Concentration Breakthrough Research Articles

Evaluation of boron uptake by anion exchange resins in tap and geothermal water matrix

The efficiency of commercial basic anion exchange resins (Amberlite IRA743, Purolite S108, Purolite A200, Dowex 1x8) in the removal of boron from natural water samples was evaluated under typical conditions of water treatment. Batch ion exchange experiments using distilled water spiked with 5 mg B/L at pH 7, indicated that weakly basic N-methylglucamine functionalized resins (Amberlite IRA743, Purolite S108) were able to reduce boron concentration below 1 mg/L, whereas strong base quaternary ammonium resins (Purolite A200, Dowex 1x8) were found completely ineffective to capture boron. The study of pH dependence of uptake efficiency suggests an optimum range of pH 4-9 for both Amberlite IRA743 and Purolite S108. The ion-exchange capacity for both resins was estimated by ion exchange column experiments to be around 3.5 mg B/g for residual concentration 1 mg B/L in tap and geothermal water matrix. Regeneration of the columns was achieved by acid washing followed by stabilization of the resin using NaOH, NaHCO3 and mixed NaHCO3/Na2SO4 solutions. Enhancement of ion exchange capacity was observed after the stabilization by NaHCO3/Na2SO4 solution. The efficiency of Purolite S108 resin was maintained for at least 4 ion exchange cycles without significant losses. Furthermore, in case of geothermal water from Anthemounta basin, which does not contain NO3-, the stabilization by the mixed 20 g/L NaHCO3/ Na2SO4 solution optimized selectivity to boron and improved uptake capacity to 4.3 mg/g at breakthrough concentration 1 mg/L.

Analytical model for straining-dominant large-retention depth filtration

A method for determining pore size distribution for a porous medium from long-time straining-dominant mono-sized suspension injection is proposed. The aim is avoiding using multiple-size particle suspensions in short-term challenge tests. We derive an exact solution for long-time non-linear injection of particles with the same size, where the non-linearity is determined by accumulation of strained particles and alternation of porous medium properties. The exact downscaling procedure, determining the evolution of pore size distribution from an exact solution of large-scale equations is developed. It shows the preferential plugging of large pores during mono-sized particle transport, explaining well-posed formulation of pore-size distribution tuning from breakthrough concentrations and retention profiles. The laboratory tests on long-term mono-sized injections, where straining dominance has been monitored by DLVO-repulsion between particles and porous media, have been performed. High quality match of the breakthrough concentrations by the analytical model has been observed. The tuned-model-based prediction of the retained profiles also shows close agreement with the experimental data, which validates the proposed method.

Pore-scale simulations of concentration tails in heterogeneous porous media

The retention of contaminants in the finest and less-conductive regions of natural aquifer is known to strongly affect the decontamination of polluted aquifers. In fact, contaminant transfer from low to high mobility regions at the back end of a contaminant plume (i.e. back diffusion) is responsible for the long-term release of contaminants during remediation operation. In this paper, we perform pore-scale calculations for the transport of contaminant through heterogeneous porous media composed of low and high mobility regions with two objectives: (i) study the effect of permeability contrast and solute transport conditions on the exchange of solutes between mobile and immobile regions and (ii) estimate the mass of contaminants sequestered in low mobility regions based on concentration breakthrough curves.

Predicting the propagation of concentration and saturation fronts in fixed-bed filters

The phenomenon of adsorption is widely exploited across a range of industries to remove contaminants from gases and liquids. Much recent research has focused on identifying low-cost adsorbents which have the potential to be used as alternatives to expensive industry standards like activated carbons. Evaluating these emerging adsorbents entails a considerable amount of labor intensive and costly testing and analysis. This study proposes a simple, low-cost method to rapidly assess the potential of novel media for potential use in large-scale adsorption filters. The filter media investigated in this study were low-cost adsorbents which have been found to be capable of removing dissolved phosphorus from solution, namely: i) aluminum drinking water treatment residual, and ii) crushed concrete. Data collected from multiple small-scale column tests was used to construct a model capable of describing and predicting the progression of adsorbent saturation and the associated effluent concentration breakthrough curves. This model was used to predict the performance of long-term, large-scale filter columns packed with the same media. The approach proved highly successful, and just 24–36 h of experimental data from the small-scale column experiments were found to provide sufficient information to predict the performance of the large-scale filters for up to three months.

A study of the distribution of the concentration of mercury ions along a fixed ionite bed in the sorption treatment of water

A new approach to the mathematical description of the breakthrough curve by using the space−time concentrations profile of contaminants along the fixed sorbent bed in the process of sorption purification of water is proposed. For the breakthrough concentration C of contaminant in water effluent from the fixed bed, an expression for its time dependence is derived. The space−time concentration profile of mercury adsorbed from a polluted water flow on Amberlite® GT-73 cationite was determined experimentally. Using the formula derived in the present paper, the time dependence C(t) is calculated, known as a breakthrough curve. The latter was compared with a curve determined experimentally using traditional methods. A close match between the two curves is observed. It is established that, during the adsorption process, adsorbed mercury ions are redistributed between different parts of the fixed bed. It was found that, up to the time of breakthrough (200 min), the concentration of mercury in water flowing out from the fixed Amberlite® GT-73 bed, ranges within 1−5 μg/L, i.e., is below the maximum permissible concentration, even if the incoming water contains mercury in a concentration of up to 70 mg/L, which corresponds to the typical level of pollution of industrial wastewater.

Transport of E. coli in saturated and unsaturated porous media: effect of physiological state and substrate availability

Saturated and unsaturated sand and soil column experiments were conducted to study the complex interaction between the effects of biological and hydrological factors on the transport of bacteria through a porous medium. These experiments were conducted with continuous input of bacteria and substrate at the inlet to reflect the groundwater contamination caused by leaking septic tanks and leach pits. Experiments were conducted with metabolically active and inactive Escherichia coli. Cell surface characteristics and batch experimental data for bacterial attachment were correlated with the transport behaviour in continuous column studies. Normalized breakthrough concentration for metabolically inactive cells (C/C 0 = 0.74 in sand) was higher than that for active cells (C/C 0 = 0.68 in sand) owing to change in cell surface characteristics. A similar trend was observed in the case of transport through soil columns. There was an increase of 29.5% in the peak C/C 0 value at the outlet when the flow velocity was increased from 0.0535 cm/h (C/C 0 = 0.61) to 0.214 cm/h (C/C 0 = 0.79) in case of sand columns. However, this difference was only 20% in case of soil columns. Peak normalized concentrations at the outlet were less in soil column as compared to those in sand column because of lesser grain size. Unlike the earlier studies with pulse input, present experiments with continuous input of metabolically active bacteria along with substrate indicated that the normalized concentration at the outlet increased with increased concentration at the inlet. It was found that unsaturated conditions led to more retention of bacteria in both sand and soil columns. In case of sand columns, the normalized concentration at the exit reduced to as much as 0.46. It was also found that the existing mathematical models based on macroscopic advection–dispersion–filtration equations could satisfactorily simulate the bacterial transport except in a case where the substrate was added to the bacteria in the column studies.

The role of interfacial tension in colloid retention and remobilization during two-phase flow in a polydimethylsiloxane micro-model

The work presented here consisted of steady-state and transient two-phase flow experiments on the role of interfacial tension in colloid transport. Experiments were performed in a polydimethylsiloxane (PDMS) micro-model containing a network of pores with a mean pore size of 30µm. The flow network covered an area of 1mm×10mm. Water and Fluorinert FC43 were used as the two immiscible liquids. Since PDMS is a hydrophobic material, Fluorinert was the wetting phase, and water was the non-wetting phase in this micro-model. The interfacial tension was changed by adding a Fluorinert-soluble surfactant into Fluorinert FC43 to change the interfacial tension from 55mN/m to 30mN/m. The colloids were fluorescent carboxylate-modified polystyrene microspheres and 300nm in diameter. We directly observed colloid movement using confocal microscopy. We also obtained colloid concentration breakthrough curves by measuring the fluorescent intensities in the outlet of the micro-model.The breakthrough curves showed that during steady-state unsaturated flow, fewer colloids were retained in the system when interfacial tension was lower. During transient flow, more colloids were remobilized by the moving Fluorinert-water interfaces (FWIs) and Fluorinert-water-solid contact lines (FWSCs) under high interfacial tension. Visualization results showed that, at low interfacial tension, the fluid-fluid interfaces were almost flat; thus, less interfacial area was available for colloid attachment.Generally, confocal images and measured breakthrough curves clearly demonstrate the effect of interfacial tension on colloid retention and remobilization.

Tracking effluent discharges in undisturbed stony soil and alluvial gravel aquifer using synthetic DNA tracers

With the intensification of human activities, fresh water resources are increasingly being exposed to contamination from effluent disposal to land. Thus, there is a greater need to identify the sources and pathways of water contamination to enable the development of better mitigation strategies. To track discharges of domestic effluent into soil and groundwater, 10 synthetic double-stranded DNA (dsDNA)33Double-stranded DNA (dsDNA); single-stranded DNA (ssDNA); base-pair (bp); bromide (Br); polymerase chain reaction (PCR); colony-forming units (cfu); relative mass recovery (RB); relative transport speed (RV); relative dispersion (RS); concentration breakthrough curve (BTC). tracers were developed in this study. Laboratory column experiment and field groundwater and soil lysimeter studies were carried out spiking DNA with oxidation-pond domestic effluent. The selected DNA tracers were compared with a non-reactive bromide (Br) tracer with respect to their relative mass recoveries, speeds of travel and dispersions using the method of temporal moments. In intact stony soil and gravel aquifer media, the dsDNA tracers typically showed earlier breakthrough and less dispersion than the Br tracer, and underwent mass reduction. This suggests that the dsDNA tracers were predominantly transported through the network of larger pores or preferential flow paths. Effluent tracking experiments in soil and groundwater demonstrated that the dsDNA tracers were readily detectable in effluent-contaminated soil and groundwater using quantitative polymerase chain reaction. DNA tracer spiked in the effluent at quantities of 36μg was detected in groundwater 37m down-gradient at a concentration 3-orders of magnitude above the detection limit. It is anticipated it could be detected at far greater distances. Our findings suggest that synthetic dsDNA tracers are promising for tracking effluent discharges in soils and groundwater but further studies are needed to investigate DNA-effluent interaction and the impact of subsurface environmental conditions on DNA attenuation. With further validation, synthetic dsDNA tracers, especially when multiple DNA tracers are used concurrently, can be an effective new tool to track effluent discharge in soils and groundwater, providing spatial estimation on the presence or absence of contamination sources and pathways.

Experimental study on retardation of a heavy NAPL vapor in partially saturated porous media

Abstract. Non-aqueous-phase liquid (NAPL) contaminants introduced into the unsaturated zone spread as a liquid phase; however, they can also vaporize and migrate in a gaseous state. Vapor plumes migrate easily and thus pose a potential threat to underlying aquifers. Large-scale column experiments were performed to quantify partitioning processes responsible for the retardation of carbon disulfide (CS2) vapor in partially saturated porous media. The results were compared with a theoretical approach taking into account the partitioning into the aqueous phase as well as adsorption to the solid matrix and to the air–water interface. The experiments were conducted in large, vertical columns (i.d. of 0.109 m) of 2 m length packed with different porous media. A slug of CS2 vapor and the conservative tracer argon was injected at the bottom of the column followed by a nitrogen chase. Different seepage velocities were applied to characterize the transport and to evaluate their impact on retardation. Concentrations of CS2 and argon were measured at the top outlet of the column using two gas chromatographs. The temporal-moment analysis for step input was employed to evaluate concentration breakthrough curves and to quantify dispersion and retardation. The experiments conducted showed a pronounced retardation of CS2 in moist porous media which increased with water saturation. The comparison with an analytical solution helped to identify the relative contributions of partitioning processes to retardation. Thus, the experiments demonstrated that migrating CS2 vapor is retarded as a result of partitioning processes. Moreover, CS2 dissolved in the bulk water is amenable to biodegradation. The first evidence of CS2 decay by biodegradation was found in the experiments. The findings contribute to the understanding of vapor-plume transport in the unsaturated zone and provide valuable experimental data for the transfer to field-like conditions.

Efficiency of Iron-Based Oxy-Hydroxides in Removing Antimony from Groundwater to Levels below the Drinking Water Regulation Limits

This study evaluates the efficiency of iron-based oxy-hydroxides to remove antimony from groundwater to meet the requirements of drinking water regulations. Results obtained by batch adsorption experiments indicated that the qualified iron oxy-hydroxide (FeOOH), synthesized at pH 4 for maintaining a high positive charge density (2.5 mmol OH−/g) achieved a residual concentration of Sb(III) below the EU drinking water regulation limit of 5 μg/L by providing an adsorption capacity of 3.1 mg/g. This is more than twice greater compared either to similar commercial FeOOHs (GFH, Bayoxide) or to tetravalent manganese feroxyhyte (Fe-MnOOH) adsorbents. In contrast, all tested adsorbents failed to achieve a residual concentration below 5 μg/L for Sb(V). The higher efficiency of the qualified FeOOH was confirmed by rapid small-scale column tests, since an adsorption capacity of 3 mg Sb(III)/g was determined at a breakthrough concentration of 5 μg/L. However, it completely failed to achieve Sb(V) concentrations below 5 μg/L even at the beginning of the column experiments. The results of leaching tests classified the spent qualified FeOOH to inert wastes. Considering the rapid kinetics of this process (i.e., 85% of total removal was performed within 10 min), the developed qualified adsorbent may be promoted as a prospective material for point-of-use Sb(III) removal from water in vulnerable communities, since the adsorbent’s cost was estimated to be close to 30 ± 3.4 €/103 m3 for every 10 μg Sb(III)/L removed.

Experimental investigation of the impact of compound‐specific dispersion and electrostatic interactions on transient transport and solute breakthrough

AbstractThis study investigates the effects of compound‐specific diffusion/dispersion and electrochemical migration on transient solute transport in saturated porous media. We conducted laboratory bench‐scale experiments, under advection‐dominated regimes (seepage velocity: 0.5, 5, 25 m/d), in a quasi two‐dimensional flow‐through setup using pulse injection of multiple tracers (both uncharged and ionic species). Extensive sampling and measurement of solutes' concentrations (∼1500 samples; >3000 measurements) were performed at the outlet of the flow‐through setup, at high spatial and temporal resolution. The experimental results show that compound‐specific effects and charge‐induced Coulombic interactions are important not only at low velocities and/or for steady state plumes but also for transient transport under high flow velocities. Such effects can lead to a remarkably different behavior of measured breakthrough curves also at very high Péclet numbers. To quantitatively interpret the experimental results, we used four modeling approaches: classical advection‐dispersion equation (ADE), continuous time random walk (CTRW), dual‐domain mass transfer model (DDMT), and a multicomponent ionic dispersion model. The latter is based on the multicomponent formulation of coupled diffusive/dispersive fluxes and was used to describe and explain the electrostatic effects of charged species. Furthermore, we determined experimentally the temporal profiles of the flux‐related dilution index. This metric of mixing, used in connection with the traditional solute breakthrough curves, proved to be useful to correctly distinguish between plume spreading and mixing, particularly for the cases in which the sole analysis of integrated concentration breakthrough curves may lead to erroneous interpretation of plume dilution.

Numerical simulation and experimental study on farmland nitrogen loss to surface runoff in a raindrop driven process

It has been widely recognized that surface runoff from agricultural field is an important non-point pollution source, which however, the chemical transfer amount in the process is very difficult to be quantified in field since some variables and natural factors are hard to control, such as rainfall intensity, temperature, wind speeds and soil spatial heterogeneity, which may significantly affect the field experimental results. Therefore, a physically based nitrogen transport model was developed and tested with the so called semi-field experiments (i.e., artificial rainfall was used instead of natural rainfall, but other conditions were natural) in this paper. Our model integrated the raindrop driven process and diffusion effect with the simplified nitrogen chain reactions. In this model, chemicals in the soil surface layer, or the ‘exchange layer’, were transformed into the surface runoff layer due to raindrop impact. The raindrops also have a significant role on the diffusion process between the exchange layer and the underlying soil. The established mathematical model was solved numerically through the modified Hydrus-1d source code, and the model simulations agreed well with the experimental data. The modeling results indicate that the depth of the exchange layer and raindrop induced water transfer rate are two important parameters for the simulation results. Variation of the water transfer rate, er, can strongly influence the peak values of the NO−3-N and NH+4-N concentration breakthrough curves. The concentration of NO−3-N is more sensitive to the exchange layer depth, de, than NH+4-N. In general, the developed model well describes the nitrogen loss into surface runoff in a raindrop driven process. Since the raindrop splash erosion process may aggravate the loss of chemical fertilizer, choosing an appropriate fertilization time and application method is very important to prevent the pollution.

Determining model parameters for non-linear deep-bed filtration using laboratory pressure measurements

Particle capture in porous media and the consequent permeability reduction occur in oilfields during water injection or produced water re-injection, migration of mobilized reservoir fines, and invasion of drilling and completion fluids into formations. Reliable modelling-based prediction of particle propagation in natural reservoirs is an essential step in the planning and design of waterflooding.The mathematical model for suspension-colloidal transport in natural reservoirs contains two empirical functions of retained particle concentration. The filtration function expresses the particle capture rate. The formation-damage function determines the permeability decline due to particle capture and retention. Previous works developed an inverse-problem solution to recover both functions from breakthrough concentration and the pressure drop. The present paper develops a new method that determines the filtration and formation-damage functions from pressure measurements only. The method uses pressure data at an intermediate point of the porous column (core), which supplements pressure measurements at the core inlet and outlet. The proposed method furnishes two retained-concentration functions for filtration and formation-damage. The method is validated by comparison with laboratory experiments. A high fit with the pressure data at three core points was observed. Moreover, the fitted model predicts the pressure measured at other core points with high precision.

Dynamic adsorption of nitrogen dioxide on zeolites

The dynamic adsorption of nitrogen dioxide on Zeolon 900H, Zeolon 900K, Zeolon 900Na, Zeolon 200H, Zeolon 500H, H/ZSM-5, Na/ZSM-5, and CaA zeolites is studied. Breakthrough curves C/C0 = σ(t) (C 0 is the initial concentration of NO2 at the adsorption column inlet and C is the breakthrough concentration at the column outlet) is measured over a temperature range of 290–430 K. It has been shown that the time dependence of C/C 0 is described by the Wheeler–Jonas equation. The parameters of this equation, the adsorption constant rate k v and dynamic adsorption capacity (DAC) are determined by fitting the theoretical curves to the experimental data for the aforementioned zeolites at 290–430 K. The Hertz–Knudsen equation is used to obtain theoretical temperature dependence of k v. For this dependence, the curve fitting method is also used to determine the adsorption activation energy E ad. It is found that the activation energy ranges within 300–500 cal/mol, which shows that the adsorption of NO2 on these zeolites is a physical process, with the electronic structure of the molecules being perturbed only slightly during the adsorption process. A theoretical formula for calculating the time of protective action of the sorbent is derived. It is shown that the DAC and the corresponding protective action time for the studied sorbents depend on the temperature and increase in the series Zeolon 900Na < Na/ZSM-5 < Zeolon 900K < H/ZSM-5 < Zeolon 500H < Zeolon 200H < Zeolon 900H for gas mask filters at room temperature (293 K), and increase in the series CaA < H/ZSM-5 < Zeolon 900H < Zeolon 500H < Na/ZSM-5 < Zeolon 200H < Zeolon 900K < Zeolon 900Na for systems of purification of flue gases from thermal power plants at 350 K.

Influence of pumping operational schedule on solute concentrations at a well in randomly heterogeneous aquifers

We investigate the way diverse groundwater extraction strategies affect the history of solute concentration recovered at a pumping well while taking into account random spatial variability of the system hydraulic conductivity. Considering the joint effects of spatially heterogeneous hydraulic conductivity and temporally varying well pumping rates leads to a realistic evaluation of groundwater contamination risk at the pumping well location. We juxtapose the results obtained when the pumping well extracts a given amount of water operating (a) at a uniform pumping rate and (b) under a transient regime. The analysis is performed within a numerical Monte Carlo framework. Our results show that contaminant concentration breakthrough curves (BTCs) at the well are markedly affected by the transient pumping strategy according to which the well is operated. Our results document the occurrence in time of multiple peaks in the mean and variance of flux-averaged concentrations at the extraction well operating at a transient rate. Our findings suggest that lowest and largest values of mean and variance of flux-averaged concentration at the well tend to occur at the same time. We show that uncertainty associated with detected BTCs at the well increases for pumping regimes displaying a high degree of temporal variability. As such, the choice of the type of engineering control to the temporal sequence of pumping rates could represent a key factor to drive quantification of uncertainty of the contaminant concentration detected at the well. It is documented that pumping rate fluctuations induce a temporally oscillating risk pattern at the well, thus suggesting that the selection of a dynamic pumping regime has a clear influence on the temporal evolution of risk at the well.

Synthesis, Characterization and Performance Validation of Hybrid Cation Exchanger Containing Hydrated Ferric Oxide Nanoparticles (HCIX-Fe) for Lead Removal from Battery Manufacturing Wastewater

This study is aimed to synthesize, characterize and validate the performance of a novel hybrid nanoadsorbent for selective removal of lead from a battery manufacturing wastewater. The hybrid nanosorbent, named as HCIX-Fe, was prepared by impregnating hydrated Fe (III) oxide (HFO) nanoparticles inside polymeric cation exchange resin containing negatively charged sulfonic acid (-SO3-) fixed functional groups. HCIX-Fe was characterized by SEM-EDX and XRD to confirm the distribution and determination of phase of HFO dispersed inside the hybrid nanosorbent. Fixed-bed column runs with HCIX-Fe beads were carried out using wastewater from a battery manufacturing plant. The wastewater had a pH of 1.8 and contained of 3.5 mg/L of Pb2+ coexisted with 250 mg/L Ca2+ ions. The results have shown that HCIX-Fe column could treat lead-contaminated water up to 6,500 bed volumes (BVs) before the occurrence of breakthrough concentration of 0.2 mg/L Pb2+ resulting in a removal capacity of 6.85 mg Pb2+/ml of the HCIX-Fe bed. Under similar condition, adsorbent columns with cation exchange resin (C100), granulated activated carbon (GAC) and granulated activated carbon impregnated with HFO (GAC-Fe), could treat the same wastewater only until 400, 900 and 1,500 BVs, respectively. When compared with the parent adsorbents, impregnation by HFO greatly enhanced the Pb2+ removal capacity of C100 and GAC by 1,625% and 167%, respectively. Both HFO and high density of sulfonic acid (-SO3-) in the host cation exchanger are individually capable of selective removal of Pb2+ ions; however the hybrid material demonstrated a synergistic effect for Pb2+ removal through the Donnan Membrane effect. Due to amphoteric behavior of HFO, the HCIX-Fe could be regenerated and reused with 10 BVs of 2% HNO3 and 1% FeCl3·6H2O solution.

Adsorption performance of coconut shell activated carbon for the removal of chlorate from chlor-alkali brine stream.

Activated carbon from coconut shell was used to investigate the adsorption of chlorate from a chlor-alkali plant's brine stream. The effect of pH, flowrate, chlorate and chloride concentration on the breakthrough curves were studied in small-scale column trials. The results obtained show enhanced adsorption at low flowrates, higher chlorate concentrations, and at a pH of 10. These studies show that introducing an activated carbon adsorption column just before the saturator would remove sufficient quantities of chlorate to allow more of the chlor-alkali plant's brine stream to be reused. From column dynamic studies, the Thomas model showed close approximation when the chlorate in the effluent was higher than breakthrough concentrations and there was close correlation at high influent concentration. The qo (maximum adsorption capacity) values were close to those obtained experimentally, indicating close representation of the breakthrough curve by the Thomas model.

Techno-economic assessment of biogas plant upgrading by adsorption of hydrogen sulfide on treated sewage–sludge

Biogas plant upgrading by adsorption of hydrogen sulfide on treated sewage–sludge was techno-economically assessed. Three different processes were included in the study: the desulfurization of biogas by adsorption, the in-situ regeneration of the adsorbent and its production from sewage-sludge. Biogas plant upgrading was performed for a flow rate of 1000Nm3/h of biogas with a H2S concentration of 2000ppmv and a breakthrough concentration of 200ppmv, which is the technical limit value for internal combustion engines. The cost due to the steam required for the in-situ regeneration was evaluated in two different scenarios: as a bought external utility and as an in-situ produced utility, installing an electric or a biogas steam boiler. According to the cash flow analysis carried out, all the options require a similar minimum selling price for the upgraded biogas (about 0.27–0.29€/Nm3), with a cost of the overall desulfurization process between 2.5 and 4.0c€/Nm3.

Water and solute transport in agricultural soils predicted by volumetric clay and silt contents

Solute transport through the soil matrix is non-uniform and greatly affected by soil texture, soil structure, and macropore networks. Attempts have been made in previous studies to use infiltration experiments to identify the degree of preferential flow, but these attempts have often been based on small datasets or data collected from literature with differing initial and boundary conditions. This study examined the relationship between tracer breakthrough characteristics, soil hydraulic properties, and basic soil properties. From six agricultural fields in Denmark, 193 intact surface soil columns 20cm in height and 20cm in diameter were collected. The soils exhibited a wide range in texture, with clay and organic carbon (OC) contents ranging from 0.03 to 0.41 and 0.01 to 0.08kgkg−1, respectively. All experiments were carried out under the same initial and boundary conditions using tritium as a conservative tracer. The breakthrough characteristics ranged from being near normally distributed to gradually skewed to the right along with an increase in the content of the mineral fines (particles ≤50μm). The results showed that the mineral fines content was strongly correlated to functional soil structure and the derived tracer breakthrough curves (BTCs), whereas the OC content appeared less important for the shape of the BTC. Organic carbon was believed to support the stability of the soil structure rather than the actual formation of macropores causing preferential flow. The arrival times of 5% and up to 50% of the tracer mass were found to be strongly correlated with volumetric fines content. Predicted tracer concentration breakthrough points as a function of time up to 50% of applied tracer mass could be well fitted to an analytical solution to the classical advection-dispersion equation. Both cumulative tracer mass and concentration as a function of time were well predicted from the simple inputs of bulk density, clay and silt contents, and applied tracer mass. The new concept seems promising as a platform towards more accurate proxy functions for dissolved contaminant transport in intact soil.

One-dimensional coupled model for landfill gas and water transport in layered unsaturated soil cover systems

Cover systems are used to prevent water infiltration into a waste body. They also play an important role in controlling landfill gas transport from the waste body to the atmosphere. It is important to assess the flux of landfill gas at the surface of a cover system by considering the coupled effects of rainwater infiltration and gas transport in the cover soils. We have developed a 1D mathematical model for coupled transient gas and water transport in unsaturated cover soils. The coupled model was solved by the finite element method. Results obtained by the proposed model agreed well with experimental data. Based on the proposed solution, the influences of gas pressure, gas permeability, and the thickness of the cover soils on soil gas concentration profiles were investigated. The difference in soil gas concentration reached up to 31% as the thickness of cover increased from 1 to 2 m. Gas concentration at a depth of 0.2 m decreased by 6% as the amplitude of atmospheric gas pressure fluctuation increased from 20 to 100 Pa. The gas concentration increased by only 3% when gas permeability increased by a factor of 2 for a relatively long period of gas migration (e.g., 60 h) under the given conditions. Results suggest that both diffusion and advection should be considered when estimating gas transport in unsaturated cover soils. The numerical model can be used in the design of cover systems in relation to gas breakthrough time, breakthrough concentration, and flux.