Predicting DNAPL mass discharge from pool-dominated source zones

Characterization of DNAPL source zones in clay-sand media via joint inversion of DC resistivity, induced polarization and borehole data

Toxic organic contaminants in groundwater are pervasive at many industrial sites worldwide. These contaminants, such as chlorinated solvents, often appear as dense non-aqueous phase liquids (DNAPLs). To design efficient remediation strategies, detailed characterization of DNAPL Source Zone Architecture (SZA) is required. Since invasive borehole-based investigations suffer from limited spatial coverage, a non-intrusive geophysical method, direct current (DC) resistivity, has been applied to image the DNAPL distribution; however, in clay-sand environments, the ability of DC resistivity for DNAPLs imaging is limited since it cannot separate between DNAPLs and surrounding clay-sand soils. Moreover, the simplified parameterization of conventional inversion approaches cannot preserve physically realistic patterns of SZAs, and tends to smooth out any sharp spatial variations. In this paper, the induced polarization (IP) technique is combined with DC resistivity (DCIP) to provide plausible DNAPL characterization in clay-sand environments. Using petrophysical models, the DCIP data is utilized to provide tomograms of the DNAPL saturation (SN) and hydraulic conductivity (K). The DCIP-estimated K/SN tomograms are then integrated with borehole measurements in a deep learning-based joint inversion framework to accurately parameterize the highly irregular SZA and provide a refined DNAPL image. To evaluate the performance of the proposed approach, we conducted numerical experiments in a heterogeneous clay-sand aquifer with a complex SZA. Results demonstrate the standalone DC resistivity method fails to infer the DNAPL in complex clay-sand environments. In contrast, the combined DCIP technique provides the necessary information to reconstruct the large-scale features of K/SN fields, while integrating DCIP data with sparse but accurate borehole data results in a high resolution characterization of the SZA.

Mobilization pilot test of PCE sources in the transition zone to aquitards by combining mZVI and biostimulation with lactic acid

The potential toxic and carcinogenic effects of chlorinated solvents in groundwater on human health and aquatic ecosystems require very effective remediation strategies of contaminated groundwater to achieve the low legal cleanup targets required. The transition zones between aquifers and bottom aquitards occur mainly in prograding alluvial fan geological contexts. Hence, they are very frequent from a hydrogeological point of view. The transition zone consists of numerous thin layers of fine to coarse-grained clastic fragments (e.g., medium sands and gravels), which alternate with fine-grained materials (clays and silts). When the transition zones are affected by DNAPL spills, free-phase pools accumulate on the less conductive layers. Owing to the low overall conductivity of this zone, the pools are very recalcitrant. Little field research has been done on transition zone remediation techniques. Injection of iron microparticles has the disadvantage of the limited accessibility of this reagent to reach the entire source of contamination. Biostimulation of indigenous microorganisms in the medium has the disadvantage that few of the microorganisms are capable of complete biodegradation to total mineralization of the parent contaminant and metabolites. A field pilot test was conducted at a site where a transition zone existed in which DNAPL pools of PCE had accumulated. In particular, the interface with the bottom aquitard was where PCE concentrations were the highest. In this pilot test, a combined strategy using ZVI in microparticles and biostimulation with lactate in the form of lactic acid was conducted. Throughout the test it was found that the interdependence of the coupled biotic and abiotic processes generated synergies between these processes. This resulted in a greater degradation of the PCE and its transformation products. With the combination of the two techniques, the mobilization of the contaminant source of PCE was extremely effective.

Chlorinated solvents source identification by nonlinear optimization method

In this work, chloride ions were used as conservative tracers and supplemented with conservative amounts of chloroethenes (PCE, TCE, Cis-DCE, 1,1-DCE), chloroethanes (1,1,1-TCA, 1,1-DCA), and the carbon isotope ratios of certain compounds, the most representative on the sites studied, which is a novelty compared to the optimization methods developed in the scientific literature so far. A location of the potential missing sources is then proposed in view of the balances of the calculated mixing fractions. A test of the influence of measurement errors on the results shows that the uncertainties in the calculation of the mixture fractions are less than 11%, indicating that the source identification method developed is a robust tool for identifying sources of chlorinated solvents in groundwater.

An approach for quantification of the heterogeneity of DNAPL source zone geometries

Many studies have investigated the migration and entrapment processes of source zones from dense non-aqueous phase liquid (DNAPL) contamination under different conditions. However, the characterization of occupying area by source zone (or source shape) in water-saturated aquifers is still rudimentarily considered. In this study, we demonstrated this issue (1) by providing a brief review of existing approaches for source shape consideration, (2) by proposing an approach with simple shape parameters based on the non-uniformity of source widths, and (3) by providing exemplary applications of our proposed approach on shapes already published in previous research works. Our literature review suggested that the source zone in mathematical approaches is generally characterized as simple geometrical shapes (arbitrary lines or rectangles) or system-defined parameters that contrast to complex and discontinuous shapes observed in the real world. But the characterization of such complex shapes is still not possible with acceptable efforts. Therefore, we proposed an approach to parameterize the source shape by considering the variation of width and midpoints over the depth of the entire source zone and formulate four parameters based on population statistics (mean, standard deviation). To illustrate the suitability of our approach, we applied it to the results of lab experiments, and by analyzing these complex shapes, we highlighted the potential for improving the characterization techniques of non-uniformity of the source zones.

Upscaled modeling of complex DNAPL dissolution

A straightforward, upscaled DNAPL mass dissolution model is developed using relatively simple input consisting of characteristic dimensions and saturations of a DNAPL accumulation. Multiple accumulations are aggregated into a single source zone volume. Physically, the dissolution process is a combination of flow through the mass (advective component) and flow around the mass (dispersive component). The contribution of each component is based on initial characteristic length scales and the average initial saturation. Changes over time with the depletion of mass are captured with a changing relative permeability and a power law relationship for the fraction of initial mass remaining. The utility of the upscaled process model is demonstrated with data from three studies: numerical simulation of multiple pools, two-dimensional test cell experiments with mixed architecture and with heterogeneous soil, and a controlled field study of multicomponent DNAPL release and depletion. Use of the model successfully reproduced the observed multistage mass discharge in each study and illuminated the governing processes. The power law exponent was relatively constant for the various conditions and relative permeability changes were integral to the success. The numerical and experimental studies were run to complete mass depletion which the upscaled model matched. The input parameters are minimal and are found in typical DNAPL source zone characterization data.

Integrating hydraulic tomography, electrical resistivity tomography, and partitioning interwell tracer test datasets to improve identification of pool-dominated DNAPL source zone architecture

High-resolution characterization of complex dense non-aqueous phase liquid (DNAPL) contaminated sites is crucial for developing effective remediation strategies. The DNAPL source zone is usually characterized by hydraulic/partitioning tracer tomography (HPTT). However, the HPTT method may fail to capture the highly saturated pool-dominated DNAPL source zone architecture (SZA), because partitioning tracers tend to bypass the low-permeability zones where the pool DNAPL accumulates, resulting in a low-resolution DNAPL estimation. With a limited number of measurements, the estimation errors may be significant. To overcome these difficulties, time-lapse electrical resistivity tomography (ERT) was integrated with the partitioning interwell tracer test (PITT) and hydraulic tomography (HT) to characterize the pool-dominated DNAPL SZA. Herein, we proposed an iterative joint inversion framework coupling the multiphase flow model with the ERT forward model to estimate the heterogeneous permeability distribution and DNAPL SZA. Under this framework, permeability was estimated using the hydraulic head data from HT in stage 1, and the DNAPL SZA was subsequently estimated by assimilating both the PITT and ERT observations in stage 2. The permeability estimated from stage 1 was used as prior information for stage 2, and the DNAPL saturation estimated from stage 2 was served as prior information for stage 1 in the next loop to form an iterative loop to improve the estimation of both permeability and DNAPL SZA. The iterative joint inversion framework was evaluated in two numerical experiments with different heterogeneous structures by assimilating multi-source datasets, including hydraulic head, partitioning interwell tracer concentration, and electrical resistivity. Results show that with limited measurements of HPTT method, one can roughly capture the DNAPL distribution, missing the fine structure of the DNAPL SZA. In contrast, by incorporating multi-source datasets, the heterogeneous permeability and DNAPL SZA can be reconstructed with a higher resolution. Furthermore, the inversion accuracy of the DNAPL SZA improves progressively as the iteration proceeds, which demonstrates the advantage of utilizing complementary information from permeability and DNAPL distribution through the iteration framework. Comparing with the results without loop iteration, the estimation error is reduced by 17.3% for permeability and 8.6% for DNAPL saturation in Experiment 1; by 14.7% for permeability and 11.2% for DNAPL saturation in Experiment 2 through the iterative framework. To further evaluate our framework, we preformed the prediction of the depletion process of the DNAPL source zone and plume based on the estimated DNAPL SZA. Results show that using the iterative framework, the prediction of the SZA depletion is greatly improved, i.e., the estimation error of the dissolved downstream plume from the DNAPL source zone after 3 years is reduced by 20.9% in Experiment 1, and by 43.2% in Experiment 2, respectively, through the iterative framework. This significant improvement is because the iterative method can better capture the spread of DNAPL pool.

Towards predicting DNAPL source zone formation to improve plume assessment: Using robust laboratory and numerical experiments to evaluate the relevance of retention curve characteristics

We conducted multiple laboratory trials in a robust and repeatable experimental layout to study dense non-aqueous phase liquid (DNAPL) source zone formation. We extended an image processing and analysis framework to derive DNAPL saturation distributions from reflective optical imaging data, with volume balance deviations < 5.07%. We used a multiphase flow model to simulate source zone formation in a Monte Carlo approach, where the parameter space was defined by the variation of retention curve parameters. Integral and geometric measures were used to characterize the source zones and implemented into a multi-criteria objective function. The latter showed good agreement between observation data and simulation results for effective DNAPL saturation values > 0.04, especially for early stages of DNAPL migration. The common hypothesis that parameters defining the DNAPL-water retention curves are constant over time was not confirmed. Once DNAPL pooling started, the optimal fit in the parameter space was significantly different compared to the earlier DNAPL migration stages. We suspect more complex processes (e.g., capillary hysteresis, adsorption) to become relevant during pool formation. Our results reveal deficits in the grayscale-DNAPL saturation relationship definition and laboratory estimation of DNAPL-water retention curve parameters to overcome current limitations to describe DNAPL source zone formation.

Mathematical modeling of organic liquid dissolution in heterogeneous source zones

A simple one-dimensional heterogeneous-source model was used to simulate dissolution of organic liquid that was non-uniformly distributed in physically heterogeneous porous media. The permeability field was depicted as a pseudo-homogeneous medium. The source zone was discretized into multiple domains representing different organic-liquid configurations and hydraulic accessibilities, each with a different representative upscaled mass transfer rate coefficient that is temporally variable. This simplified approach represents a system where minimal information is available regarding system heterogeneities. All factors that influence dissolution were incorporated into the calibrated mass transfer terms. The mass transfer terms were calibrated for each zone separately. The one-dimensional, heterogeneous-source model adequately simulated the multi-stage dissolution behavior observed for column-scale systems that were packed with different natural soils, as well as for flow-cell systems wherein the source zone consisted of both a residual zone and pool. The results indicate that the model adequately simulated the presence of multiple organic-liquid zones in porous media with different configurations and hydraulic accessibilities, which accounts for the non-ideal dissolution behavior observed. The calibrated mass transfer terms for each source type were consistent with those obtained for systems that contained only one of either source type.

Integrated hydrogeophysical modelling and data assimilation for geoelectrical leak detection

Time-lapse electrical resistivity tomography (ERT) measurements provide indirectobservations of hydrological processes in the Earth's shallow subsurface at high spatial and temporal resolution. ERT has been used in the past decades to detect leaks and monitor the evolution of associated contaminant plumes. Specifically, inverted resistivity images allow visualization of the dynamic changes in the structure of the plume. However, existing methods do not allow the direct estimation of leak parameters (e.g. leak rate, location, etc.) and their uncertainties. We propose an ensemble-based data assimilation framework that evaluates proposed hydrological models against observed time-lapse ERT measurements without directly inverting for the resistivities. Each proposed hydrological model is run through the parallel coupled hydro-geophysical simulation code PFLOTRAN-E4D to obtain simulated ERT measurements. The ensemble of model proposals is then updated using an iterative ensemble smoother. We demonstrate the proposed framework on synthetic and field ERT data from controlled tracer injection experiments. Our results show that the approach allows joint identification of contaminant source location, initial release time, and solute loading from the cross-borehole time-lapse ERT data, alongside with an assessment of uncertainties in these estimates. We demonstrate a reduction in site-wide uncertainty by comparing the prior and posterior plume mass discharges at a selected image plane. This framework is particularly attractive to sites that have previously undergone extensive geological investigation (e.g., nuclear sites). It is well suited to complement ERT imaging and we discuss practical issues in its application to field problems.

Long-term mass flux assessment of a DNAPL source area treated using bioremediation

This study assessed the long-term effectiveness of bioremediation as a remedial strategy for a chlorinated, ethene dense, non-aqueous phase liquid (DNAPL) source area, consisting of a higher- and a lower-permeability zone at Alameda Point, California. The evaluation was performed over 3.7 years after cessation of active source area bioremediation using passive flux meters (PFMs), push-pull tracer tests, and soil cores. PFMs showed that total chlorinated ethene molar discharge emanating from the source area remained relatively unchanged pre-and post-bioremediation, but molar discharge compositions shifted from trichloroethene (TCE) and cis-1,2-dichloroethene (cis-DCE) to vinyl chloride (VC) and ethene dominated during post-remedial monitoring. First-order rate constants, derived from PFM data at the edge of the source area and describing the complete dechlorination of TCE at 3.7 years following active bioremediation, were approximately 1.05 yr^-1, which was over three times lower than the rate 3.6 yr^-1 determined using compound stable isotope analysis (CSIA). Soil cores and push-pull tracer test data showed that DNAPL volume estimates were relatively unchanged pre- and post-bioremediation due to the remaining presence of DNAPL in the lower-permeability zone. These data suggest biotransformation processes are continuing in the higher-permeability zone, whereas DNAPL in the lower-permeability zone continues to serve as a significant source of groundwater contamination. The results suggest that it will take many years under current conditions to attain the United States Environmental Protection Agency (EPA) Maximum Contaminant Levels (MCLs) cleanup objectives.

The fate of DNAPL contaminants in non-consolidated subsurface systems - Discussion on the relevance of effective source zone geometries for plume propagation

Dense non-aqueous phase liquids, i.e., DNAPLs and the evolving contaminant plumes in aquifers provide significant potential to pose hazards affecting both environment and human health. Therefore, a proper assessment of contaminant spreading within the subsurface is critical. This includes a sufficient characterization of governing parameters describing both the subsurface and the contaminant itself. Thereby, knowledge on the contaminant source zone and especially the source zone geometry, i.e., SZG is critically required, yet very uncertain. This study identifies current limitations and open research questions in the formation and shape determination of source zone geometry, as well as its relevance for contaminant plumes. Our literature review reveals that existing characterization methods are subject to data interpretation uncertainties, while the application of these methods on field scale is limited by technical demands and accompanied efforts. In a next step, methods to implement increased source zone information into calculation methods are discussed. By means of an exemplary application of selected assessment tools, i.e., plume response models, results clearly proof the relevance of SZGs for site assessment. However, existing plume response models consider over-simplified geometries that may compromise their suitability. Our findings identify the demand for improved characterization of complex SZGs and the need to better evaluate the dependency of DNAPL migration on system properties and external influences. With emphasized knowledge on the most relevant SZG features, the delineation of "effective" SZGs allowing for straightforward implementation into plume response models and an adaption of the latter to incorporate more information on SZGs should be possible.

Hydrogeochemical Model Supporting the Remediation Strategy of a Highly Contaminated Industrial Site

Delineation and understanding the geology and the hydrogeology of a contaminated site, considering its chemical and its biological aspects, are fundamental requirements for successful environmental remediation. The aim of this research is to provide some evidence about the effectiveness of a hydrogeochemical geodatabase to facilitate the integrated management, representation and analysis of heterogeneous data, enabling the appropriate selection, design and optimization of an effective remediation strategy. This study investigates a new technology for the remediation of a dense non-aqueous phase liquid aged source zone, with the aim of enhancing in situ bioremediation by coupling groundwater circulation wells with a continuous production system of electron donors. The technology was verified through a pilot test carried out at an industrial site highly contaminated by chlorinated aliphatic hydrocarbons. The multidisciplinary conceptual model confirmed a complex hydrogeological situation, with the occurrence of active residual sources in low permeability layers. The pilot test results clearly demonstrate a significant mobilization of contaminants from the low permeability zone, and the possibility of favoring the in situ natural attenuation mechanisms based upon biological reductive dechlorination. Different information related to the hydrogeochemical sphere must be integrated and taken into consideration when developing a reliable remediation strategy for contaminated sites.

An empirical model for the evaluation of the dissolution rate from a DNAPL-contaminated area

This paper investigates dynamic variation in the morphologic distribution of dense non-aqueous phase liquids (DNAPLs), which take into account the coupled mass transfer. Experiments were carried out in a 2D tank representing a reconstructed aquifer model. DNAPL dissolution rates were investigated over a wide range of DNAPL saturations, several source configurations, and different hydraulic conditions. Morphometric indexes are presented that take into consideration further factors affecting the dissolution process. Local information regarding transport parameters related to the characteristics of the medium was obtained through a neural network and an optimization algorithm applied to experimental tracer tests. The history of DNAPL source architecture, in terms of saturation, indentation grade, and orientation, was determined by image analysis. Dissolved concentrations were registered and mass transfer rate coefficients were obtained for a wide range of source-zone configurations. A statistical analysis was performed to develop a constitutive equation that is descriptive of the mass transfer rate as a function of source-zone metric characteristics. A new empirical dissolution model using the proposed morphometric parameters is presented and compared with other models. The mass transfer correlation reported incorporates morphometric parameters and considers the complex and variable architecture of non-miscible contaminants. The proposed correlation can be used for an initial assessment of non-aqueous phase liquid (NAPL) dissolution rates over a wide range of saturation (residual and non-residual) conditions and different aqueous phase velocities within the NAPL source zone.

In-situ determination of field-scale NAPL mass transfer coefficients: Performance, simulation and analysis

Better estimates of non-aqueous phase liquid (NAPL) mass, its persistence into the future, and the potential impact of source reduction are critical needs for determining the optimal path to clean up sites impacted by NAPLs. One impediment to constraining time estimates of source depletion is the uncertainty in the rate of mass transfer between NAPLs and groundwater. In this study, an innovative field test is demonstrated for the purpose of quantifying field-scale NAPL mass transfer coefficients (kl(N)) within a source zone of a fuel-contaminated site. Initial evaluation of the test concept using a numerical model revealed that the aqueous phase concentration response to the injection of clean groundwater within a source zone was a function of NAPL mass transfer. Under rate limited conditions, NAPL dissolution together with the injection flow rate and the radial distance to monitoring points directly controlled time of travel. Concentration responses observed in the field test were consistent with the hypothetical model results allowing field-scale NAPL mass transfer coefficients to be quantified. Site models for groundwater flow and solute transport were systematically calibrated and utilized for data analysis. Results show kl(N) for benzene varied from 0.022 to 0.60d(-1). Variability in results was attributed to a highly heterogeneous horizon consisting of layered media of varying physical properties.

Spatial and temporal dynamics of organohalide-respiring bacteria in a heterogeneous PCE-DNAPL source zone

Effective treatment of sites contaminated with dense non-aqueous phase liquids (DNAPLs) requires detailed understanding of the microbial community responses to changes in source zone strength and architecture. Changes in the spatial and temporal distributions of the organohalide-respiring Dehalococcoides mccartyi (Dhc) strains and Geobacter lovleyi strain SZ (GeoSZ) were examined in a heterogeneous tetrachloroethene- (PCE-) DNAPL source zone within a two-dimensional laboratory-scale aquifer flow cell. As part of a combined remedy approach, flushing with 2.3 pore volumes (PVs) of 4% (w/w) solution of the nonionic, biodegradable surfactant Tween® 80 removed 55% of the initial contaminant mass, and resulted in a PCE-DNAPL distribution that contained 51% discrete ganglia and 49% pools (ganglia-to-pool ratio of 1.06). Subsequent bioaugmentation with the PCE-to-ethene-dechlorinating consortium BDI-SZ resulted in cis-1,2-dichloroethene (cis-DCE) formation after 1 PV (ca. 7 days), while vinyl chloride (VC) and ethene were detected 10 PVs after bioaugmentation. Maximum ethene yields (ca. 90 μM) within DNAPL pool and ganglia regions coincided with the detection of the vcrA reductive dehalogenase (RDase) gene that exceeded the Dhc 16S rRNA genes by 2.0±1.3 and 4.0±1.7 fold in the pool and ganglia regions, respectively. Dhc and GeoSZ cell abundance increased by up to 4 orders-of-magnitude after 28 PVs of steady-state operation, with 1 to 2 orders-of-magnitude increases observed in close proximity to residual PCE-DNAPL. These observations suggest the involvement of these dechlorinators the in observed PCE dissolution enhancements of up to 2.3 and 6.0-fold within pool and ganglia regions, respectively. Analysis of the solid and aqueous samples at the conclusion of the experiment revealed that the highest VC (≥155 μM) and ethene (≥65 μM) concentrations were measured in zones where Dhc and GeoSZ were predominately attached to the solids. These findings demonstrate dynamic responses of organohalide-respiring bacteria in a heterogeneous DNAPL source zone, and emphasize the influence of source zone architecture on bioremediation performance.

Architecture, persistence and dissolution of a 20 to 45 year old trichloroethene DNAPL source zone

A detailed field-scale investigation of processes controlling the architecture, persistence and dissolution of a 20 to 45year old trichloroethene (TCE) dense non-aqueous phase liquid (DNAPL) source zone located within a heterogeneous sand/gravel aquifer at a UK industrial site is presented. The source zone was partially enclosed by a 3-sided cell that allowed detailed longitudinal/fence transect monitoring along/across a controlled streamtube of flow induced by an extraction well positioned at the cell closed end. Integrated analysis of high-resolution DNAPL saturation (Sn) (from cores), dissolved-phase plume concentration (from multilevel samplers), tracer test and permeability datasets was undertaken. DNAPL architecture was determined from soil concentration data using partitioning calculations. DNAPL threshold soil concentrations and low Sn values calculated were sensitive to sorption assumptions. An outcome of this was the uncertainty in demarcation of secondary source zone diffused and sorbed mass that is distinct from trace amounts of low Sn DNAPL mass. The majority of source mass occurred within discrete lenses or pools of DNAPL associated with low permeability geological units. High residual saturation (Sn>10-20%) and pools (Sn>20%) together accounted for almost 40% of the DNAPL mass, but only 3% of the sampled source volume. High-saturation DNAPL lenses/pools were supported by lower permeability layers, but with DNAPL still primarily present within slightly more permeable overlying units. These lenses/pools exhibited approximately linearly declining Sn profiles with increasing elevation ascribed to preferential dissolution of the uppermost DNAPL. Bi-component partitioning calculations on soil samples confirmed that the dechlorination product cDCE (cis-dichloroethene) was accumulating in the TCE DNAPL. Estimated cDCE mole fractions in the DNAPL increased towards the DNAPL interface with the uppermost mole fraction of 0.04 comparable to literature laboratory data. DNAPL dissolution yielded heterogeneous dissolved-phase plumes of TCE and its dechlorination products that exhibited orders of magnitude local concentration variation. TCE solubility concentrations were relatively localised, but coincident with high saturation DNAPL lens source areas. Biotic dechlorination in the source zone area, however, caused cDCE to be the dominant dissolved-phase plume. The conservative tracer test usefully confirmed the continuity of a permeable gravel unit at depth through the source zone. Although this unit offered significant opportunity for DNAPL bypassing and decreased timeframes for dechlorination, it still transmitted a significant proportion of the contaminant flux. This was attributed to dissolution of DNAPL-mudstone aquitard associated sources at the base of the continuous gravel as well as contaminated groundwater from surrounding less permeable sand and gravel horizons draining into this permeable conduit. The cell extraction well provided an integrated metric of source zone dissolution yielding a mean concentration of around 45% TCE solubility (taking into account dechlorination) that was equivalent to a DNAPL mass removal rate of 0.4tonnes per annum over a 16m(2) cell cross sectional area of flow. This is a significant flux considering the source age and observed occurrence of much of the source mass within discrete lenses/pools. We advocate the need for further detailed field-scale studies on old DNAPL source zones that better resolve persistent pool/lens features and are of prolonged duration to assess the ageing of source zones. Such studies would further underpin the application of more surgical remediation technologies.

Enhanced aqueous dissolution of a DNAPL source to characterize the source strength function

Simplified analytical solutions, developed as source strength functions (SSFs), are capable of describing the temporal dissolution of nonaqueous phase liquids in groundwater, which is useful for predicting source longevity and can serve as a guide for remedial activities. Here, SSF parameters were estimated by fitting enhanced aqueous dissolution data from a flow cell consisting of three injection and four extraction wells to analytical dissolution models (power law model (PLM) and equilibrium streamtube model (EST)) at a trichloroethene (TCE) contaminated site, Alameda Point, California. Both the PLM and the EST model were able to characterize the observed aqueous TCE dissolution during enhanced water flooding. Additional field activities conducted at the site included soil core collection, a recirculated partitioning tracer test, passive flux meter transects, and push-pull tracer tests. The additional site characterization data were used to independently estimate the observed SSF parameters using information such as the TCE mass, distribution and porous media heterogeneity. The exponential decay model (a subset of the PLM) accurately predicted the enhanced dissolution, likely because the site was significantly aged (most of the mass in the plume rather than in the source zone) or middle stage, and the mass in the source zone could be approximately estimated. The EST tracer-based model, when combined with data from the recirculated partitioning tracer test, soil cores, and the push-pull tracer test, was capable of accurately predicting the observed aqueous dissolution. The mass in the source zone and the fraction of contaminated flowpaths were the most important site characteristics, requiring the greatest accuracy to predict aqueous dissolution. Establishing steady state dissolution was essential to provide a more accurate estimate of the fraction contaminated and high resolution data from soil cores in the source zone were needed to estimate the mass present.

Assessing contaminant-removal conditions and plume persistence through analysis of data from long-term pump-and-treat operations

Historical groundwater-withdrawal and contaminant-concentration data collected from long-term pump-and-treat operations were analyzed and used to examine contaminant mass discharge (CMD) and mass-removal behavior for multiple sites. Differences in behavior were observed, and these differences were consistent with the nature of contaminant distributions and subsurface properties of the sites. For example, while CMD exhibited a relatively rapid decline during the initial stage of operation for all three sites, the rate of decline varied. The greatest rate was observed for the PGN site, whereas the lowest rate was observed for the MOT site. In addition, the MOT site exhibited the lowest relative reduction in CMD. These results are consistent with the actuality that the MOT site likely contains the greatest proportion of poorly accessible contaminant mass, given that it comprises a combined alluvium and fractured-bedrock system in which solvent and dissolved mass are present directly in the bedrock. The relative contributions of the source zones versus the plumes to total CMD were determined. Constrained contaminant mass removal was observed to influence the plumes for all three sites, and was attributed to a combination of uncontrolled (or imperfectly controlled) sources, back diffusion, and well-field hydraulics. The results presented herein illustrate that detailed analysis of operational pump-and-treat data can be a cost-effective method for providing value-added characterization of contaminated sites.

Long-term mass transfer and mixing-controlled reactions of a DNAPL plume from persistent residuals

Understanding and being able to predict the long-term behavior of DNAPL (i.e., PCE and TCE) residuals after active remediation has ceased have become increasingly important as attention at many sites turns from aggressive remediation to monitored natural attenuation and long-term stewardship. However, plume behavior due to mass loading and reactions during these later phases is less studied as they involve large spatial and temporal scales. We apply both theoretical analysis and pore-scale simulations to investigate mass transfer from DNAPL residuals and subsequent reactions within the generated plume, and, in particular, to show the differences between early- and late-time behaviors of the plume. In the zone of entry of the DNAPL entrapment zone where the concentration boundary layer in the flowing groundwater has not fully developed, the pore-scale simulations confirm the past findings based on laboratory studies that the mass transfer increases as a power-law function of the Peclét number, and is enhanced due to reactions in the plume. Away from the entry zone and further down gradient, the long-term reactions are limited by the available additive and mixing in the porous medium, thereby behave considerably differently from the entry zone. For the reaction between the contaminant and an additive with intrinsic second-order bimolecular kinetics, the late-time reaction demonstrates a first-order decay macroscopically with respect to the mass of the limiting additive, not with respect to that of the contaminant. The late-time decay rate only depends on the intrinsic reaction rate and the solubility of the entrapped DNAPL. At the intermediate time, the additive decays exponentially with the square of time (t(2)), instead of time (t). Moreover, the intermediate decay rate also depends on the initial conditions, the spatial distribution of DNAPL residuals, and the effective dispersion coefficient.

Persistence of a Groundwater Contaminant Plume after Hydraulic Source Containment at a Chlorinated-Solvent Contaminated Site

The objective of this study was to characterize the behavior of a groundwater contaminant (trichloroethene) plume after implementation of a source-containment operation at a site in Arizona. The plume resides in a quasi three-layer system comprising a sand/gravel unit bounded on the top and bottom by relatively thick silty clayey layers. The system was monitored for 60 months beginning at start-up in 2007 to measure the change in contaminant concentrations within the plume, the change in plume area, the mass of contaminant removed, and the integrated contaminant mass discharge. Concentrations of trichloroethene in groundwater pumped from the plume extraction wells have declined significantly over the course of operation, as have concentrations for groundwater sampled from 40 monitoring wells located within the plume. The total contaminant mass discharge associated with operation of the plume extraction wells peaked at 0.23 kg/d, decreased significantly within one year, and thereafter began an asymptotic decline to a current value of approximately 0.03 kg/d. Despite an 87% reduction in contaminant mass and a comparable 87% reduction in contaminant mass discharge for the plume, the spatial area encompassed by the plume has decreased by only approximately 50%. This is much less than would be anticipated based on ideal flushing and mass-removal behavior. Simulations produced with a simplified 3-D numerical model matched reasonably well to the measured data. The results of the study suggest that permeability heterogeneity, back diffusion, hydraulic factors associated with the specific well field system, and residual discharge from the source zone are all contributing to the observed persistence of the plume, as well as the asymptotic behavior currently observed for mass removal and for the reduction in contaminant mass discharge.

Use of Historical Pump-and-Treat Data to Enhance Site Characterization and Remediation Performance Assessment

Groundwater withdrawal and contaminant concentration data are routinely collected for pump-and-treat operations conducted at hazardous waste sites. These data sets can be mined to produce a wealth of information to support enhanced site characterization, optimization of remedial system operations, and improved decision making regarding long-term site management and closure. Methods that may be used to analyze and interpret pump-and-treat data to produce such assessments are presented, along with a brief illustration of their application to a site. The results presented herein illustrate that comprehensive analysis of pump-and-treat data is a powerful, cost-effective method for providing higher-resolution, value-added characterization of contaminated sites.

Field-scale prediction of enhanced DNAPL dissolution based on partitioning tracers

The equilibrium streamtube model (EST) has demonstrated the ability to accurately predict dense nonaqueous phase liquid (DNAPL) dissolution in laboratory experiments and numerical simulations. Here the model is applied to predict DNAPL dissolution at a tetrachloroethylene (PCE)-contaminated dry cleaner site, located in Jacksonville, Florida. The EST model is an analytical solution with field-measurable input parameters. Measured data from a field-scale partitioning tracer test were used to parameterize the EST model and the predicted PCE dissolution was compared to measured data from an in-situ ethanol flood. In addition, a simulated partitioning tracer test from a calibrated, three-dimensional, spatially explicit multiphase flow model (UTCHEM) was also used to parameterize the EST analytical solution. The EST ethanol prediction based on both the field partitioning tracer test and the simulation closely matched the total recovery well field ethanol data with Nash-Sutcliffe efficiency E=0.96 and 0.90, respectively. The EST PCE predictions showed a peak shift to earlier arrival times for models based on either field-measured or simulated partitioning tracer tests, resulting in poorer matches to the field PCE data in both cases. The peak shifts were concluded to be caused by well screen interval differences between the field tracer test and ethanol flood. Both the EST model and UTCHEM were also used to predict PCE aqueous dissolution under natural gradient conditions, which has a much less complex flow pattern than the forced-gradient double five spot used for the ethanol flood. The natural gradient EST predictions based on parameters determined from tracer tests conducted with a complex flow pattern underestimated the UTCHEM-simulated natural gradient total mass removal by 12% after 170 pore volumes of water flushing indicating that some mass was not detected by the tracers likely due to stagnation zones in the flow field. These findings highlight the important influence of well configuration and the associated flow patterns on dissolution.

Characterizing long-term contaminant mass discharge and the relationship between reductions in discharge and reductions in mass for DNAPL source areas

The objective of this study was to characterize the temporal behavior of contaminant mass discharge, and the relationship between reductions in contaminant mass discharge and reductions in contaminant mass, for a very heterogeneous, highly contaminated source-zone field site. Trichloroethene is the primary contaminant of concern, and several lines of evidence indicate the presence of organic liquid in the subsurface. The site is undergoing groundwater extraction for source control, and contaminant mass discharge has been monitored since system startup. The results show a significant reduction in contaminant mass discharge with time, decreasing from approximately 1 to 0.15 kg/d over five years. Two methods were used to estimate the mass of contaminant present in the source area at the initiation of the remediation project. One was based on a comparison of two sets of core data, collected 3.5 years apart, which suggests that a significant (~80%) reduction in aggregate sediment-phase TCE concentrations occurred between sampling events. The second method was based on fitting the temporal contaminant mass discharge data with a simple exponential source-depletion function. Relatively similar estimates, 784 and 993 kg, respectively, were obtained with the two methods. These data were used to characterize the relationship between reductions in contaminant mass discharge (CMDR) and reductions in contaminant mass (MR). The observed curvilinear relationship exhibits a reduction in contaminant mass discharge essentially immediately upon the initiation of mass reduction. This behavior is consistent with a system wherein significant quantities of mass are present in hydraulically poorly accessible domains for which mass removal is influenced by rate-limited mass transfer. The results obtained from the present study are compared to those obtained from other field studies to evaluate the impact of system properties and conditions on mass-discharge and mass-removal behavior. The results indicate that factors such as domain scale, hydraulic-gradient status (induced or natural), and flushing-solution composition had insignificant impact on the CMDR-MR profiles and thus on underlying mass-removal behavior. Conversely, source-zone age, through its impact on contaminant distribution and accessibility, was implicated as a critical factor influencing the nature of the CMDR-MR relationship.

The use of mass depletion-mass flux reduction relationships during pumping to determine source zone mass of a reactive brominated-solvent DNAPL

Mass depletion-mass flux relationships usually applied to a groundwater plume were established at field scale for groundwater pumped from within the source zone of a dense non-aqueous phase liquid (DNAPL). These were used as part of multiple lines of evidence in establishing the DNAPL source mass and architecture. Simplified source mass-dissolved concentration models including those described by exponential, power, and error functions as well as a rational mass equation based on the equilibrium stream tube approach were fitted to data from 285 days of source zone pumping (SZP) from a single well which removed 152 kg of dissolved organics from a multi-component, reactive brominated solvent DNAPL. The total molar concentration of the source compound, tetrabromoethane and its daughter products was used as a single measure of contaminant concentration to relate to source mass. A partitioning inter-well tracer test (PITT) conducted prior to the SZP provided estimates of groundwater travel times, enabling parameterisation of the models. After accounting for capture of the down-gradient dissolved plume, all models provided a good fit to the observed data. It was shown that differentiation between models would only emerge after appreciably more pumping from the source zone. The model fits were not particularly sensitive to the exponent parameters and variance of groundwater travel time. In addition, the multi-component nature of the DNAPL did not seem to affect the utility of the models for the period examined. Estimates of the DNAPL mass prior to the start of SZP from the models were greatest where the log of the variance of travel time was used explicitly in the source depletion models (mean 295kg) compared to where the associated power exponent and variance was fitted freely (mean 258 kg). The estimates of source mass were close to that of 220kg determined from the PITT. In addition to the PITT, multi-level groundwater sampling from within the source zone provided important supporting information for developing the conceptual model of the source zone. It is concluded that SZP may be an effective and relatively simple means for characterising DNAPL source zones.

ZVI-Clay remediation of a chlorinated solvent source zone, Skuldelev, Denmark: 2. Groundwater contaminant mass discharge reduction

The impact of source mass depletion on the down-gradient contaminant mass discharge was monitored for a 19-month period as a part of a field demonstration of the ZVI-Clay soil mixing remediation technology. Groundwater samples were collected from conventional monitoring wells (120 samples) and a dense network of multilevel samplers (640 samples). The hydraulic gradient and conductivity were determined. Depletion of the contaminant source is described in the companion paper (Fjordbøge et al., 2012). Field data showed four distinct phases for PCE mass discharge: (1) baseline conditions, (2) initial rapid reduction, (3) temporary increase, and (4) slow long-term reduction. Numerical modeling was utilized to develop a conceptual understanding of the four phases and to identify the governing processes. The initial rapid reduction of mass discharge was a result of the changed hydraulic properties in the source zone after soil mixing. The subsequent phases depended on the changed accessibility of the contaminant mass after mixing, the rate of source depletion, and the concentration gradient at the boundaries of the mixed source zone. Overall, ZVI-Clay soil mixing resulted in a significant down-gradient contaminant mass discharge reduction (76%) for the parent compound (PCE), while the overall reduction of chlorinated ethenes was smaller (21%).

The legacy of chlorinated solvents in the Birmingham aquifer, UK: observations spanning three decades and the challenge of future urban groundwater development

Licensed abstraction well data collected during 1986-2008 from a total of 77 wells mainly located at industrial sites combined with historic land use data from 1975 has allowed insight into the legacy of chlorinated solvent contamination in the Birmingham aquifer that underlies the UK's second largest city. This legacy, expected to be reasonably symptomatic of those occurring in other urban aquifers, was characterised by: dominance of parent solvents, particularly TCE (trichloroethene) that widely exceeded drinking-water quality criteria; greater TCE occurrence in wells in proximity to increased historic land use by the metal/engineering solvent-user industry (the relationship providing a first-pass indicator of future resource development potential); regional groundwater vulnerability controls; well abstraction changes (over months to decades) influential of observed concentration transients and anticipated plume capture or release; persistence of contamination over decades (with less soluble PCE (perchloroethene) showing increased persistence relative to TCE) that was reasonably ascribed to slow contaminant release from DNAPL (dense non-aqueous phase liquid) sources and, or low permeability layers; presence of dechlorination products arising from solvent (bio)degradation, although this key attenuation process appeared to have moderate to weak influence regionally on plumes; and, inadvertent, but significant solvent mass removal from the aquifer by industrial abstractions. Key challenges to realising future urban groundwater development were identified based on the observed legacy and well capture zone simulations. Despite the extensive contamination of the aquifer, it should still be possible to develop wells of high (several megalitres per day) capacity for drinking water supply (or other lower grade uses) without the requirement for solvent treatment. In those areas with higher risk of contamination, our dataset, together with application of emergent risk assessment approaches (that our dataset may serve to validate), could be used to inform potential abstractors as to whether solvent treatment is likely to be required at a particular abstraction site with time. Challenges identified that were relevant to the future development of Birmingham and urban aquifers more generally include the adequacy of groundwater quality monitoring data and uncertainties in contaminant source terms, abstraction well capture zone predictions and plume natural attenuation, in particular degradation rates. The study endorses that despite significant solvent contamination encountered, strategies can, and need, to be increasingly found to reclaim urban aquifer resources and more sustainably meet urban water demands.

Multicomponent NAPL source dissolution: evaluation of mass-transfer coefficients

Mass transfer rate coefficients were quantified by employing an inverse modeling technique to high-resolution aqueous phase concentration data observed following an experimental release of a multicomponent nonaqueous phase liquid (NAPL) at a field site. A solute transport model (SEAM3D) was employed to simulate advective-dispersive transport over time coupled to NAPL dissolution. Model calibration was demonstrated by accurately reproducing the observed breakthrough times and peak concentrations at multiple observation points, observed mass discharge at pumping wells, and the reported mass depletions for three soluble NAPL constituents. Vertically variable NAPL mass transfer coefficients were derived for each constituent using an optimized numerical solute transport model, ranging from 0.082 to 2.0 day(-1) across all constituents. Constituent-specific coefficients showed a positive correlation with liquid-phase diffusion coefficients. Application of a time-varying mass transfer coefficient as NAPL mass depleted showed limited sensitivity during which over 80% of the most soluble NAPL constituent dissolved from the source. Long-term simulation results, calibrated to the experimental data and rendered in terms of mass discharge versus source mass depletion, exhibited multistage behavior.

Development of a clusterwise-linear-regression-based forecasting system for characterizing DNAPL dissolution behaviors in porous media

Groundwater contamination by dense non-aqueous phase liquids (DNAPLs) has become an issue of great concern in many industrialized countries due to their serious threat to human health. Dissolution and transport of DNAPLs in porous media are complicated, multidimensional and multiphase processes, which pose formidable challenges for investigation of their behaviors and implementation of effective remediation technologies. Numerical simulation models could help gain in-depth insight into complex mechanisms of DNAPLs dissolution and transport processes in the subsurface; however, they were computationally expensive, especially when a large number of runs were required, which was considered as a major obstacle for conducting further analysis. Therefore, proxy models that mimic key characteristics of a full simulation model were desired to save many orders of magnitude of computational cost. In this study, a clusterwise-linear-regression (CLR)-based forecasting system was developed for establishing a statistical relationship between DNAPL dissolution behaviors and system conditions under discrete and nonlinear complexities. The results indicated that the developed CLR-based forecasting system was capable not only of predicting DNAPL concentrations with acceptable error levels, but also of providing a significance level in each cutting/merging step such that the accuracies of the developed forecasting trees could be controlled. This study was a first attempt to apply the CLR model to characterize DNAPL dissolution and transport processes.

Chlorinated ethene source remediation: lessons learned

Chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene (PCE) are widespread groundwater contaminants often released as dense nonaqueous phase liquids (DNAPLs). These contaminants are difficult to remediate, particularly their source zones. This review summarizes the progress made in improving DNAPL source zone remediation over the past decade, and is structured to highlight the important practical lessons learned for improving DNAPL source zone remediation. Experience has shown that complete restoration is rare, and alternative metrics such as mass discharge are often useful for assessing the performance of partial restoration efforts. Experience also has shown that different technologies are needed for different times and locations, and that deliberately combining technologies may improve overall remedy performance. Several injection-based technologies are capable of removing a large fraction of the total contaminant mass, and reducing groundwater concentrations and mass discharge by 1 to 2 orders of magnitude. Thermal treatment can remove even more mass, but even these technologies generally leave some contamination in place. Research on better delivery techniques and characterization technologies will likely improve treatment, but managers should anticipate that source treatment will leave some contamination in place that will require future management.

Kinetic limitations on tracer partitioning in ganglia dominated source zones

Quantification of the relationship between dense nonaqueous phase liquid (DNAPL) source strength, source longevity and spatial distribution is increasingly recognized as important for effective remedial design. Partitioning tracers are one tool that may permit interrogation of DNAPL architecture. Tracer data are commonly analyzed under the assumption of linear, equilibrium partitioning, although the appropriateness of these assumptions has not been fully explored. Here we focus on elucidating the nonlinear and nonequilibrium partitioning behavior of three selected alcohol tracers - 1-pentanol, 1-hexanol and 2-octanol in a series of batch and column experiments. Liquid-liquid equilibria for systems comprising water, TCE and the selected alcohol illustrate the nonlinear distribution of alcohol between the aqueous and organic phases. Complete quantification of these equilibria facilitates delineation of the limits of applicability of the linear partitioning assumption, and assessment of potential inaccuracies associated with measurement of partition coefficients at a single concentration. Column experiments were conducted under conditions of non-equilibrium to evaluate the kinetics of the reversible absorption of the selected tracers in a sandy medium containing a uniform entrapped saturation of TCE-DNAPL. Experimental tracer breakthrough data were used, in conjunction with mathematical models and batch measurements, to evaluate alternative hypotheses for observed deviations from linear equilibrium partitioning behavior. Analyses suggest that, although all tracers accumulate at the TCE-DNAPL/aqueous interface, surface accumulation does not influence transport at concentrations typically employed for tracer tests. Moreover, results reveal that the kinetics of the reversible absorption process are well described using existing mass transfer correlations originally developed to model aqueous boundary layer resistance for pure-component NAPL dissolution.

Effectiveness of nanoscale zero-valent iron for treatment of a PCE-DNAPL source zone

Nanoscale zero-valent iron (nZVI) has received considerable attention as a potential in situ remediation technology for treating chlorinated solvent source zones. Experimental and mathematical modeling studies were conducted to investigate the performance of nZVI in the transformation of tetrachloroethene (PCE) entrapped as a dense nonaqueous phase liquid (DNAPL). Injection of a 60 g/L suspension of nZVI into a column containing 20-30 mesh Ottawa sand and PCE-DNAPL at a residual saturation of 5.5% resulted in a uniform distribution of nZVI and minimal displacement of PCE. Subsequent flushing with 267 pore volumes of water containing 3mM CaCl(2) at a Darcy velocity of 0.75 m/day resulted in steady-state effluent concentrations of PCE near the solubility limit (ca. 200mg/L) and production of dissolved-phase ethene (10-30 mg/L). Over the duration of the experiment, approximately 30% of the initial PCE-DNAPL mass reacted to form ethene, 50% was eluted as dissolved-phase PCE, and 20% remained in the column as PCE-DNAPL. To further explore the implications of the nZVI column results, a multiphase transport model was developed that incorporated rate-limited PCE-DNAPL dissolution and reactions with nZVI. Using a fitted pseudo first-order transformation rate coefficient of 1.421/h, the model accurately captured observed trends in effluent concentrations of PCE and ethene and overall mass balance. A model sensitivity study reveals a strong dependence of treatment effectiveness on system characteristics. The sensitivity analysis suggests that an increase in the extent of PCE transformation is facilitated by decreasing flow rate, emplacement of nZVI down-gradient of the DNAPL source zone, and decreasing length of the DNAPL source zone. These findings indicate that, although emplacement of high concentrations of nZVI within a PCE-DNAPL source zone can result in substantial transformation of the parent compound, careful attention to design parameters (e.g. flow rate, location and amount nZVI delivered) will be required to achieve complete conversion to benign reaction products.