Transport In Soil Columns Research Articles

Phosphorus Fate and Transport in Soil Columns Loaded Intermittently with Influent of High Phosphorus Concentrations

In this study, several columns of different lengths were filled with composite soils sampled from the field at corresponding depths and then loaded intermittently with influent of a high phosphorus concentration to evaluate phosphorus fate and transport in soil. The results indicate that the height of the mass transfer zone, solvent pore velocity, and soil's life expectancy for phosphorus removal increased with depth, while the retained phosphorus per kilogram of soil and the linear adsorption equilibrium coefficient, R, decreased with depth. An equation was developed to link liquid-phase phosphorus with solvent traveling time and soil depth. The results of X-ray diffraction and washout tests indicate that calcium-phosphorus precipitation and/or crystal growth occurred in the columns. The new protocol is useful for evaluation of phosphorus fate and transport in other subsurface systems, because it allows flexible adjustments in hydraulic loadings, feed solution, and sampling schemes.

The Fate of Cryptosporidium Parvum Oocysts in Reclaimed Water Irrigation-history and Non-history Soils Irrigated with Various Effluent Qualities

The present study aimed to look at the fate of protozoan parasite Cryptosporidium parvum oocysts applied through surface drip irrigation on reclaimed water irrigation-history and non-history sandy-loam (Hamra) soil columns. A new and simple isolation method for recovery of oocysts from soil samples was developed and used along this study. The new soil isolation method of oocysts is based on the "two phase separation method" formerly used to recover Clostridium perfringens spores from sedi- ments and soil samples with minor modifications. The range recovery achieved by this method was 64-95% (mean 61.2±17.4). The objectives of the second part of this study were to investigate several physical and chemical factors governing transport and survival of C. parvum oocysts in sandy-loam soil columns by breakthrough curves. Comparison of fresh water and reclaimed water irrigation revealed that reclaimed water irrigated-history soil was more hydrophobic allowingwater flowthrough channelswith poor oocysts retention and fast flow. Examination of the organic matter effect (originating from reclaimed water irriga- tion) on oocysts breakthrough revealed that their soil infiltration increased. Calculations of oocysts concen- tration at different columns depths showed that most of the oocysts were retained in the first 5 cm of soil column. In the present study, comparing the two soil types (history and non-history of effluents irrigation) beside the surface electrostatic charge, one of the main elements found to affect oocysts infiltration and transport in soil columns was soil hydrophobicity caused by soluble organic matter originating from reclaimed water irrigation. Therefore, prior to applica- tion in soil irrigation, reclaimed water should be treated to high quality (i.e. membrane technology as the best option) to prevent enhanced transport of various pathogens through those irrigated soils.

Colloid Mobilization and Arsenite Transport in Soil Columns: Effect of Ionic Strength

Colloid generation and transport in soils is of significance because of suspected colloid-facilitated transport of contaminants to the groundwater. In this study, colloid mobilization and its effect on the transport of arsenite [As(III)] were investigated in Olivier (fine-silty, mixed, active, thermic Aquic Fraglossudalfs) and Windsor (mixed, mesic typic Udipsamments) soil columns. Input solution of 10 mg L(-1) As(III) in 0.01 M NaCl was applied to water-saturated columns, and followed by leaching with deionized water (DIW). Flow interruptions were performed during the As(III) input and DIW leaching phases. Turbidity, electrical conductivity (EC), and pH of column effluents were monitored with time. Total and dissolved concentrations of As, Fe, and Al were analyzed. Effluent results demonstrated that colloid-facilitated transport contributed little to arsenic movement when the solution ionic strength was maintained constant. Mobilization of colloidal amorphous material and enhanced transport of As(III) were observed as a result of changes in ionic strength of the input solution. The peak of colloid generation coincided with peak concentrations of Fe, suggesting mobilization of Fe oxides and facilitated transport of As(III) adsorbed on oxide surfaces. Colloid mobilization was enhanced due to flow interruption in the Olivier column, which suggests slow dissociation of aggregated colloidal particles. Moreover, effluent results indicate significant effect of organic matter in stabilizing aggregates of colloidal particles.

A reaction-based paradigm to model reactive chemical transport in groundwater with general kinetic and equilibrium reactions

This paper presents a reaction-based water quality transport model in subsurface flow systems. Transport of chemical species with a variety of chemical and physical processes is mathematically described by M partial differential equations (PDEs). Decomposition via Gauss–Jordan column reduction of the reaction network transforms M species reactive transport equations into two sets of equations: a set of thermodynamic equilibrium equations representing N E equilibrium reactions and a set of reactive transport equations of M– N E kinetic-variables involving no equilibrium reactions (a kinetic-variable is a linear combination of species). The elimination of equilibrium reactions from reactive transport equations allows robust and efficient numerical integration. The model solves the PDEs of kinetic-variables rather than individual chemical species, which reduces the number of reactive transport equations and simplifies the reaction terms in the equations. A variety of numerical methods are investigated for solving the coupled transport and reaction equations. Simulation comparisons with exact solutions were performed to verify numerical accuracy and assess the effectiveness of various numerical strategies to deal with different application circumstances. Two validation examples involving simulations of uranium transport in soil columns are presented to evaluate the ability of the model to simulate reactive transport with complex reaction networks involving both kinetic and equilibrium reactions.

E. coli transport in soil columns: implications for reuse of treated wastewater in irrigation

Reuse of treated wastewater in irrigation is gaining recognition as a vital element in the water resources management plan of developing countries, especially those situated in arid and semi-arid regions. An understanding of the transport of residual pollutants from treated wastewater, such as bacteria, in soil as a result of irrigation is critical to assessing health risks and the possible contamination of limited groundwater resources. In this work, retention of E. coli is evaluated for a soil that is irrigated by treated wastewater for growth of non-food crops near Egypt's Red Sea coast. In particular, the effects of soil organic fraction (SOF) and hydraulic loading rate (HLR) were investigated in laboratory soil columns. The matrix of experiments included three HLRs and three SOFs. The retention of bacteria by adsorption was observed at HLRs of 5 and 13 cm/h, with the magnitude of the adsorption increasing proportionally to the SOF. The impact of SOF was greater for the lower HLR. At the lowest HLR investigated (5 cm/h), filtration was also observed for the two higher SOFs (0.674 and 2.04 per cent). At a high HLR (66 cm/h) simulating flood irrigation, retention of bacteria was minimal regardless of the SOF. Since the bacterial solution is applied to a dry soil column to simulate field conditions, E. coli breakthrough after two pore volumes of throughput (vs. one) provided a meaningful comparison of bacterial retention as a function of HLR and SOF.

Equilibrium and Nonequilibrium Transport of Boron in Soil

Though B adsorption on soil is considered to be reversible and rapid, the use of models based on the assumption of local equilibrium often provide poor descriptions of B transport in soil columns. This study was conducted to reconcile inconsistencies between the findings of transport and batch‐adsorption experiments. The B displacement experiments in the loamy sand soil were conducted at various pH values (6.9, 8.3, and 9.3) and pore‐water velocities (3.6 and 0.16 cm h−1). The B transport in soil was strongly controlled by the pH‐dependent and rate‐limited adsorption (the soil heterogeneity was insignificant). The impact of rate‐limited adsorption was dependent on pore‐water velocity. The two‐site (local equilibrium–nonequilibrium [LE–NE]) model accounting for the existence of equilibrium and nonequilibrium adsorption sites was used to describe nonideal transport of B in loamy sand soil. The Keren's phenomenological equation was used to simulate B adsorption on equilibrium sites and the Langmuir rate equation was applied for the rate‐limited sites. The B adsorption parameters in the model were obtained from batch experiments. The fraction parameter f (representing the fraction of soil in which B adsorption is assumed to be rate‐limited) and the dimensionless rate coefficients γ0 (the Damkohler number) for B adsorption–desorption reactions were calculated by fitting the LE–NE model to the breakthrough curves (BTCs) for B measured from the fast‐velocity experiments. The fraction parameter was >0.9, indicating that most of B adsorption sites on the loamy sand soil are rate‐limited. The γ0 values calculated from B adsorption BTCs were greater than that for desorption, indicating that hysteresis in B adsorption–desorption processes can be observed during nonequilibrium B transport in soil. The LE–NE model well reproduced the general B transport behavior in the soil over the observed pH and velocity ranges.

Modeling contaminant transport in soil column and ground water pollution control

A mathematical and computer model for the transport and transformation of solute contaminants through a soil column from the surface to the groundwater is presented. The model simulates selenium species such as selenate, selenite, and selenomethionine as well as pesticides and nitrogen. This model is based on the mass balance equation including convective transport, dispersive transport, surface adsorption, oxidation and reduction, volatilization, chemical and biological transformation. The governing equations are solved numerically by the method of implicit finite difference. The simulation results are in good agreement with measured values. The major finding in the present study indicates that as the time of simulation increases, the concentration of different selenium species approaches the measured values.

Mobility of sulfate in forest soils: kinetic modeling.

Understanding sulfate transport and retention dynamics in forest soils is a prerequisite in predicting SO4 concentration in the soil solution and in lake and stream waters. In this study forest soil samples from the Gårdsjön catchments, Sweden, were used to study SO4 transport in soil columns from the upper three soil horizons (E, Bs, and BC). The columns were leached using a sequential leaching technique. The input solutions were CaSO4 equilibrated with forest floor material. Leaching behavior of SO4 and concentration in the effluent were measured from columns from individual horizons. Sulfate was always retained in the Bs and BC horizons, while the pattern for the E horizon varied. Attempts were also made to model SO4 breakthrough results based on miscible displacement approaches and solute convection-dispersion equation (CDE) in porous media. Several retention mechanisms were incorporated into the CDE to account for possible reversible and irreversible SO4 reactions in individual soil layers. Our modeling efforts were inadequate in describing the mobility of SO4 in the top (E) horizon. Moreover, a linear equilibrium approach was generally inadequate for describing SO4 sorption during transport in the Bs and BC horizons. In contrast, we found that the model provided good descriptions of all breakthrough results when SO4 reactivity was accounted for based on nonlinear equilibrium or first-order kinetic processes. Moreover, based on model parameter estimates, the reactivity or retention of SO4 during transport is concentration dependent. We conclude that sulfate retention during transport in this forest soil is most likely controlled by kinetic reactivity of SO4 of the reversible and irreversible mechanisms.

Deriving Boron Adsorption Isotherms from Soil Column Displacement Experiments

This study was conducted to elucidate the applicability of the column technique to obtain B adsorption isotherms. A column technique based on miscible displacement was used to investigate B equilibrium adsorption and B transport in soil at various pHs and B concentrations. Boron adsorption by soil at equilibrium was described by the Langmuir equation with the pH‐dependent apparent adsorption coefficient. This equation, combined with one‐dimensional convection–dispersion equation (CDE), was used to simulate B transport in soil columns. The results indicated that adsorption equilibrium was reached during the B transport in the homogeneously packed soil columns. The adsorption isotherms that were obtained from the column breakthrough curves (BTCs) and the batch experiments were identical at any given pH. This indicates that under equilibrium conditions, the column technique can be used for calculating B adsorption isotherm parameters for any given pH.

Deriving Boron Adsorption Isotherms from Soil Column Displacement Experiments

This study was conducted to elucidate the applicability of the column technique to obtain B adsorption isotherms. A column technique based on miscible displacement was used to investigate B equilibrium adsorption and B transport in soil at various pHs and B concentrations. Boron adsorption by soil at equilibrium was described by the Langmuir equation with the pH-dependent apparent adsorption coefficient. This equation, combined with one-dimensional convection–dispersion equation (CDE), was used to simulate B transport in soil columns. The results indicated that adsorption equilibrium was reached during the B transport in the homogeneously packed soil columns. The adsorption isotherms that were obtained from the column breakthrough curves (BTCs) and the batch experiments were identical at any given pH. This indicates that under equilibrium conditions, the column technique can be used for calculating B adsorption isotherm parameters for any given pH.

Flow and Reactivity Effects on Dissolved Organic Matter Transport in Soil Columns

AbstractDissolved organic matter (DOM) plays a prominent role in the transport of contaminants in porous media. As DOM has to be considered as a reactive component, flow regime and sorbent reactivity should affect overall DOM transport in an important way. We focused on DOM transport in unsaturated column experiments using quartz sand (QS) and goethite‐coated quartz sand (GS). Rate constrictions to DOM sorption were investigated by varying the volumetric flow rate, while extent and reversibility of sorption were studied in consecutive adsorption and desorption steps. In the QS, DOM retention was low and unaffected by changes in flow rate. Desorption‐step breakthrough curves (BTCs) and mass balances show full reversibility of the sorption process. However, DOM retention in GS was significant and sensitive to flow variation, indicative of nonequilibrium sorption. At lower flow rates, DOM breakthrough exhibited a change in curvature (shoulder) due to the superimposition of two BTCs representing reactive and nonreactive DOM fractions. Transport was successfully modeled assuming these two fractions governed overall DOM mobility. At higher flow rates, the BTC shoulder vanished due to reduced contact time between the DOM and the solid phase (rate‐limited sorption). Sorption of DOM on GS is accompanied by a marked rise in effluent pH, indicative of a ligand‐exchange mechanism. Recovery of DOM during desorption was incomplete due to either partially irreversible sorption or strongly rate‐limited desorption. Increased DOM mobility in the consecutive adsorption step resulted from partial blocking of sorption sites by the initial pulse of DOM.

Controlled Release of Pesticides into Soils from Clay−Polymer Formulations

Alachlor was released from a controlled-release formulation (CRF) when applied to a sandy loam soil under conditions of constant temperature and moisture and achieved concentrations of 0.1−0.2 mg/kg after 1 week. Soil concentrations of 0.2−0.8 mg/kg were maintained for at least an additional 8 weeks. A large portion (35−70%) of the active ingredient was still present in the CRFs at the conclusion of the experiment. Application of alachlor as the commercial formulation resulted in 50% dissipation after only 3−4 days. Alachlor released from CRFs with a smaller diameter resulted in higher soil concentrations (1.2 mg/kg) after 18 days. In column leaching studies the leaching of alachlor, atrazine, and trifluralin from CRFs was considerably reduced with respect to leaching of the same compounds from commercial formulations. The release of alachlor from an Al-pillared clay CRF and its subsequent transport in soil columns showed that pillared clays can be used for controlled release of herbicides, but more work ...

Sorption and desorption of fluoride in soil polluted from the aluminium smelter at Årdal in western Norway

Fluoride (F) accumulation and transport in soil columns near the aluminium smelter at Ardal in Western Norway are studied together with fluoride sorption capacity, and the effect of pH and ionic strength on F solubility in soil. Unpolluted soil columns of 50 cm height placed at different distances from the smelter (1–0 km) accumulated from 0.27 g F m-2 to 1.5 g F m-2 during a 5 months period. Fluoride accumulation was high in the upper 0–10 cm of the soil columns where 50–90% of the accumulated F was found. Laboratory sorption experiments showed that the sorption of added F in the Ah-horizons increased with increased distance from the smelter at Ardal, in accordance with decreasing previous pollution from the smelter. The B-horizons sorbed considerably more F than the Ah-horizons, due to higher content of Al-oxides/hydroxides. Maximum sorption of F occurred at pH 4.8–5.5. Fluoride solubility increased with increased ionic strength. Continued deposition of F may increase the availability to plants and soil organisms. Leaching of F from soil to groundwater or surface water will be strongly dependent on the presence and thickness of a B-horizon, and probably also on underlying horizons.

Intercomparison between TRIO-EF and IMPACT codes with reference to experimental strontium migration data

This work presents an intercomparison exercise between two geochemical migration codes, TRIO-EF (an object-oriented finite element code) and IMPACT (a chemical engineering code using mixing cells in series). The predictions of the two codes are compared with the reference experimental results obtained in a previous study of strontium transport in soil columns. This simulated geochemical system is well documented and includes ion exchange and dissolution-precipitation reactions. The solution transport is simulated by a one-dimensional advection-dispersion model. The predictions of TRIO-EF and IMPACT are both in good agreement with the experimental results. However, slight differences can be observed between the two codes, especially when concentration discontinuities are involved, such as precipitation fronts or changes in boundary conditions. These discrepancies between the two codes can mainly be attributed to the different discretisation approaches.

Technical Notes: An Automated Sampling System for Large Soil Column Transport Studies

An automated column effluent sampling system for large unsaturated soil columns was constructed and evaluated. The automated collection system consist of a programmable three-solenoid valve system that maintains a constant matric potential at the bottom of the soil column and periodically switches column effluent from a waste receptacle to a sampling vial. We compared the automated system to a traditional vacuum chamber-fraction collector system that continuously sampled all column effluent. Comparisons were made by measuring 3H2O breakthrough curves (BTCs) generated by the two collection systems using the same intact soil columns. No significant differences were observed in 3H2O BTCs between collection systems. These results indicate that the automated collection system is effective for large column transport studies and provides additional advantages by maintaining effluent resolution (sample volume), yet reduces sample size (numbers) and labor required for sample collection.

Survival and Transport of lac-lux Marked Pseudomonas fluorescens Strain in Uncontaminated and Chemically Contaminated Soils

Summary Survival of a lac ZY-lux AB strain and rifampicin, ampicillin resistant strain of Pseudomonas fluorescens UG16 in uncontaminated soil and soils containing polycyclic aromatic hydrocarbons (PAHs) and chlorinated phenols was conducted over 7 weeks at 10 and 22°C. Survival of cells containing lac ZY-lux AB reporter gene system determined by viable plating and detection with a charge coupled camera was generally better or similar to the rifampicin, ampicillin resistant strain. Microcosms containing remediated soil with decreased levels of PAHs and chlorinated phenols incubated at 10°C allowed better survival of introduced cells than soil incubated at 22°C, containing higher concentrations of PAHs and chlorinated phenols. Bacterial transport in soil columns of contaminated, bioremediated and uncontaminated control soils was carried out using the lac ZY-lux AB strain of P. fluorescens UG16. Soil columns were flushed with non-sterile rainwater and microorganisms enumerated in the soil profile and leachate. Non-remediated soil containing chemical contaminants displayed lower transport of the inoculated strain. Remediated and uncontaminated soils allowed transport of the introduced bacterial cells through the soil column in into leachate samples.

Simulating Cation Transport during Water Flow in Soil: Two Approaches

Two numerical models were used to simulate observed ternarycation transport in soil columns during unsteady, unsaturated flow with variable total solution concentration (CT). Transport of Ca, Na, and K cation species was simulated for constant-flux infiltration of aqueous electrolyte (NaCl-KCl) solutions in columns of water-unsaturated clay soil that was initially saturated with Ca. A simplified transport model where selectivity coefficients and CT were assumed to be constants and steady flow was imposed described observed spatial distributions of concentrations for all species in both solution and exchange phases. A detailed numerical model that combines transient, unsaturated flow and transport that includes variable CT and binary exchange selectivity coefficients, which vary with CT and with ion concentration within the solution phase, provided an equally good description of concentrations in solution and exchange phases but was also able to simultaneously provide excellent descriptions of volumetric water content (θ) and CT distributions. Thus approximations of constant CT, constant binary exchange selectivities, and steady liquid flow were sufficient to describe cation transport observed during experiments with variable CT and transient, unsaturated liquid flow in soil columns.

Modeling the Transport and Retention of Cadmium in Soils: Multireaction and Multicomponent Approaches

AbstractA major challenge in modeling the mobility of solutes in soils is to take into account the time‐ dependent retention of reactive solutes in a transport model that can be evaluated by independent experiments. The kinetics of retention and transport of Cd in two soils were investigated in this study. Time‐dependent batch experiments were carried out to describe the extent of Cd sorption by two soils. Miscible‐displacement experiments were conducted to quantify Cd transport in soil columns that received single and multiple Cd pulses. A multi‐reaction convective‐dispersive transport model inadequately predicted effluent concentrations for both soils when independently measured parameters of retention‐release rate coefficients were obtained from batch data sets. We developed a multicomponent approach that accounts for reversible and irreversible Cd retention during transport in soils. Retention mechanisms considered include instantaneous equilibrium ion exchange and a specific sorption process that was based on a second‐order (Langmuir) kinetic reaction. The multicomponent model adequately predicted the observed breakthrough results. The use of kinetic ion exchange and specific sorption for irreversible retention provided improved overall predictions of the results. Parameter values used for the rates of specific sorption (kt) and that for kinetic ion exchange (α), which provided improved predictions for single‐pulse applications, were similar to those used for multiple‐pulse applications. This finding was consistent for both soils and was considered to add credence to our multicomponent model.

Simulation of phosphate transport in soil columns. II. Simulation results

Phosphate (P) transport in vertical soil columns was simulated, assuming steady-state flow. The retention of P in soil was modeled assuming two mechanisms for the reaction, i.e., adsorption and precipitation. In a companion paper we showed that P displaces in a travelling wave like fashion for a homogeneous soil column and exhibits a relatively sharp front for total P when parameter values as determined in laboratory experiments are used. Reasonable changes in the flow velocity, the volumetric water fraction at which drainage occurs, and the diffusion/dispersion coefficient have only minor effects on the front steepness. Of the chemical parameters notably a change in the precipitation kinetics has a large effect on the front shape. In case the lower bound concentration c e below which no precipitation occurs is increased, the double front that is intrinsic to distinguishing two sorption processes may start to spread out. For large enough values of c e the downstream part of the front, which is affected only by adsorption, moves faster than the upstream part, which is also retarded by precipitation. A criterion when such front splitting occurs is presented and depends on the chemistry parameters. When the inflow of P is discontinuous, the concentration front is highly variable in time, but the total-P front responds only slightly on the variation of c in time. Also for layered soils steep P-fronts were simulated. For layered soils the front depth depends on the properties of different layers situated upstream, while the shape mostly depends on the layer properties where the front is situated. Discontinuation of P-applications leads to a perturbation of the front shape. In that case, as well as when a homogeneous uptake of P by crops following discontinuation is simulated, mobile P (dissolved and adsorbed reversibly) decreases rapidly. Discontinuation of P-input and intensive cropping may significantly limit P-leaching.

Simulation of phosphate transport in soil columns. I. model development

A phosphate (P) transport model is developed assuming one-dimensional steady-state flow. It is assumed that P-adsorption follows the Langmuir isotherm and is reversible. Besides adsorption also non-equilibrium and irreversible precipitation may occur, which is modeled using a semi-empirical description from the unreacted shrinking core concept. The used P-sorption model is in agreement with short- and long-term batch experiments and has been validated with laboratory-scale transport experiments in previous work. The equations for transport in combination with adsorption and precipitation are solved numerically using the modular approach, in which the chemical and transport equations are decoupled. The additional dispersion due to this decoupling is quantified and methods to decrease computation times resulting from a fine discretization in time and space are discussed. The numerical program is verified with analytical solutions for the transport equations in case of linear and nonlinear (Langmuir) sorption with excellent results. Neglecting precipitation a very sharp front of P is found. The dependence of front thickness on parameter values is illustrated. When taking precipitation into account a relatively sharp front may also develop, which has travelling wave characteristics (a front shape and velocity independent of time for a uniform soil column).