical discharge and can consider partial DNAPL source remediation (depletion) at any time after the initial DNAPL release. The source functions are used as a time-dependent boundary condition in an idealized chemical transport model to develop leading order approximations of the plume response to DNAPL source removal. The results suggest that partial DNAPL remediation does not tend to have a dramatic impact on the maximum extent of the plume if very low concentration values are used to define the plume boundaries. However, the solutions show that partial DNAPL removal from the source zone is likely to lead to large reductions in plume concentrations and mass, and it reduces the longevity of the plume. When the mass discharge from the source zone is linearly related to the DNAPL mass, it is shown that partial DNAPL depletion leads to linearly proportional reductions in the plume mass and concentrations.     

Natural attenuation of chlorinated ethene compounds: model development and field-scale application at the Dover site

A multi-dimensional and multi-species reactive transport model was developed to aid in the analysis of natural attenuation design at chlorinated solvent sites. The model can simulate several simultaneously occurring attenuation processes including aerobic and anaerobic biological degradation processes. The developed model was applied to analyze field-scale transport and biodegradation processes occurring at the Area-6 site in Dover Air Force Base, Delaware. The model was calibrated to field data collected at this site. The calibrated model reproduced the general groundwater flow patterns, and also, it successfully recreated the observed distribution of tetrachloroethene (PCE), trichloroethene (TCE), dichloroethylene (DCE), vinyl chloride (VC) and chloride plumes. Field-scale decay rates of these contaminant plumes were also estimated. The decay rates are within the range of values that were previously estimated based on lab-scale microcosm and field-scale transect analyses. Model simulation results indicated that the anaerobic degradation rate of TCE, source loading rate, and groundwater transport rate are the important model parameters. Sensitivity analysis of the model indicated that the shape and extent of the predicted TCE plume is most sensitive to transmissivity values. The total mass of the predicted TCE plume is most sensitive to TCE anaerobic degradation rates. The numerical model developed in this study is a useful engineering tool for integrating field-scale natural attenuation data within a rational modeling framework. The model results can be used for quantifying the relative importance of various simultaneously occurring natural attenuation processes.     

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.     

Comment on “Analysis of groundwater contamination using concentration-time series recorded during an integral pumping test: Bias introduced by strong concentration gradients within the plume” by Allelign Zeru and Gerhard Schäfer

We consider the results of a recent paper in this journal [Zeru, A. and Schäfer, G., 2005. Analysis of groundwater contamination using concentration–time series recorded during an integral pumping test: Bias introduced by strong concentration gradients within the plume. Journal of Contaminant Hydrology 81 (2005) 106–124], which addresses the field-scale characterisation of contaminant plumes in groundwater. There, it is concluded that contaminant concentration gradients can bias Integral Pumping Test (IPT) interpretations considerably, in particular if IPTs are conducted in advective fronts of contaminant plumes. We discuss implications of this setting and also argue that the longitudinal and transverse dispersivities used in the examples of Zeru and Schäfer (2005) of up to 30 m and 3 m, respectively, are generally very high for the here relevant capture zone scale (< 20 m). However, regardless of both longitudinal and transverse concentration gradients, we further show through a counter-example that IPT results are unbiased as long as the concentration attenuation along the flow direction is linear over the capture zone extent.