The Impact of Well-Field Configuration and Permeability Heterogeneity on Contaminant Mass Removal and Plume Persistence
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The purpose of this study is to investigate the effects of well-field hydraulics and permeability heterogeneity on mass-removal efficiency for systems comprising large groundwater contaminant plumes. A three-dimensional (3D) numerical model was used to simulate the impact of different well-field configurations on pump-and-treat mass removal for heterogeneous domains. The relationship between reduction in contaminant mass discharge (CMDR) and mass removal (MR) was used as the metric to examine remediation efficiency. The impacts of well-field configuration on mass removal behavior are attributed to mass-transfer constraints related to regions of low flow associated with the well field, which can be muted by the influence of permeability heterogeneity. These impacts are reflected in the associated CMDR-MR profiles. Systems whose CDMR-MR profiles are below the 1:1 relationship line are associated with more efficient well-field configurations. The impact of domain heterogeneity on mass-removal effectiveness was investigated in terms of both variance and correlation scale of the random permeability distributions and indexed by the CMDR-MR relationship. Data collected from pump-and-treat operations conducted in a section of the Tucson International Airport Area (TIAA) federal Superfund site were used as a case study. The comparison between simulated and measured site data supports the general validity of the numerical model, and results from the case study are consistent with the conclusions of the theoretical study. These results illustrate that the CMDR-MR relationship can be an effective way to quantify the impacts of different factors on mass-removal efficiency.Keywords:
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Groundwater flow equation
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Modeling groundwater flow and contaminant transport in industrialized and urbanized alluvial plains is challenging because of the dynamic nature of the strongly interacting river – groundwater system and the scarcity of data. An original combined regional – local scale approach is proposed and illustrated using data from a detailed hydrogeological characterization of a brownfield located next to a water-level controlled river in Belgium. An original combined regional-scale (zonation) and local-scale (pilot points) approach is proposed to develop and calibrate a groundwater flow model in this context, using the finite difference code MODFLOW-2000, with an automatic parameter estimation using PEST. Groundwater flows are calibrated in transient conditions using river water level fluctuations as the hydrologic forcing of the groundwater – surface water system, resulting in a detailed, spatially heterogeneous distribution of the hydraulic conductivity field. The model reproduces with great accuracy groundwater head variations and continuous Darcy flux changes at the aquifer-river interface as a consequence of river fluctuations. The resulting hydraulic conductivity pattern is validated afterwards using field observations on hydrogeochemistry and tracer test results. Based on the calibrated groundwater flow model and the spatial heterogeneity of hydraulic conductivity field, benzene transport simulations were performed using MT3DMS, considering benzene attenuation as quantified in-situ, along the groundwater flow path, using carbon isotopic fractionation. The model indicated that fluctuations in the river water were at the origin of back and forward movements of benzene plumes and it allowed explaining the absence of benzene close to the river-aquifer interface by ongoing sulfate-reducing processes and aquifer heterogeneity.
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Modeling Three-Dimensional Groundwater Flow and Advective Contaminant Transport at a Heterogeneous Mountainous Site in Support of Remediation Strategy Quanlin Zhou, Jens T. Birkholzer, Iraj Javandel, and Preston D. Jordan Ernest Orlando Lawrence Berkeley National Laboratory Earth Sciences Division 1 Cyclotron Road, MS 90-1116, Berkeley CA 94720 USA Abstract A calibrated groundwater flow model for a contaminated site can provide substantial information for assessing and improving hydraulic measures implemented for remediation. A three-dimensional transient groundwater flow model was developed for a contaminated mountainous site, at which interim corrective measures were initiated to limit further spreading of contaminants. This flow model accounts for complex geologic units that vary considerably in thickness, slope, and hydrogeologic properties, as well as large seasonal fluctuations of the groundwater table and flow rates. Other significant factors are local recharge from leaking underground storm drains and recharge from steep uphill areas. The zonation method was employed to account for the clustering of high and low hydraulic conductivities measured in a geologic unit. A composite model was used to represent the bulk effect of thin layers of relatively high hydraulic conductivity found within bedrock of otherwise low conductivity. The inverse simulator ITOUGH2 was used to calibrate the model for the distribution of rock properties. The model was initially calibrated using data collected between 1994 and 1996. To check the validity of the model, it was subsequently applied to predicting groundwater level fluctuation and groundwater flux between 1996 and 1998. Comparison of simulated and measured data demonstrated that the model is capable of predicting the complex flow reasonably well. Advective transport was approximated using pathways of particles originating from source areas of the plumes. The advective transport approximation was in good agreement with the trend of contaminant plumes observed over the years. The validated model was then refined to focus on a subsection of the large system. The refined model was subsequently used to assess the efficiency of hydraulic measures implemented for remediation.
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Compared with commonly used deterministic methods,the stochastic hydrogeology method is a more rational resort for solving the flow and transport problems in the heterogeneous aquifers.On the assumption that the hydraulic conductivity field follows a lognormal distribution,the direct Fourier transform is introduced to generate multiple realizations of hydraulic conductivity field.Then the Monte Carlo method,based on the stochastic theory,is applied to investigate the effect of the spatial variation of hydraulic conductivity on the fate of contaminant plume.For the contaminant plume examined in this synthetic example study the spatial second moments defining the spread of the contaminant plume around its centroid increase with the enlargement of the variance of hydraulic conductivity distribution,while the first moment specifying the centroid of the plume is not influenced by the variance of hydraulic conductivity but dominated by the mean of hydraulic conductivity.Additional analysis demonstrates the sensitivity of both variance and coefficient of variation of plume concentration to the various variances of hydraulic conductivity field.
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Abstract This study evaluated the effect of heterogeneity in hydraulic conductivity on the tendency for contaminant plumes to attenuate via dilution, hydrodynamic dispersion, and molecular diffusion in simulated aquifers. Simulations included one homogeneous and four increasingly heterogeneous hydraulic conductivity fields. A numerical mass transport model generated an initial contaminant plume for each case; all initial plumes had the same mass. Next, the model simulated plume migrations through the simulated aquifers. Results suggest that highly heterogeneous settings are potentially effective at plume attenuation. Low‐velocity zones in heterogeneous settings delay plume travel, enabling more time for natural processes to lower contaminant concentrations in groundwater. © 2012 Wiley Periodicals, Inc.
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Mass transport
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Contaminant transport in natural geological formations at the field-scale is strongly affected by spatial variations in the hydraulic and chemical-sorption properties of the host medium. The hydraulic conductivity of subsurface strata varies by many orders of magnitude. The presence of fractures in clays and consolidated rocks and discontinuities such as root holes or macropores in near-surface soils represent another form of heterogeneity. This work examines the sensitivity of the reactive-solute transport process at the field-scale to the type and degree of correlation between the heterogeneous hydraulic and sorption parameter fields and the form of competing models commonly used to describe the local-scale sorption process. The influence of factors leading to transport nonidealities such as the presence of fractures will also be addressed as they pertain to contaminant plume evolution and remediation by pump-and-treat. The general approach taken involves a series of exceedingly detailed, multi-dimensional Monte Carlo simulations of reactive and nonreactive solute transport in synthetic heterogeneous geological materials, as well as comparison of the numerical results to field observations and theoretical predictions based on stochastic-analytic theory. Differences between the evolution of nonreactive and reactive contaminant plumes at the field-scale are highlighted and issues related to prediction uncertainty and aquifer remediationmore » due to imperfect knowledge of the properties of hydrogeological systems are discussed.« less
Classification of discontinuities
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