Effects Of Pore Water Velocity Research Articles

Coupled Effects of Pore Water Velocity and Soil Heterogeneity on Bacterial Transport: Intact vs. Repacked Soils.

Transport of pathogenic bacteria from land surface to groundwater is largely influenced by rainfall intensity and geochemical and structural heterogeneities of subsurface sediments at different depths. It has been assumed that the change in rainfall intensity has different effects on bacterial transport as a function of soil depth. In this study, repacked and intact column systems were used to investigate the influences of pore water velocity on the transport of Escherichia coli 652T7 through a loamy soil collected from varying soil depths. The soils differed in geochemical properties and soil structures. The concentrations of bacteria in soil and liquid samples were measured using plate counting method. The breakthrough percentages of E. coli 652T7 increased with pore water velocity at each depth in both intact and disturbed soils. Among the different soil depths, the largest velocity effect was observed for the transport through the top soil (0–5 cm) of both disturbed and intact soil profiles. This depth-dependent effect of pore water velocity was attributed to down gradients of soil organic matter (SOM) and iron oxide contents with depth because SOM and iron oxides were favorable for bacterial attachment on soil surfaces. In addition, less bacteria broke through the disturbed soil than through the intact soil at the same depth, and the pore water velocity effect was stronger with the disturbed than intact soils. Specifically, the maximum C/C0 (i.e., ratio of effluent to influent concentration) doubled (i.e., from 0.36 to 0.76) in the 0–5 cm intact soil columns and tripled (i.e., from 0.16 to 0.43) in the 0–5 cm repacked soil columns. This structure-dependent effect of pore water velocity was attributed to larger pore tortuosity and a narrower range of pore sizes in the disturbed soil than in the intact soil. These findings suggest that change in pore water velocity could trigger bacterial remobilization especially in surface soils, where more bacteria are retained relative to deep soils.

Modeling and Experimental Study of the Effect of Pore Water Velocity on the Spectral Induced Polarization Signature in Porous Media

AbstractInduced polarization (IP) is increasingly applied for hydrological, environmental and agricultural purposes. Interpretation of IP data is based on understanding the relationship between the IP signature and the porous media property of interest. Mechanistic models on the IP phenomenon rely on the Poisson‐Nernst‐Plank equations, where diffusion and electromigration fluxes are the driving forces of charge transport and are directly related to IP. However, to our knowledge, the impact of advection flux on IP was not investigated experimentally and was not considered in any IP model. In this work, we measured the spectral IP (SIP) signature of porous media under varying flow conditions, in addition to developing and solving a model for SIP signature of porous media, which takes flow into consideration. The experimental and the model results demonstrate that as bulk velocity increases, polarization and relaxation time decrease. Using a numerical model, we established that fluid flow near the particle deforms the electrical double layer (EDL) structure, accounting for the observed reduction in polarization. We found a qualitative agreement between the model and the measurements. Still, the model overestimates the impact of flow rate on SIP signature, which we explain in terms of the flow boundary conditions. Overall, our results demonstrate the sensitivity of the SIP signature to fluid flow, highlighting the need to consider fluid velocity in the interpretation of the SIP signature of porous media, and opening an exciting new direction for noninvasive measurements of fluid flow at the EDL scale.

Transport of bisphenol-A in sandy aquifer sediment: Column experiment.

The present paper aims to study the transport behavior of bisphenol-A (BPA) in sandy aquifer so as to provide important parameters for the prediction and control of contaminant plume in aquifer. Miscible displacement experiments were conducted and the breakthrough curves (BTCs) were simulated using HYDRUS-1D software. The effects of pore-water velocity (10-52 cm h(-1)) and initial concentration (2.5-40 mg L(-1)) on the sorption were also investigated. The BTCs of BPA fit the linear first-order non-equilibrium two-site model. The parameters such as partition coefficient (K(d)), the fraction of instantaneous adsorption on "Type-1" sites (F), the first order sorption rate coefficient for the kinetic non-equilibrium (type-2) sites (α), the retardation coefficient (R), and sorption capacity (q(column)) were computed. Results showed that BPA transported 0.11-0.83 m with various pore water velocity in sandy sediment column when water flowed 1 m. The sorption of BPA was mainly caused by the instantaneous surface adsorption as F varied from 0.596 to 0.908. The transport velocity of BPA was affected by pore water velocity (v) and followed the linear equation 1/R = 0.0600 + 0.0110v (r(2) = 0.9724). The parameter K(d) were also closely related to v and followed the equation LnK(d) = 1.0023-0.0482v (r(2) = 0.9690). The sorption capacity was more related to the initial BPA concentration (C0) and followed the linear equation q(column) = 0.265 + 0.253C0 (r(2) = 0.9727). The parameter α was affected by both v and C0 whereas F was not dramatically affected by both.

Transport of Human Adenoviruses in Water Saturated Laboratory Columns.

Groundwater may be contaminated with infective human enteric viruses from various wastewater discharges, sanitary landfills, septic tanks, agricultural practices, and artificial groundwater recharge. Coliphages have been widely used as surrogates of enteric viruses, because they share many fundamental properties and features. Although a large number of studies focusing on various factors (i.e. pore water solution chemistry, fluid velocity, moisture content, temperature, and grain size) that affect biocolloid (bacteria, viruses) transport have been published over the past two decades, little attention has been given toward human adenoviruses (hAdVs). The main objective of this study was to evaluate the effect of pore water velocity on hAdV transport in water saturated laboratory-scale columns packed with glass beads. The effects of pore water velocity on virus transport and retention in porous media was examined at three pore water velocities (0.39, 0.75, and 1.22cm/min). The results indicated that all estimated average mass recovery values for hAdV were lower than those of coliphages, which were previously reported in the literature by others for experiments conducted under similar experimental conditions. However, no obvious relationship between hAdV mass recovery and water velocity could be established from the experimental results. The collision efficiencies were quantified using the classical colloid filtration theory. Average collision efficiency, α, values decreased with decreasing flow rate, Q, and pore water velocity, U, but no significant effect of U on α was observed. Furthermore, the surface properties of viruses and glass beads were used to construct classical DLVO potential energy profiles. The results revealed that the experimental conditions of this study were unfavorable to deposition and that no aggregation between virus particles is expected to occur. A thorough understanding of the key processes governing virus transport is pivotal for public health protection.

Influence of Pore-Water Velocity on Transport Behavior of Cadmium: Equilibrium versus Nonequilibrium

The effect of pore-water velocity (6–82 cm∕h) on transport behavior of cadmium was investigated in three soils using column experiments. The symmetrical bromide breakthrough curves indicated the absence of immobile water region at the applied pore-water velocities. The equilibrium model could not describe the breakthrough curves of cadmium, particularly at high pore-water velocities. The shorter residence times at higher pore-water velocities result in the more significant early breakthrough and long tailing, reflecting a higher degree of nonequilibrium. While both the two-site nonequilibrium model and the effective dispersion equilibrium model could describe the transport behavior of cadmium, the latter appeared to be inappropriate because it overestimated the dispersion coefficients as high as 6.3-fold compared with the coefficients obtained from the bromine breakthrough curves. Thus, the nonequilibrium model was used to analyze the results. With increasing pore-water velocity, the sorption rate coeffic...

Cadmium desorption in sand

Desorption of cadmium (Cd) from sand was studied by both batch and flow-through methods. Batch experiments were conducted at three pH values (5.5, 6.0 and 6.5). In each case, the amount of Cd desorbed was low compared with the quantity of Cd adsorbed previously. Desorption of Cd in the batch experiments can be described adequately by a Freundlich isotherm. The Freundlich isotherm coefficient, K f, increased with pH. Hysteresis between the sorption/desorption isotherms was observed in all batch experiments. Flow-through experiments in soil columns were conducted for the same three pH values, with the results used to determine transport and sorption/desorption parameters. Again, the desorption isotherms bore little resemblance to the corresponding adsorption isotherms. The experimental breakthrough curves were well fitted by a nonequilibrium desorption model, however the time scale of the desorption process was much larger than measured in batch experiments. This model was therefore rejected as lacking realism. A simple linear retardation (including hysteresis) model that utilises different isotherms was found to simulate column breakthrough curves well. The Freundlich isotherm coefficients, K f, in all batch and flow-through desorption experiments were different to values evaluated from the corresponding adsorption experiments. However, in contrast to adsorption, desorption in flow-through experiments was not noticeably affected by changes in pH. The effect of pore-water velocity on desorption was also studied at pH 6.0. No trend was established between flow velocity and the desorption coefficient.

Field-scale physical non-equilibrium transport in an alluvial gravel aquifer

The effects of pore-water velocity ( v), transport distance ( L), and hydraulic residence time ( L/ v m, in which subscript m refers to mobile region) on the fraction of mobile water ( β), mass transfer rate ( α), and longitudinal dispersivity ( λ) were evaluated from 53 breakthrough curves of a conservative tracer (rhodamine WT), observed from tracer experiments conducted at a field site. The individual effects of hydraulic conductivity ( K) and hydraulic gradient ( I), which determine v, were also evaluated. A three-dimensional non-equilibrium analytical model, N3DADE, was used to analyze the data, combined with a modified method of moments for estimation of pore-water velocity. At v values of 5–104 m/day in an alluvial gravel aquifer, the following significant tendencies were found: (1) β and α increased while λ decreased with increasing v and K; (2) α decreased while λ increased with increasing L and L/ v m; (3) there was no significant relationship between β and α and between β and λ, but α had a strong inverse correlation with λ. The results of this study suggest that both aquifer properties ( K) and fluid dynamics ( I and L/ v m), as well as transport scale ( L), were controlling factors in physical non-equilibrium transport and parameter interrelationships. β was largely a property of the aquifer medium, while α and λ appeared to be properties of both aquifer medium and fluid. There was significant `immobile water' in the alluvial gravel aquifer system, probably resulting from aquifer heterogeneity, in which slow mixing between zones of contrasting permeability was important.

Effects of Pore Water Velocity on the Transport of Arsenate

The potential for AsO4 to exhibit sorption-related nonequilibrium during transport has not been previously determined. Consequently, the objectives of this study were to determine the applicability of transport models based on the advec tion−dispersion equation (ADE) using various equilibrium and kinetic sorption expressions for describing AsO4 mobility. Saturated column transport experiments were performed using an applied AsO4 pulse at pore water velocities (PWVs) of 0.2, 1.0, 10, and 90 cm h-1 through a sand where the principal reactive phase was amorphous or poorly crystalline iron oxides. Observed AsO4 breakthrough curves (BTCs) demonstrated sorption-related nonequilibrium as evidenced by a leftward shift in observed BTCs and an increase in observed effluent recovery with increasing PWV. The use of independently derived equilibrium and kinetic sorption parameters in the ADE failed to describe observed AsO4 BTCs at all PWVs. Apparent sorption rate coefficients obtained by fitting observed BTCs to an nth-order kinetic model increased with increasing PWV. The time scales of the fitted rate coefficients are significantly longer than previously determined rate coefficients describing the chemical step of arsenate sorption by iron oxide minerals, suggesting that slower diffusional processes control the rate of arsenate sorption−desorption during transport.

Cadmium adsorption at different pore water velocities

Batch and column experiments were conducted to study the effect of pore water velocity on the adsorption of cadmium (Cd) onto a homogeneous nonaggregated porous medium under controlled environmental conditions. A wide range (5–214 cm/hr) of pore water velocities was imposed to observe adsorption effects. The results of batch experiments showed that adsorption coefficients are affected by the solid-to-liquid ratio, the adsorption coefficient increasing with this ratio. The batch-determined adsorption isotherm constants differed from those determined from column experiments. There was no evidence of nonequilibrium processes occurring in the flow-through system. For most of the column breakthrough curves, the linear equilibrium model adequately described the data. The fits of this model to the observed breakthrough data indicated that the adsorption coefficient remains fairly constant with velocity. However, flow-determined adsorption coefficients were clearly higher than batch-determined ones at the same solid-to-liquid ratio.

Sorption nonequilibrium during solute transport

The effects of pore-water velocity, solute hydrophobicity, and sorbent organic-carbon content on sorption nonequilibrium during solute transport were evaluated. Nonequilibrium transport was observed to increase with pore-water velocity, solute hydrophobicity, and sorbent organic-carbon content. Nonequilibrium transport of neutral organic compounds was not detected with low organic-carbon (TOC = 0.33 g kg −1) aquifer material, but was detected on higher organic sorbents from the unsaturated zone (TOC = 2.6 g kg −1) and the soil surface (TOC = 6.9 g kg −1). For solute-sorbent combinations yielding retardation factors > 2, nonequilibrium during transport was observed. After experimentally accounting for slow solute diffusion in the aqueous phase and isotherm nonlinearity as potential contributors to nonequilibrium solute transport, sorption nonequilibrium was attributed to slow solute diffusion within the organic-carbon matrix.