Suspended Wash Load Transport of Nonuniform Sediments

The results of an experimental study on transport of suspended wash load through a coarse-bed stream are presented. The experiments were conducted under different concentrations of fine suspended sediment (wash load of uniform size, 0.064 mm diameter) and with three different coarse-bed sediments: two having uniform sizes and one with nonuniform size distribution. For any equilibrium concentration of wash load in suspension, a definite proportion of the wash material was observed to be present within the bed material. No difference is found in this regard between wash load and suspended load transport. Therefore, the relationship, as stated by Samaga et al., for the parameter representing sheltering—exposure and interference effects in the suspended load transport of nonuniform sediments was applied in a modified form by using the present data and the data collected from the literature.     

Application of an Intraparticle Diffusion Model to Describe the Release of Polyaromatic Hydrocarbons from Field Soils

The fate and transport of chemicals of concern released from field soils must be known to protect human and ecological receptors. A mechanistic approach to modeling chemical release from soil is advantageous to implement effective remediation strategies at an impacted site. The focus of this research was to gain an understanding of the processes causing slow release of polyaromatic hydrocarbons (PAHs) from field soils collected at sites with historical releases. A mechanistically based intraparticle diffusion model was applied to experimentally measured hydrocarbon release data and particle size distributions obtained from three field soils. For these field soils, the intraparticle diffusion model was able to describe the measured chemical release data. Fitted effective diffusion coefficients (Deff) of the intraparticle diffusion model correlated to expected results. Trends were found to exist between the Deff and both the molecular weight (MW) and the octanol–water partition coefficient (Kow) of the PAH analyzed for the field soils with low organic carbon content. For these soils, the relationships suggest that intraparticle diffusion processes may be responsible for slow desorption and it may be possible to estimate Deff values for a soil or contaminated media with similar intraparticle properties using a readily measured chemical characteristic such as MW and Kow.     

The development of a new method to detect the adulteration of commercial aloe gel powders

Recent high-resolution analysis of tubulin's structure has led to the prediction that the taxol binding site and a tubulin acetylation site are on the interior of microtubules, suggesting that diffusion inside microtubules is potentially a biologically and clinically important process. To assess the rates of transport inside microtubules, predictions of diffusion time scales and concentration profiles were made using a model for diffusion with parameters estimated from experiments reported in the literature. Three specific cases were considered: 1) diffusion of alpha beta-tubulin dimer, 2) diffusion/binding of taxol, and 3) diffusion/binding of an antibody specific for an epitope on the microtubule's interior surface. In the first case tubulin is predicted to require only approximately 1 min to reach half the equilibrium concentration in the center of a 40 microns microtubule open at both ends. This relatively rapid transport occurs because of a lack of appreciable affinity between tubulin and the microtubule inner surface and occurs in spite of a three-fold reduction in diffusivity due to hindrance. By contrast the transport of taxol is much slower, requiring days (at nM concentrations) to reach half the equilibrium concentration in the center of a 40 microns microtubule having both ends open. This slow transport is the result of fast, reversible taxol binding to the microtubule's interior surface and the large capacity for taxol (approximately 12 mM based on interior volume of the microtubule). An antibody directed toward an epitope in the microtubule's interior is predicted to require years to approach equilibrium. These results are difficult to reconcile with previous experimental results where substantial taxol and antibody binding is achieved in minutes, suggesting that these binding sites are on the microtubule exterior. The slow transport rates also suggest that microtubules might be able to serve as vehicles for controlled-release of drugs.     

Effects of Background Water Composition on Stream–Subsurface Exchange of Submicron Colloids

While very fine sediments (colloids) are normally assumed to be readily transported downstream without deposition, recent evidence suggests that these particles will often deposit into streambeds due to a combination of physical and chemical mechanisms. This study investigates a regime of particle deposition where settling is unimportant and thus where particle deposition can only result from advective stream–subsurface exchange followed by deep-bed filtration. Laboratory flume experiments were conducted to examine the deposition of 0.45 μm diameter silica colloids into a silica sand bed. This system was selected for study because submicron sized colloids will not settle and silica colloid filtration by silica sand is generally quite low. Despite the lack of settling and the weak particle–particle interactions, the ongoing interfacial flux of colloids to the subsurface still produced significant filtration of silica colloids over the course of the experiments. Variation of the background ionic strength caused significant modification of filtration behavior and silica colloid deposition. In addition, cleaning the sand surface with mild acid and base washes reduced both filtration and net colloid exchange. These experimental results are interpreted in terms of a fundamentally based physicochemical model which predicts net particle deposition based on stream and subsurface hydrodynamic conditions and subsurface filtration. These results show that both particle surface chemical conditions and background water chemistry play a critical role in controlling the net transport and deposition of fine sediments. It is important to recognize the effects of physicochemical processes both when designing laboratory experiments and when analyzing environmental particle transport.     

Removal of Hexachlorobenzene from Kaolin by Electrokinetics Coupled with Cu/Fe PRB

This study deals with the remediation of clayed soils contaminated with hydrophobic organic compounds (HOCs) by using electrokinetics (EK) coupled with a permeable reactive barrier (PRB) technique. Hexachlorobenzene (HCB) and kaolin were selected as the typical HOCs and clay, respectively, and microscale Cu/Fe particles were synthesized as the functional materials of PRB. Furthermore, Triton X-100 was used as the solubility-enhanced agent to promote the movement and removal of contaminants. Four bench-scale EK tests were conducted with or without Cu/Fe PRB. The results reveal that coupling EK with Cu/Fe PRB greatly promoted the overall removal of HCB from soil, compared with EK alone. Other than the electroosmotic flow, the adsorption/reductive degradation by Cu/Fe PRB may also be responsible for the HCB removal (over 82% of HCB passing by the PRB was removed). Our investigation suggests that the integration of EK and Cu/Fe PRB is of great promise to promote the application of EK technique in the remediation of HOC-contaminated soils.     

Nodal Point Relation for the Distribution of Sediments at Channel Bifurcation

This technical note explores the applicability of the general nodal point relation to describe the distribution of sediments at channel bifurcation. Experiments were conducted in a physical model of channel bifurcation to estimate the coefficient and exponent of the nodal point relation for different nose angles and upstream discharges. It was found that the nose angle is the major variable for the distribution of sediments to the downstream branches. The value of the exponent k found from experimental results was compared with that of a theoretical analysis. For a stable equilibrium, the value of k is greater than 5/3 from theoretical consideration when the Engelund–Hansen sediment transport formula is used. This was confirmed from the experimental results. This suggests that the distribution of sediments at channel bifurcation can be expressed by the general nodal point relation.     

Long-Term Tritium Transport through Field-Scale Compacted Soil Liner

A 13-year study of tritium transport through a field-scale earthen liner was conducted by the Illinois State Geological Survey to determine the long-term performance of compacted soil liners in limiting chemical transport. Two field-sampling procedures (pressure-vacuum lysimeter and core sampling) were used to determine the vertical tritium concentration profiles at different times and locations within the liner. Profiles determined by the two methods were similar and consistent. Analyses of the concentration profiles showed that the tritium concentration was relatively uniformly distributed horizontally at each sampling depth within the liner and thus there was no apparent preferential transport. A simple one-dimensional analytical solution to the advective–dispersive solute transport equation was used to model tritium transport through the liner. Modeling results showed that diffusion was the dominant contaminant transport mechanism. The measured tritium concentration profiles were accurately modeled with an effective diffusion coefficient of 6×10?4?mm2/s, which is in the middle of the range of values reported in the literature.     

Centrifuge Modeling of Moisture Migration in Silty Soils

An effort has been made to demonstrate use of a small geotechnical centrifuge as a research tool to understand moisture migration in a silty soil. Such a basic study is essential for understanding the much more complex phenomenon of solute transport through a soil, where the physical, chemical, and biological properties of the soil-solute system play an important role. Present study indicates that the advection in soils is dependent on the state of the soil and, in particular, on its degree of saturation. The effect of pore structure on the advection process has also been demonstrated. The trends of experimental data indicate that modeling of models may be valid only for saturated soils.     

Investigation of Cadmium Desorption from Different-Sized Sediments

Using the adsorptive reaction kinetics equation and the mass conservation equation, formulas for calculating the particulate heavy metal concentration (or the residual adsorption content of unit weight of sediment) and the dissolved heavy metal concentration were deduced. Furthermore, equations for calculating the equilibrium particulate heavy metal concentration (or the equilibrium residual adsorption content of unit weight of sediment) and the equilibrium dissolved heavy metal concentration were formulated. On the basis of these theoretically deduced formulas and through laboratory reactor modeling of artificially contaminated sediments with cadmium in suspension, heavy metal desorption rates from nonuniform sediment particles were studied. Both theoretical analysis and experimental results indicate that adsorption and desorption of heavy metal pollutants to and from sediment particles are partly reversible. It is also noticed from the experiments that the adsorption and desorption rates of heavy metals to and from sediments are dependent on the content of active adsorption components in the sediments. The greater the content of active adsorption components, the higher the adsorption rate of the sediments and the lower the desorption rate. The total amount of heavy metal desorbed from sediments is directly proportional to the concentration of suspended sediment particles.     

Sediment Control of Facilitated Transport and Enhanced Desorption

Laboratory column experiments examined the facilitated transport and enhanced desorption of benz(a)anthracene [B(a)A] by dissolved natural organic matter (OM) in sediments of low organic carbon content. The two-component experiments examining OM-sediment interaction and B(a)A-sediment interaction were modeled to determine the value of the linear rate constants describing transfer of B(a)A and OM between water and sediment. It was found that a two-rate approach better simulated B(a)A breakthrough and elution in the sediment relative to a one-rate approach. In contrast, OM-sediment interaction was well-simulated with a one-rate approach due to low OM sorption by sediment. The three-component experiments examining facilitated transport and enhanced desorption of B(a)A by dissolved OM, showed rapid linear reversible B(a)A-OM interaction. The value, within a factor of 2, of the equilibrium distribution constant for benz(a)anthracene distribution between water and OM was ～1E6 for soil humic acid and ～1E5 for Suwannee River humic acid. Simulations of the three-component experiments based on the equilibrium distribution constants for B(a)A-OM interaction and the rate constants determined from the two-component experiments were performed to determine whether rate constants differed in the two-component versus three-component systems. The simulations captured the major features of the facilitated transport and enhanced desorption data; however, discrepancies indicated that either the two-rate model for solute-sediment interaction was inappropriate, or that B(a)A transfer from sediment to dissolved OM was altered in the three-component system relative to the two-component system.     

Influence of Al,Si and P on the kinetics of intercritical annealing of TRIP‐aided steels: thermodynamical prediction and experimental verification

TRIP‐aided steels offer an excellent combination of strength and formability, which makes them particularly interesting for use in automotive applications. Recent investigations have shown that while the typical high CMnSi TRIP‐aided steel composition offers good mechanical properties, alloying with other elements or a modification of the processing are required to make this steel readily galvanizable without loss of the TRIP properties. Al‐alloying seems especially promising to realize this goal and P could also be an alternative. Due to the very specific thermal processing needed to obtain a TRIP microstructure, it is important to know the influence of these alloying elements on the re‐austenitization kinetics during the annealing. This paper aims at identifying the differences in the influence of Si, Al and P on the intercritical annealing of TRIP‐aided steels. The equilibrium thermodynamics calculations and diffusion‐controlled transformation simulations were used in order to predict the transformation behaviour, and experimental verification was done based on dilatometric experiments.     

Metal Ion Leaching from Contaminated Soils: Model Calibration and Application

A previously developed model that describes leaching of heavy metals from contaminated soils is applied to four hazardous-waste-site soils contaminated with Pb. Processes included in the model are intraparticle diffusion, rate expressions for irreversibly and reversibly sorbed fractions, and metal complexation by ions in solution. The model is calibrated using laboratory experimental data in the pH 1–3 range, liquid-to-solid mass ratios from 5 to 20, and leaching times of 24 h. Parameters for the model are estimated through a combination of independent experiments, literature correlations, and mathematical optimization. Equilibrium data were used to estimate site density and an adsorption equilibrium constant. Two kinetic rate coefficients, a particle tortuosity factor, and a distribution coefficient (αa) that defined the amount of Pb in two contaminant fractions were adjusted to match kinetic leaching data. Using one set of parameter estimates for each soil, the model successfully simulated experimental data collected under different leaching conditions. The fraction of Pb associated with easily leachable, irreversibly sorbed fraction (1 ?αa) provides some insight to the geochemical distribution of Pb in the soils tested. The model is used to explore effects of process variables such as liquid-to-solid ratio and sequential washes. The model should be useful for simulating ex-situ soil washing processes and may, with further development, have applications for in-situ flushing processes.     

Scour in Long Contractions

Experimental results on local scour in long contractions for uniform and nonuniform sediments (gravels and sands) under clear-water scour are presented. An emphasis was given to conduct the experiments on scour in long contractions for gravels. The findings of the experiments are used to describe the effects of various parameters (obtained from dimensional analysis) on equilibrium scour depth under clear-water scour. The equilibrium scour depth increases with decrease in opening ratio and with increase in sediment size for gravels. But the curves of scour depth versus sediment size have considerable sag at the transition of sand and gravel. The scour depth decreases with increase in densimetric Froude number, for larger opening ratios, and increases with increase in approaching flow depth at lower depths. However, it becomes independent of approaching flow depth at higher flow depths. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to the formation of armor layer in the scour hole. Using the continuity and energy equations, a simple analytical model for the computation of clear-water scour depth in long contractions is developed with and without sidewall correction for contracted zone. The models agree satisfactorily with the present and other experimental data. Also, a new empirical equation of maximum equilibrium scour depth, which is based on the experimental data at the limiting stability of sediments in approaching channel under clear-water scour, is proposed. The potential predictors of the maximum equilibrium scour depth in long contractions are compared with the experimental data. The comparisons indicate that the equations given by Komura and Lim are the best predictors among those examined.     

Evaluation of Multidimensional Transport through a Field-Scale Compacted Soil Liner

A field-scale compacted soil liner was constructed at the University of Illinois at Urbana-Champaign by the U.S. Environmental Protection Agency (USEPA) and Illinois State Geological Survey in 1988 to investigate chemical transport rates through low permeability compacted clay liners (CCLs). Four tracers (bromide and three benzoic acid tracers) were each added to one of four large ring infiltrometers (LRIs) while tritium was added to the pond water (excluding the infiltrometers). Results from the long-term transport of Br? from the localized source zone of LRI are presented in this paper. Core samples were taken radially outward from the center of the Br? LRI and concentration depth profiles were obtained. Transport properties were evaluated using an axially symmetric transport model. Results indicate that (1) transport was diffusion controlled; (2) transport due to advection was negligible and well within the regulatory limits of ksat ? 1×10?7?cm/s; (3) diffusion rates in the horizontal and vertical directions were the same; and (4) small positioning errors due to compression during soil sampling did not affect the best fit advection and diffusion values. The best-fit diffusion coefficient for bromide was equal to the molecular diffusion coefficient multiplied by a tortuosity factor of 0.27, which is within 8% of the tortuosity factor (0.25) found in a related study where tritium transport through the same liner was evaluated. This suggests that the governing mechanisms for the transport of tritium and bromide through the CCL were similar. These results are significant because they address transport through a composite liner from a localized source zone which occurs when defects or punctures in the geomembrane of a composite system are present.     

Mechanisms of Small-Strain Shear-Modulus Anisotropy in Soils

In this paper, experimental studies using a true triaxial apparatus and a bender element system, and numerical simulations based on the discrete element method (DEM) were used to investigate the stress- and fabric-induced shear-stiffness anisotropy in soils at small strains. Verified by experiments and DEM simulations, the shear modulus was found to be relatively independent of the out-of-plane stress component, which can be revealed by the indistinctive change in the contact normal distribution and the normal contact forces on that plane in the DEM simulations. Simulation and experimental results also demonstrated that the shear modulus is equally contributed by the two principal stress components on the associated shearing planes. Fabric-induced stiffness anisotropy, i.e., the highest Gxy or Ghh, can be explained by simulation findings in which more contact normals prefer to distribute along the horizontal direction. The experiments and simulations also reveal that the fabric-induced stiffness anisotropy increases with an increasing aspect ratio of the particles. The assumption of transversely isotropic fabric in soils is valid based on the DEM simulation results; however, this assumption is not completely supported by the experimental results.     

Predicting Interfacial Diffusion Coefficients for Fluxes across the Sediment-Water Interface

Calculations of interfacial diffusion coefficients are often based on tracer flux or penetration into permeable media or sediments. Based on previous investigations, a new empirical relationship for tracer-based interfacial diffusion coefficients is derived. This relationship is a powerful tool for estimating interfacial fluxes over a range of environmentally relevant conditions. Support for this relationship was found in experiments targeting the slip velocity at porous media–boundary layer interfaces. Slip measurements from flume experiments using flat permeable sediment beds and from previous studies using high permeability media were converted to interfacial diffusion coefficients based on the momentum flux needed to drive the observed interstitial flow. Slip-based estimates compared well with the tracer-based predictions over the entire range of flow-permeability conditions. This study presents a relationship for predicting fluxes across the sediment-water interface that is driven by permeability-scale processes. Predicted fluxes are comparable to those documented for other processes driving interfacial transport, such as bed topography and sediment transport.     

Effects of Flood Flow on Flood Plain Soil and Riparian Vegetation in a Gravel River

Field observations were performed to determine the effects of flood flow on the geometrical and chemical characteristics of flood plain soil and the distribution of riparian vegetation in a gravel river. The results of the observations show a decrease in the amounts of the particulate nutrients in the flood plain soil during fairly large flood, because the fine sands that serve as a nutrient source were removed by the flood flow. Numerical simulations for the transport of suspended sediments were performed by varying the peak discharge of the flood, and the change of the particulate nutrients in flood plain soil was estimated by using the results of the simulations. The numerical analysis predicts the reduction of the particulate nutrients in the flood plain soils well. The particulate nutrients on the flood plains decrease if the discharge exceeds the flood of approximately 2 year return period in the observation area.     

Multilayer diffusional growth in silver-zinc alloys

Multilayer diffusional growth experiments were performed on silver-zinc binary alloys at 400 °C.The growth of the different phase layers was evaluated in terms of alternative theories, which were demonstrated to be equivalent algebraically but complementary because they are expressed in terms of different experimental variables. Parabolic growth constants and integral values of the chemical diffusion coefficients over the widths of the phases were calculated. In all instances of diffusion-controlled growth, the diffusion coefficients were in excellent agreement with conventionally measured values. The γ phase (Ag₅Zn₈) exhibited nondiffusion-controlled growth kinetics. The influence of the theoretical assumptions of the narrow range of stoichiometry, equilibrium phase boundary compositions and constant molar volumes is discussed. Formerly Research Associate in the Metallurgical Engineering Department, The Ohio State University, Columbus     

Sand Transport on Steeply Sloping Plane and Rippled Beds

Experiments on sand transport have been carried out in the Sloping Sediment Duct at HR Wallingford. The aim of the experiments was to investigate sediment transport mechanisms, for sand of varying degree of grading, on sloping beds. The Sloping Sediment Duct is a steady flow, recirculating duct, capable of generating mean flow speeds of up to 1 m/s and tilting to +/?30°. Twenty-two tests with two different sediments were conducted. Both sediments had a median grain size of about 0.23 mm but different standard deviations. Bed slopes up to +/?20° were used in the experiments. The results show that bedforms have a significant effect on the transport rate. Since the bedforms, in turn, are affected significantly by the slope, the relation between transport rate and slope is not a monotonic function. Maximum suspended transport rates were attained for downslope flows at angles of about 10°. The transport rate for widely graded sediment was significantly larger than that for well-sorted sediment for almost all flows and slopes.