Pore Water Velocity Research Articles

Experimental and mathematical investigation of cotransport of clay and microplastics in saturated porous media

Microplastics in the subsurface cause groundwater contamination, thereby posing potential risks to human health and the ecosystem. Clay particles are ubiquitous in the subsurface and can interact and alter the transport behavior of microplastics. Hence, it is essential to understand the effect of clays on the transport behavior of microplastics to estimate the groundwater contamination potential. This study investigated the individual transport and cotransport of clay and microplastics under different pore-water velocities and sand types in saturated porous media through column experiments and mathematical modeling. Copresence of suspended microplastics retarded the transport of clay due to the preferential attachment of clay over microplastics on grain surfaces and the formation of clay-microplastic heteroaggregates which have a greater retention in sand than free clay and free microplastics. However, in contrast, cotransport with clay enhanced the transport of microplastics due to the lower affinity of microplastics than clay for deposition on grain surfaces and the lesser mass fraction of microplastics than clay in the heteroaggregates. The cotransport of clay and microplastics was successfully simulated using a two-way coupled model, which accounted for the retention of free clay and free microplastics in the sand, kinetics of clay-microplastics heteroaggregation, and heteroaggregate retention in the sand. The rates of heteroaggregation and heteroaggregate retention in sand decreased with increasing velocity and grain size, resulting in increased transport of clay and microplastics.

Electrical conductivity fluctuations as a tracer to determine time-dependent transport characteristics in hyporheic sediments

Assessing solute transport in riverbed sediments is important for quantifying the effective reactivity of hyporheic sediments and the magnitude of exchange flows between rivers and their river beds. A typical approach of estimating transport in riverbed sediments is by measuring natural tracers such as fluctuations of temperature or electrical conductivity (EC) and fitting models to them that assume time-independent travel time distributions, implying steady-state flow. Here, we use a transport parameterization that is based on the advection–dispersion equation (ADE) with coefficients that continuously vary in time. The ADE is solved numerically and its solution is fitted to measured EC time series using Bayesian parameter inference. A continuous function of model parameters is constructed by smoothly interpolating between point values with different temporal resolution, and Tikhonov regularization is used to avoid spurious parameter fluctuations. The approach is tested using EC time series synchronously measured in river water and hyporheic porewater of two urban rivers in Germany and one urban river in South Australia. For all datasets the goodness of fit was improved by introducing a time-dependent EC offset. Estimated porewater velocities were highly transient in three out of the four datasets with values increasing by up to a factor of six over the course of a day. Flux transients were likely related to both variations of hydraulic gradients along and spatial shifting of flow paths. Comparison to stationary, non-parametric deconvolution indicated that transient flow may induce multimodality in stationary travel time distributions. Given the high temporal dynamics of transport in the streambed sediments of the three investigated urban rivers, we envision that the presented model is also a valuable tool to improve the assessment of reactive transport in riverbed sediments.

Slope stability considering multi-fissure seepage under rainfall conditions

Fissures form the channel for rainwater infiltration, which accelerate the infiltration of rainwater into slope bodies, hence its important impact on the seepage field and stability of the slope. In this paper, taking one landslide of Liang-Wan freeway as the research object, firstly, the equivalent permeability coefficient method is used to homogenize the fissured soil. Then considering the boundary conditions of rainfall infiltration and groundwater level, a fluid–structure coupling model is established based on saturated–unsaturated seepage theory, and evolution characteristics of seepage, displacement and stress of the slope are studied. Based on these, the slope stability coefficient is determined. The results show that the rising rate of pore water pressure and volume water content of topsoil increases when multi-fissure seepage is considered, and the pore water velocity is larger in the local seepage range of fissures. With the increase of buried depth, the closer to groundwater level, the influence of multi-fissure seepage gradually weakens. The theoretical calculation results of slope displacement are more consistent with the field monitoring results. With the increase of rainfall time, the stability coefficient of slope decreases gradually, and the rate and range of decrease are greater.

Bacterial transport mediated by micro-nanobubbles in porous media

Determining the role of micro-nanobubbles (MNBs) in controlling the risk posed by pathogens to soil and groundwater during reclaimed water irrigation requires clarification of the mechanism of how MNBs block pathogenic bacteria. In this study, real-time bioluminescence imaging was used to investigate the effects of MNBs on the transport and spatiotemporal distribution of bioluminescent Escherichia coli 652T7 strain in porous media. The presence of MNBs significantly increased the retention of bacteria in the porous media, decreasing the maximum relative effluent concentration (C/C0) by 78 % from 0.97 (without MNBs) to 0.21 (with MNBs). The results suggested that MNBs provided additional sites at the air-water interface (AWI) for bacterial attachment and acted as physical obstacles to reduce bacterial passage. These effects varied with environmental conditions such as solution ionic strength and pore water velocity. The results indicated that MNBs enhanced electrostatic attachment of bacteria at the AWI and their mechanical straining in pores. This study suggests that adding MNBs in pathogen-containing water is an effective measure for increasing filtration efficiency and reducing the risk of pathogenic contamination during agricultural irrigation.

Spatial, Seasonal, and Diel Controls of Nitrogen‐Carbon‐Oxygen Cycling During Lake‐Water Infiltration to an Aquifer

AbstractMany freshwater lakes are groundwater flow‐through systems. Although lakes commonly are considered to be sinks for nitrogen inputs, relatively little is known about carbon and nitrogen export from lakes to groundwater. The current study focused on lake‐bottom biogeochemical processes accompanying the transport of nitrogen, dissolved oxygen (O2), and dissolved organic carbon (DOC) during lake‐water recharge from a groundwater flow‐through lake. Lake‐water and porewater (15–100 cm below lakebed) samples were collected along transects within the lake downwelling zone. Infiltrating porewater O2 and DOC concentrations decreased with depth while nitrate (NO3−) concentrations increased, indicating nitrification of organic matter within the profiles. The depth of NO3− production and transport was seasonally dependent. In winter, NO3− and O2 were exported beyond 100‐cm depth; whereas in summer, shallow nitrification zones were underlain by deeper NO3− reduction zones, and diel patterns of O2 and NO3− penetration depths were observed. Microbial community compositions and stable isotope profiles (δ15N[NO3−], δ18O[NO3−], δ18O[O2]) were consistent with apparent C–N–O reaction stoichiometries indicating O2 reduction and nitrification in shallower porewater, followed by varying NO3− reduction at depth. Maximum porewater NO3− concentrations (∼10–20 μM) were limited by infiltrating O2 concentrations and C/N ratios of reacting organic matter. Lake‐water level variations caused changes in shoreline position and porewater velocities, while variations in lake‐water temperature, DOC, and O2 contributed to changes in reaction rates and depth of O2 and NO3− penetration into the lakebed. The quality of groundwater recharged by lake water reflected temporally and spatially varying physical and biogeochemical processes in the sediment porewater.

ASSESSING LOCAL PORE WATER VELOCITY ALONG PREFERENTIAL FLOW IN EARTHEN DAM USING SALT TRACER

The local pore water velocity along the preferential flow path signifies the hydraulic parameter responsible for erosion within an earthen dam. This study introduces an empirical approach to ascertain the local pore water velocity within the earth dam's leakage zones by monitoring the travel time of the salt tracer through the corresponding electric potential anomalies in the ground. The alignment of electric potential anomalies with the movement of the salt tracer plume over time was confirmed through experiments on a physical model coupled with numerical simulations. The pore water velocity, calculated based on the location of the maximum electric potential anomaly, demonstrated excellent agreement with the experimental value, with an error of under 6%. For illustrative purposes, a field-scale salt tracer test was conducted at a leaking earthen dam in Vietnam. The tracer breakthrough curve originating from the leakage point revealed that the seepage water's travel time is approximately 40 days. The results of electric potential anomalies over time indicate that the pathway of seepage flow from upstream to the leakage point forms a horizontal V-shape, with the local pore water velocity ranging from 1.7 to 9.9x10-5 m/s. These local pore water velocities are subsequently compared with the critical seepage velocity to assess in-situ information regarding the internal erosion status of the target dam.

Performance of Six Methods in Determining Solute-Transport Parameters from Breakthrough Curves

ABSTRACT The solute-transport parameters are usually determined by fitting solute-transport models to measured solute breakthrough curves (BTCs). But due to different initial and boundary conditions and approximations, the solute-transport models perform with different accuracy levels, making it difficult to select the most appropriate one for a particular type of breakthrough experiment. This study evaluated the performances of four commonly used solute-transport models and two methods by analyzing the BTCs of 81 pulse-type breakthrough experiments with an inert solute (CaCl2) in three different soil-types and identified their strengths and weaknesses. The employed models were: the equilibrium (EQ) and non-equilibrium (NEQ) models of the CXTFIT codes, and analytical solutions of Lapidus and Amundson (LA) and Lindstrom and coauthors (LI), while the two methods were: the method of moments (MOM), and transfer-function (TF) method, the latter still remains less studied. The MOM accurately determined mean velocity (V) and travel time () of the solute but poorly determined its mass-dispersion number (N), dispersion coefficient (D), and dispersivity (); note that, for inert solutes, V is identical to the mean pore-water velocity. The NEQ model always provided inconsistent solute-transport parameters, while the LA and LI models sometimes provided inconsistent parameters; all three models suffered from the problems of convergence during the fitting process. The EQ model and the TF method resulted in the consistent solute-transport parameters, with the latter method performing better. The results of this study will guide to select appropriate model/method to determine reliable solute-transport parameters from pulse-type solute-transport experiments.

Measuring pore water velocities and dynamic contact angles at unstable wetting fronts

The imbibition of fluids in porous media has been studied widely. Still, processes of preferential flow under gravity due to instability at the wetting front, crucial in groundwater contamination, have yet to be fully understood. Recent theories using dynamic contact angles could describe unstable flow phenomena but have not been proven experimentally. Therefore, infiltration experiments in small sand-filled chambers were conducted to explore the effect of dynamic contact angles. A high-speed camera recorded pore invasion at the unstable imbibition liquid front. A tensiometer recorded the matric potential. The results show that water moved in milliseconds through a small pore at the wetting front, followed by a stationary period. The maximum observed pore water velocity was 25 cm/s, exceeding the saturated hydraulic conductivity by three orders of magnitude. The relationship between dynamic contact angle and water velocity found by Hoffman in glass tubes could describe which was observed in soils.

Macropore flow in relation to the geometry and topology of soil macropore networks: Re-visiting the kinematic wave equation

The rapid flow of water through soil macropores significantly affects the partitioning of precipitation between surface runoff and infiltration and also the rate of solute transport in soil, both of which have an impact on the risk of contamination of surface water and groundwater. The kinematic wave equation is often employed as a model of gravity-driven water flow through soil macropores. The exponent in this simple model influences the pore water velocity attained in the macropores at any given input rate and is usually estimated by inverse modelling against measured flow rates or water contents. In theory, the exponent in the kinematic wave equation should depend on the geometry and topology of the conducting macropore networks, although these relationships have not so far been investigated. In this study, we related metrics of soil structure derived from X-ray images to values of the kinematic exponent estimated from drainage experiments on twenty-two columns sampled at three different field sites under two contrasting land uses and at three different depths.We found that smaller values of the exponent in the kinematic wave equation, which would equate to more rapid flow of water through soil macropores, were found in plough pan and subsoil columns of smaller macroporosity, for which biopores comprised a significant fraction. The macroporosity in these columns was more vertically oriented and poorly inter-connected, though still continuous across the sample. In contrast, topsoil columns from both arable land and grassland had better connected, denser and more isotropically-distributed macropore networks and larger values of the kinematic exponent. Our results suggest that for predictive modelling at large scales, it may be feasible to estimate the kinematic exponent using class pedotransfer functions based on pedological information such as land use and horizon type.

Hyporheic exchange in a compound channel under unsteady flow: Numerical simulations

Hyporheic exchange (HE) within compound channels under unsteady surface water flow conditions plays an important role in the river-floodplain ecosystem. Numerical simulations were applied to investigate the HE in a compound channel under unsteady flow. High velocity regions were observed in the hyporheic zone near the banks of the floodplain and of the main channel, and the distribution of the pore water velocity in this region was seen to significantly change under the unsteady conditions. The velocity peaked during the rising and falling phases and had the smallest value when the inlet discharge of surface water had the maximum and minimum values. The changes in the relationship between surface groundwater recharge and discharge were observed in the context of a lateral water level threshold of approximately 1.2 m. About 75 % of the hyporheic exchange flux was due to the dynamic pressure. Comparing the hyporheic exchange in the compound channel driven by static pressure with that in a rectangular channel, it was found that the floodplain decreased the hyporheic exchange. These findings provide a better understanding of the HE driven by unsteady flow in a compound channel and may be applied in combined flood management and river ecological restoration.

Biotransformation of 6:2 fluorotelomer sulfonate and microbial community dynamics in water-saturated one-dimensional flow-through columns

Nearly all per- and polyfluoroalkyl substances (PFAS) biotransformation studies reported to date have been limited to laboratory-scale batch reactors. The fate and transport of PFAS in systems that more closely represent field conditions, i.e., in saturated porous media under flowing conditions, remain largely unexplored. This study investigated the biotransformation of 6:2 fluorotelomer sulfonate (6:2 FTS), a representative PFAS of widespread environmental occurrence, in one-dimensional water-saturated flow-through columns packed with soil obtained from a PFAS-contaminated site. The 305-day column experiments demonstrated that 6:2 FTS biotransformation was rate-limited, where a decrease in pore-water velocity from 3.7 to 2.4 cm/day, resulted in a 21.7–26.1 % decrease in effluent concentrations of 6:2 FTS and higher yields (1.0–1.4 mol% vs. 0.3 mol%) of late-stage biotransformation products (C4C7 perfluoroalkyl carboxylates). Flow interruptions (2 and 7 days) were found to enhance 6:2 FTS biotransformation during the 6–7 pore volumes following flow resumption. Model-fitted 6:2 FTS column biotransformation rates (0.039–0.041 cmw3/gs/d) were ∼3.5 times smaller than those observed in microcosms (0.137 cmw3/gs/d). Additionally, during column experiments, planktonic microbial communities remained relatively stable, whereas the composition of the attached microbial communities shifted along the flow path, which may have been attributed to oxygen availability and the toxicity of 6:2 FTS and associated biotransformation products. Genus Pseudomonas dominated in planktonic microbial communities, while in the attached microbial communities, Rhodococcus decreased and Pelotomaculum increased along the flow path, suggesting their potential involvement in early- and late-stage 6:2 FTS biotransformation, respectively. Overall, this study highlights the importance of incorporating realistic environmental conditions into experimental systems to obtain a more representative assessment of in-situ PFAS biotransformation.

Minimizing the exposure risk from groundwater pollution by optimizing the temporal extraction patterns.

Optimization models for minimizing pollutant exposure from groundwater resources require time and resources that many communities might not have ready access to due to their economic conditions. In such cases, it might be useful to develop a "rule of thumb" approach for suggestions in case of uncertainties and inadequate means to address these uncertainties. Monte Carlo analysis was performed for a simplified groundwater system, and the effects of temporal extraction patterns, distance to the pollution source, dispersivity, pollutant pulse period, pore water velocity, and decay were examined for minimizing the high pollutant exposure risk from the extracted groundwater. Results indicate that, in a high uncertainty scenario, the best bet for minimizing the risk of high pollutant exposure would be to adopt a frequent temporal extraction pattern and supply the water as a mixture of extractions from multiple wells spread over an area. These findings can be used as a "rule of thumb" wherever time and resources might be the limiting factors.

Performance expectation of coal waste in permeable reactive barrier for removal of cadmium considering contamination level and pore water velocity

The effectiveness and longevity of permeable reactive barriers (PRBs) depend on the performance of the reactive materials and the subsurface environment. The relationship of the groundwater velocity on performance of coal waste for the heavy metal removal was reported in our previous study. In this study, we investigated the performance and longevity of coal waste as a PRB material for the removal of Cd considering subsurface environmental conditions such as contamination level and groundwater velocity. The artificial groundwater contaminated by Cd were prepared with various concentrations ranging from 10 to 100 mg L−1. Lab-scale column experiments were conducted using coal waste filled columns by injecting the artificial groundwater. The breakthrough curves were analyzed advection dispersion equation coupled with equilibrium sorption model to determine the retardation factor. The Cd breakthrough curves exhibited different retardation with respect to the contamination levels. The Cd transport was more retarded as the contamination level lowered. The relationship between the retardation factor and the contamination levels could be explained with empirical equations based on non-linear sorption isotherms. By adopting the velocity dependency of sorbent performance in our previous study, transport of Cd within coal waste was simulated under various subsurface environmental conditions to construct the longevity function. The function could be used for the longevity prediction of coal waste as a PRB material considering groundwater velocity and contamination level in subsurface environment.

Percolation characteristics of pore fluid during hydrate depressurization dissociation from multi-phase multi-field coupling analysis

The recovery of gas from hydrate-bearing reservoirs is essentially controlled by pore fluid flow. Four numerical models with different multi-phase multi-field were established in this work based on the thermo-hydro-chemical model, considering ice phase, fluidized sand phase, depositing sand phase, and ice, fluidized sand and depositing sand phases, respectively. These above models were used to simulate the dissociation of hydrate-bearing cores by depressurization and to investigate the effects of freezing and melting behavior, sand production behavior, and their combined behavior on pore fluid migration. By comparing the simulations with and without the ice phase, it can be found that the permeability decreases sharply due to ice generation resulting in a decrease of 89.16% in pore-water velocity and of 99.95% in pore-gas velocity. This indicates an inhibition of pore fluid migration by freezing behavior. Meanwhile, it can be found that the ice melting behavior can temporarily promote pore-water migration but has no obvious promotion for pore-gas migration. It is difficult for the pore fluid to percolate in the regions prone to ice generation during hydrate dissociation. In addition, the permeability at 300 min with sand detachment is 16% higher than that without sand detachment. But at the same time, the pore water velocity is 28.44% smaller since sand detachment behavior does not directly power pore fluid migration. By comparing the simulations with and without the combined behavior, it can be concluded that the freezing behavior and the sand production behavior can jointly weaken the pore fluid mobility. Significantly, the former has a stronger negative effect on the pore fluid mobility than the latter during the depressurization dissociation of the hydrate-bearing sediment.

Evaluation of Dual Domain Mass Transfer in Porous Media at the Pore Scale.

Dual-porosity models are often used to describe solute transport in heterogeneous media, but the parameters within these models (e.g., immobile porosity and mobile/immobile exchange rate coefficients) are difficult to identify experimentally or relate to measurable quantities. Here, we performed synthetic, pore-scale millifluidics simulations that coupled fluid flow, solute transport, and electrical resistivity (ER). A conductive-tracer test and the associated geoelectrical signatures were simulated for four flow rates in two distinct pore-scale model scenarios: one with intergranular porosity, and a second with an intragranular porosity also defined. With these models, we explore how the effective characteristic-length scale estimated from a best-fit dual-domain mass transfer (DDMT) model compares to geometric aspects of the flow field. In both model scenarios we find that: (1) mobile domains and immobile domains develop even in a system that is explicitly defined with one domain; (2) the ratio of immobile to mobile porosity is larger at faster flow rates as is the mass-transfer rate; and (3) a comparison of length scales associated with the mass-transfer rate (Lα ) and those associated with calculation of the Peclet number (LPe ) show LPe is commonly larger than Lα . These results suggest that estimated immobile porosities from a DDMT model are not only a function of physically mobile or immobile pore space, but also are a function of the average linear pore-water velocity and physical obstructions to flow, which can drive the development of immobile porosity even in single-porosity domains.

Contaminated water flow modelling through the porous media by using fractional advection-dispersion equation (FADE)

ABSTRACT This study aims to evaluate the validity of Fractional Advection-Dispersion equation (FADE) in breakthrough curves in saturated homogenous soil media, i.e. clay, and to examine the three primary restraints, pore water velocity, dispersion coefficient, and order of fractional differentiation which are impacting solute transport behavior (Fickian or Non-Fickian). In this study, the FADE framework used to characterize the transport process at depth 50 cm soil (clay). FADE Main based on FORTRAN, to estimate the parameters of FADE including fractional differentiation (λ), the dispersive coefficient (D), and the average pore-water velocity (v). If the value of is equal to or greater than 2, the transport is said to be Fickian otherwise is non-Fickian. In this study, the fractional differentiation of non-Fickian behavior was found 1.85 which is less than 2. On the other hand, early long-time tailing to the soil media showed an increase in the dispersion coefficient (D) for FADE and higher values in differentiation coefficient (λ = 1.85–1.99). The results of breakthrough curves (BTCs) of relative concentration (C/C0) show that it is best fitted by using FADE. For the assessment of fitting, Root Mean Square Error (RMSE) and determination Coefficient was used to find the quality of fit. .

Predicting bacterial transport through saturated porous media using an automated machine learning model.

Escherichia coli, as an indicator of fecal contamination, can move from manure-amended soil to groundwater under rainfall or irrigation events. Predicting its vertical transport in the subsurface is essential for the development of engineering solutions to reduce the risk of microbiological contamination. In this study, we collected 377 datasets from 61 published papers addressing E. coli transport through saturated porous media and trained six types of machine learning algorithms to predict bacterial transport. Eight variables, including bacterial concentration, porous medium type, median grain size, ionic strength, pore water velocity, column length, saturated hydraulic conductivity, and organic matter content were used as input variables while the first-order attachment coefficient and spatial removal rate were set as target variables. The eight input variables have low correlations with the target variables, namely, they cannot predict target variables independently. However, using the predictive models, input variables can effectively predict the target variables. For scenarios with higher bacterial retention, such as smaller median grain size, the predictive models showed better performance. Among six types of machine learning algorithms, Gradient Boosting Machine and Extreme Gradient Boosting outperformed other algorithms. In most predictive models, pore water velocity, ionic strength, median grain size, and column length showed higher importance than other input variables. This study provided a valuable tool to evaluate the transport risk of E.coli in the subsurface under saturated water flow conditions. It also proved the feasibility of data-driven methods that could be used for predicting other contaminants' transport in the environment.

Monitoring of Thick Vadose Zone Water Dynamics Under Irrigation Using a 48 m Deep Caisson at the Luancheng Critical Zone Observatory

AbstractUnderstanding soil water dynamics in the deep vadose zone (DVZ, below the root zone) of irrigated farmland is critical for estimating groundwater recharge and assessing the impacts of agricultural activities on groundwater quality. However, the state and movement of soil water in the DVZ remain poorly understood owing to the lack of in‐situ observations. Here, based on a 48‐m deep caisson (Critical Zone Observatory, CZO) established in a typical irrigated farmland in the piedmont region of the North China Plain, we extended the monitoring of soil water content and matric potential across the entire vadose zone profile. We found that a high soil water content and matric potential (close to or slightly higher than the field capacity) was maintained in the DVZ of typical irrigated cropland. The response of soil water in the DVZ to the water inputs at the ground surface was predominantly sequentially delayed with an increase in depth in the DVZ. As a result, the wetting front propagated into the soil layer to a depth of 38 m within 228 days. Moreover, high soil moisture contents and matric potentials in the DVZ resulted in a dramatic but explicable difference between the estimated average pore water velocity (0.73 m year−1) and the wetting front propagation rate (0.16 m day−1). This study demonstrates the unreported rapid response of soil water in the DVZ to water input at the ground surface under intensively irrigated cropland and could promote a better understanding of groundwater recharge processes with a thick unsaturated zone.

Long-term monitoring and modeling of PAHs in capped sediments at the Grand Calumet River

The assessment of a cap for remediation of sediments requires long-term monitoring because of the slow migration of contaminants in porous media. In this study, coring and passive sampling tools were used to assess the transport and degradation of polycyclic aromatic hydrocarbons (PAHs) in an amended cap (sand + Organoclay® PM-199) in the Grand Calumet River (Indiana, USA) during four sampling events from 2012 to 2019. Measurements of three PAHs (phenanthrene (Phe), pyrene (Pyr) and benzo[a]pyrene (BaP), representing low, medium, and high molecular weight compounds, respectively) showed a difference of at least two orders of magnitude between bulk concentrations in the native sediments and the remediation cap. Averages of pore water measurements also showed lower levels in the cap respective to the native sediments by a factor of at least 7 for Phe and 3 for Pyr. In addition, between the baseline (BL), which corresponds to observations from 2012 to 2014, and the measurements in 2019, there was a decrease in depth-averaged pore water concentrations of Phe (C2019/CBL=0.20−0.07+0.12 in sediments and 0.27−0.10+0.15 in cap) and Pyr (C2019/CBL=0.47−0.12+0.16 in sediments and 0.71−0.20+0.28 in the cap). In the case of BaP in pore water, no change was observed in native sediments (C2019/CBL=1.0−0.24+0.32) and there was an increase in the cap (C2019/CBL=2.0−0.54+0.72).Inorganic anions and estimates of pore water velocity along with measurements of PAHs were used to model the fate and transport of contaminants. The modeling suggested that degradation of Phe (t1/2=1.12−0.11+0.16 years) and Pyr (t1/2=5.34−1.8+5.3 years) in the cap is faster than migration, thus the cap is expected to be protective of the sediment-water interface indefinitely for these constituents. No degradation was noted in BaP and the contaminant is expected to reach equilibrium in the capping layer over approximately 100 years if there exists sufficient mass of BaP in the sediments and there is no deposition of clean sediment at the surface.

Lattice Boltzmann method for solute transport in dual-permeability media

Dual-permeability models are currently under active investigation for predicting the flow and transport of pollutant concentration in porous media. The basic idea of these models is usually to separate the medium into two distinct flow regions with separate hydraulic and solute movement properties. Most of the dual-permeability approaches presented to date assume the reaction terms to be independent of space. For local reaction processes in soils, however, the depth effects are important and should be incorporated into the dual-permeability models. To reveal the transport phenomenon in such case, the best way is to obtain an exact solution with a suitable analytical approach, but it may be hard or even impossible for this general scenario. As such, this paper seeks an alternative method, and propose a lattice Boltzmann method for solute transport in dual-permeability media. The Chapman–Enskog analysis shows that the present model can recover the governing equations correctly. The capability and accuracy of this model is first verified by two problems: the non-reactive mass transfer in dual-permeability porous media, and the solute transport in a depth-dependent soil. With this model, we further studied the contaminant transport in dual-permeability soil with depth-dependent reaction rates. The numerical results show that when the exchange coefficient is relatively smaller, there are usually two peaks for the breakthrough curves (BTCs), whereas it degenerates to a single peak as the exchange coefficient increases. In addition, we note that the distributions of the concentration in different domains are nearly the same for a larger exchange coefficient. As for the pore water velocity, it is found that when the difference in the pore water velocity in the two domains is significant, the BTC exhibits a bimodal distribution. However, there is only one peak for the BTCs when the above difference is insignificant. Further, as compared with the constant-reaction rate cases, we observe that the BTCs obtained for the media with depth-dependent rates lie in the intermediate zone defined by the above two extremes. Finally, we adopt the current approach to simulate the solute transport in an Andisol and observe that the difference between the numerical and experimental results is insignificant, indicating our approach is capable in modeling solute transport in porous media.