Civil and Environmental Engineering Publications

Systems Approach to Management of Disasters: Methods and Applications

Slobodan P. Simonović — Tue, 30 Aug 2011 17:05:13 PDT

The main goal of this text is to introduce the systems approach to disasters management community as an alternative approach that can provide support for interdisciplinary activities involved in the management of disasters. The systems approach draws on the fields of operations research and economics to create skills in solving complex management problems.

Site-Specific Design for Dual Phase Recovery and Stabilization of Pooled DNAPL

Jason I. Gerhard et al. — Wed, 08 Jul 2009 17:31:18 PDT

Volume reduction and lowering of capillary pressure within a large DNAPL pool are utilized as objectives in the design of a large-scale dual phase recovery system at a chemical manufacturing facility in the United States. By reducing DNAPL pool height through mass removal, capillary pressure is lowered, resulting in a reduced potential for future vertical and horizontal mobilization of the chlorinated solvent DNAPL pool. The DNAPL pool extends over an approximately 200 m by 275 m area in low permeability fill deposits overlying a clay aquitard. A three-dimensional multiphase flow model was employed to arrive at a final design incorporating nine horizontal drains (total length 664 m) and a pulsed pumping system. The numerical model was calibrated to the results of a 42-day field pilot-test involving the removal of approximately 25,000 L of DNAPL from a single, 55 m long horizontal drain. Numerical simulation revealed that gravity drainage, as opposed to hydraulic gradients in the water phase, is the dominant recovery mechanism at this site. This stems from the relatively high density and the viscosity of the DNAPL, and the relatively low permeability of the formation deposits. The use of pulsed pumping is shown to reduce the volume of contaminated ground water recovered from the 9-drain system, without significant reduction of the total volume of DNAPL recovered.

Parameter and Process Significance in Mechanistic Modeling of Cellulose Hydrolysis

B. E. Rotter et al. — Tue, 07 Jul 2009 18:22:59 PDT

A process-based model relevant to landfill and anaerobic digesters was developed, which included a novel approach to biomass transfer between a cellulose-bound biofilm and biomass in the bulk liquid. Model results highlighted the significance of the bacterial colonization of cellulose particles by attachment through contact in solution. Simulations revealed that both enhanced colonization and cellulose degradation are associated with reduced cellulose particle size, increased biomass populations in solution and increased cellulose-binding ability of the biomass. This suggests that transportation of biomass into the system from elsewhere and/or bacterial inoculation of such systems could enhance degradation significantly. A sensitivity analysis of the system parameters revealed the biological rate and yield properties of the hydrolyzing bacteria are most significant with regard to cellulose degradation in the system.

Multidimensional Validation of a Numerical Model for Simulating a DNAPL Release in Heterogeneous Porous Media

Gavin P. Grant et al. — Tue, 07 Jul 2009 18:22:58 PDT

A fixed-volume release of 1,2-DCE, tracked in space and time with a light transmission/image analysis system, provided a data set for the infiltration, redistribution, and immobilisation of a dense non-aqueous phase liquid (DNAPL) in a heterogeneous porous medium. The two-dimensional bench scale flow cell was packed with a spatially correlated, random heterogeneous distribution of six sand types. In order to provide the necessary modelling parameters, detailed constitutive relationships were measured at the local scale for the six sands. These experiments revealed that nonwetting phase (NWP) relative permeability–saturation (krN–SW) relationships are strongly correlated to sand type. Trends in the best-fit krN–SW parameters reflected a positive correlation between mean grain diameter and the maximum NWP relative permeability, krNmax. Multiphase flow simulations of the bench scale experiment best reproduced the experimental observations, producing excellent matches in both time and space, when the measured, correlated local scale krN–SW relationships were employed.

Time Scales of DNAPL Migration in Sandy Aquifers Examined via Numerical Simulation

Jason I. Gerhard et al. — Fri, 03 Jul 2009 19:19:29 PDT

The time required for dense nonaqueous phase liquid (DNAPL) to cease migrating following release to the subsurface is a valuable component of a site conceptual model. This study uses numerical simulation to investigate the migration of six different DNAPLs in sandy aquifers. The most influential parameters governing migration cessation time are the density and viscosity of the DNAPL and the mean hydraulic conductivity of the aquifer. Releases of between 1 and 40 drums of chlorinated solvent DNAPLs, characterized by relatively high density and low viscosity, require on the order of months to a few years to cease migrating in a heterogeneous medium sand aquifer having an average hydraulic conductivity of 7.4 × 10−3 cm/s. In contrast to this, the release of 20 drums of coal tar (ρD= 1061 kg/m3, μD= 0.161 Pa·s) requires more than 100 years to cease migrating in the same aquifer. Altering the mean hydraulic conductivity of the aquifer results in a proportional change in cessation times. Parameters that exhibit relatively little influence on migration time scales are the DNAPL–water interfacial tension, release volume, source capillary pressure, mean aquifer porosity, and ambient ground water hydraulic gradient. This study also demonstrates that low-density DNAPLs (e.g., coal tar) give rise to greater amounts of lateral spreading and greater amounts of pooling on capillary barriers than high-density DNAPLs such as trichloroethylene or tetrachloroethylene.

The Influence of Waterflood Design on the Recovery of Mobile DNAPLs

Jason I. Gerhard et al. — Fri, 03 Jul 2009 19:19:28 PDT

This study examines the effectiveness of various waterflooding strategies to recover pooled dense nonaqueous phase liquid (DNAPL) from the subsurface at an industrial facility. The relative influence of horizontal injection/recovery well configuration, established hydraulic gradient, and fluid properties is investigated for a site characterized by a homogeneous silty sand underlain by an impermeable clay layer. The top of the clay layer is located 5 m below the water table and supports a laterally extensive 2 m deep DNAPL pool. The sensitivity study employs a two-phase flow numerical model that simulates both DNAPL infiltration and redistribution, including the formation of immobilized DNAPL residual. This is accomplished with constitutive relations featuring hysteretic capillary pressure-saturation pathways in which the local amount of residual formed is a function of the maximum non-wetting saturation attained during infiltration. Sixteen simulations, performed in two-dimensional vertical cross-section, demonstrate that strategies effecting increased wetting phase gradients, namely increasing drawdown at the recovery drain, adding injection wells, and reducing their distance to the recovery drain, result in an increased DNAPL volume recovered with time at the expense of increased volumes of ground water removed per unit volume of DNAPL recovered. Strategies which do not increase wetting phase gradients result in DNAPL recovery with a minimum volume of produced contaminated ground water. Three pulsed pumping simulations indicate that increasing the length of pump shut-down time decreases the recovery of DNAPL with time but increases efficiency with respect to ground water pumped. Decreased nonwetting density and increased interfacial tension result in poorer DNAPL recovery with respect to both time and volume of ground water removed, while reduced nonwetting viscosity corresponds to dramatically increased efficiency in both respects.

The Influence of Precipitate Formation on the Chemical Oxidation of TCE DNAPL with Potassium Permanganate

Michael R. West et al. — Tue, 30 Jun 2009 17:27:44 PDT

A three-dimensional two-phase flow model is coupled to a non-linear reactive transport model to study the efficacy of potassium permanganate treatment on dense, non-aqueous phase liquid (DNAPL) source removal in porous media. A linear relationship between the soil permeability (k) and concentration of manganese dioxide precipitate ([MnO2(s)]), k = ko + Srind [MnO2(s)], is utilized to simulate nodal permeability reductions due to precipitate formation. Using published experimental column studies, an Srind = −5.5 × 10−16 m2 L/mg was determined for trichloroethylene (TCE) DNAPL. This Srind was then applied to treatment simulations on three-dimensional TCE DNAPL source zones comprising either DNAPL at residual saturations, or DNAPL at pooled saturations.

DNAPL dissolution without oxidation treatment, simulated using equilibrium and the Nambi and Powers [Nambi I, Powers S. Mass transfer correlations for non-aqueous phase liquid dissolution from regions with high initial saturations. Water Resour Res 2003;39(2):1–11, SBH 4] mass transfer expression, required 31 and 36 years, respectively, to eliminate the residual source zone. For equilibrium dissolution with continuous treatment and no precipitate influence (Srind = 0 m2 L/mg), the residual source zone was removed after 11 years. However, when considering the precipitate influence (i.e., Srind = −5.5 × 10−16 m2 L/mg), 21 years of treatment were necessary to remove the DNAPL. When considering pulse treatments of 1 and 2 years duration followed by only dissolution, approximately 36 and 38 years, respectively, were required before the source zone was depleted, suggesting that there is no benefit to pulse treatment. Similar trends were observed when allowing 10 years of dissolution prior to treatment initiation. The treatment behaviour of the pooled TCE source, while slightly more efficient than the residual saturation source, was similar.

Based on simulation findings, the precipitate (rind) formation significantly influences DNAPL treatment with permanganate; the most significant reductions in efficacy were observed for single pulse treatments (of 1 and 2 years), which exhibited times to source depletion similar to the case of dissolution without treatment.

Field Scale Impacts of Spatially Correlated Relative Permeability in Heterogeneous Multiphase Systems

G. P. Grant et al. — Tue, 30 Jun 2009 17:27:43 PDT

Two-dimensional numerical simulations of two-phase (DNAPL-water) flow in spatially correlated random fields demonstrate the influence of nonwetting phase (NWP) relative permeability–saturation (kr,N–SW) relationships correlated to porous media intrinsic permeability (k). Both the volume of porous media invaded by the NWP and the length of time during which the NWP is migrating are under predicted if kr,N–k correlation is not accounted for in the model formulation. Not accounting for the kr,N–k correlation resulted in under predicting the volume of porous media invaded by up to approximately 10%, which is likely not significant for many practical applications. However, not accounting for the kr,N–k correlation resulted in under predicting field scale migration times by up to a factor of 4, which is likely significant in that the migration times are on the order of years to several decades for the DNAPL (1,2-DCE) considered in this study. The under prediction of migration times was greater for lower permeability aquifers.

Modeling U(VI) Biomineralization in Single- and Dual-porosity Porous Media

B. E. Rotter et al. — Fri, 26 Jun 2009 18:36:51 PDT

Uranium extraction, processing, and storage have resulted in a legacy of uranium-contaminated groundwater aquifers worldwide. An emerging remediation technology for such sites is the in situ immobilization of uranium via biostimulation of dissimilatory metal-reducing bacteria (DMRB). While this approach has been successfully demonstrated in experimental studies, advances in understanding and optimization of the technique are needed. The motivation of this work was to understand better how dual-porosity (DP) porous media may affect immobilization efficiency via interactions with the dominant geochemical and microbial processes. A biogeochemical reactive transport model was developed for uranium immobilization by DMRB in both single- and dual-porosity porous media. The impact that microbial residence location has on the success of biostimulated U(VI) immobilization in DP porous media was explored under various porosity and mass transfer conditions. Simulations suggest that DP media are likely to show delayed U(VI) immobilization relative to single-porosity systems. U(VI) immobilization is predicted to be less when microbial activity is restricted to diffusion-dominant regions but not when restricted to advective-dominant regions. The results further highlight the importance of characterizing the bioresidency status of field sites if biogeochemical models are to predict accurately remediation schemes in physically heterogeneous media.

Simulating the Dissolution of a Complex Dense Nonaqueous Phase Liquid Source Zone: 2. Experimental Validation of an Interfacial Area–based Mass Transfer Model

G. P. Grant et al. — Fri, 26 Jun 2009 18:36:51 PDT

A multiphase flow–aqueous phase transport numerical model (DNAPL3D-MT) is developed to simulate the dissolution of complex source zones containing both pooled and residual dense nonaqueous phase liquids (DNAPLs). The multiphase flow model (DNAPL3D) is coupled to the aqueous species transport code (MT3D) via a flexible mass transfer function, which can employ the local equilibrium assumption or the single–boundary layer expression for rate-limited dissolution either incorporating a lumped (correlation function) coefficient or explicitly accounting for the interfacial area (IFA) between the fluids. For the latter, this work employs the thermodynamically based Explicit IFA Submodel (Grant and Gerhard, 2007), which provides IFA as a function of saturation and saturation history. A bench-scale experiment is presented involving the complete, natural dissolution of a DNAPL source zone emplaced by a point source release into heterogeneous porous media. DNAPL3D-MT simulations of the experiment, involving no calibration to results, are compared with the observed evolution of both (1) measured downgradient dissolved phase concentrations and (2) DNAPL source zone configuration. The model, employing a mass transfer expression equipped with the Explicit IFA Submodel, simulates the experiment more accurately than when equipped with either a local equilibrium assumption or a published empirical correlation expression. Sensitivity simulations indicate that this model validation is sensitive to a number of the key assumptions in the Submodel derivation except one: the relationship between interfacial area and residual DNAPL saturations. The employed assumption of a single mass transfer coefficient value is supported by an analysis of the evolution of Peclet numbers throughout the DNAPL source zone, which reveals that the low hydraulic gradient employed resulted in diffusion-dominated mass transfer conditions throughout the experiment. This study suggests that simulations of global mass flux from complex DNAPL source zones are sensitive to the interrelationship of rate-limited mass transfer and groundwater velocity (and thus aqueous phase relative permeability and DNAPL saturation) at the local scale.

Simulating the Dissolution of a Complex Dense Nonaqueous Phase Liquid Source Zone: 1. Model to Predict Interfacial Area

G. P. Grant et al. — Fri, 26 Jun 2009 18:36:50 PDT

A thermodynamically based model for predicting two-fluid interfacial area (IFA) within a porous medium as a function of wetting phase saturation (S W ) and saturation history is presented. The model considers consistency with multiphase flow constitutive relationships, the conversion of total to effective specific interfacial area, energy losses, and the change of interfacial area as residual nonwetting phase dissolves. The model requires as input only the capillary pressure–saturation relationships and porosity. Published, high-resolution interfacial area-saturation data sets were adequately reproduced when independent measures of these parameters were employed by the model. In particular, the model is found to reproduce key IFA(S W ) features including the IFA magnitude and S W value corresponding to the function's maximum, negligible IFA at residual S W and the observed hysteresis of IFA(S W ). Varying key model parameters reveals that the magnitude of the IFA(S W ) relationship is predicted to be linearly related to the porosity and entry pressure of the porous medium and is unaffected by interfacial tension. Interfacial area is a parameter in the single boundary layer expression of mass transfer between two immiscible liquids in porous media. The model's ability to predict local-scale IFA for a wide variety of fluid-fluid-porous media systems while accounting for saturation and saturation history thus provides an avenue for simulating the dissolution of complex source zones containing both pooled and residual dense nonaqueous phase liquids.

Influence of Constitutive Model Parameters on the Predicted Migration of DNAPL in Heterogeneous Porous Media

J. I. Gerhard et al. — Fri, 26 Jun 2009 18:36:49 PDT

This study examines the influence of constitutive models and their parameters on predictions of the spatial and temporal distribution of a finite release of a dense, nonaqueous phase liquid (DNAPL) into a two-dimensional, spatially correlated random permeability field. The base case simulation employed a comprehensive constitutive model that was validated against relevant one-dimensional laboratory experiments. The base case was perturbed in the course of nine individual simulations, where each simulation examined the consequences of simplifying a single model characteristic. None of the nine subsequent simulations was able to reproduce, within ±10%, the spatial and temporal migration characteristics of the nonwetting fluid body at late time predicted by the base case. Capillary pressure-saturation relationships that do not incorporate specific displacement and terminal pressures are demonstrated to severely overpredict the spatial extent of nonwetting fluid advancement. This suggests that van Genuchten-based models may not be suitable for predicting DNAPL migration in saturated porous media. Not accounting for any one of hysteresis, nonwetting phase trapping, or the proper curvature or end-point values of the nonwetting phase imbibition relative permeability curve profoundly influenced the time predicted for all nonwetting fluid movement to cease. The practical implication of this study is that an appropriate, comprehensive constitutive model, characterized with suitable parameter values, is necessary to accurately simulate a complete DNAPL release below the water table in both space and time.

Relative Permeability Characteristics Necessary for Simulating DNAPL Infiltration, Redistribution, and Immobilization in Saturated Porous Media

J. I. Gerhard et al. — Fri, 26 Jun 2009 18:36:49 PDT

This study presents a relative permeability-saturation (kr-S) constitutive model that incorporates the critical phenomena necessary for simulating the rates of nonwetting fluid infiltration, redistribution, and immobilization in a saturated porous medium. To develop a model validation data set, the migration of a dense, nonaqueous phase liquid (DNAPL) pool within a one-dimensional, 1 m tall, saturated sandpack was monitored under alternating drainage and imbibition conditions. A light transmission/image analysis system, able to distinguish between connected-phase and residual nonwetting phase (NWP) in the apparatus, measured the elevation of the top of the connected-phase DNAPL pool as a function of time. Light transmission calibration curves, correlating fluid saturation to transmitted color at the macroscopic scale, were found to exhibit a functional dependence on saturation history that must be taken into account. Applying the calibration curves to captured images of the experiment provided a continuous sequence of fluid saturation profiles. Numerical simulations of the bench-scale experiment, using model parameters measured independently at the macroscopic scale, predict within measurement uncertainty the observed timescales of DNAPL migration and immobilization. Additional simulations reveal that model validation for imbibition processes depends on properly accounting for NWP kr-S hysteresis, including imbibition function curvature and the abrupt extinction of NWP relative permeability.

Capillary Pressure Characteristics Necessary for Simulating DNAPL Infiltration, Redistribution, and Immobilization in Saturated Porous Media

J. I. Gerhard et al. — Fri, 26 Jun 2009 18:36:47 PDT

This study presents a capillary-pressure saturation (PC-S) constitutive model that incorporates the capillary phenomena necessary for simulating the spatial distribution of nonwetting fluid migrating in a saturated porous medium. To develop a model validation data set, a sequence of dense, nonaqueous phase liquid (DNAPL) pools were emplaced, under alternating drainage and imbibition conditions, in a one-dimensional, 1 m tall, saturated sand pack. A light transmission/image analysis system successfully distinguished between connected-phase and residual nonwetting fluid in the apparatus, thereby permitting the accurate measurement of DNAPL pool heights. These heights are found to depend on the nonzero capillary pressure across the fluid-fluid interface at the top of the pool. The terminal pressure is demonstrated to be the minimum sustainable capillary pressure in connected-phase nonwetting fluid experiencing imbibition, below which residual is formed. Additional bench-scale experiments demonstrate that a nonwetting phase pool will penetrate an underlying capillary barrier when the entry pressure is exceeded and that the resulting infiltration will terminate when the capillary pressure at the barrier reduces to the terminal pressure. At the macroscopic scale the terminal pressure corresponds to the extinction saturation (i.e., zero nonwetting phase flow) at the inflection point on the imbibition PC-S curve. A ratio of terminal to entry pressure of approximately 0.6 is found to apply at both bench and macroscopic scales and to be independent of porous media and fluid properties. The developed PC-S constitutive model, which extends the Brooks-Corey function to incorporate the terminal pressure, successfully predicted the behavior observed in the laboratory experiments. Constitutive models that do not incorporate both an entry and a terminal pressure, such as those based upon the standard van Genuchten function, are demonstrated to be unable to predict the observed equilibrium DNAPL pool heights in homogeneous media or above capillary barriers.

Variability of Point Source Infiltration Rates for Two-Phase Flow in Heterogeneous Porous Media

Bernard H. Kueper et al. — Fri, 26 Jun 2009 18:36:46 PDT

This study examines the influence of source release location, size, and strength on the infiltration rate and degree of lateral spreading of a dense nonwetting liquid infiltrating into an initially wetting liquid saturated, heterogeneous porous medium. It is demonstrated through numerical simulation in 25 realizations of a spatially correlated random hydraulic conductivity field that infiltration rates for point source releases are lognormally distributed with a variance equal to that of the underlying hydraulic conductivity distribution. The variability in infiltration rates is shown to decrease for sources larger than the correlation scale of hydraulic conductivity. In all cases, individual infiltration rates showed no tendency to converge to the ensemble average. A moment analysis demonstrates that the degree of lateral spreading of the nonwetting body for individual realizations varied greatly and also did not display a tendency to converge to an ensemble average. Numerical simulations carried out in an equivalent homogeneous porous medium incorporating large-scale anisotropy of intrinsic permeability provided infiltration rates below the ensemble average. For point source releases the degree of lateral spreading exhibited in the equivalent homogeneous porous medium was below the entire ensemble of heterogeneous results. A series of 10 simulations conducted in a single realization demonstrates that the degree of lateral spreading (second moment) along main drainage is a function of the average nonwetting phase saturation with greater degrees of lateral spreading at low capillary pressures. The practical implication of this study is that in addition to fluid and media properties the specific order of encounter of varying permeability lenses must be known in the immediate vicinity of a nonwetting phase release if infiltration rates are to be accurately predicted.