selenium transport and transformation modelling in soil columns and ground water contamination...

HYDROLOGICAL PROCESSESHydrol. Process. 22, 2475–2483 (2008)Published online 23 January 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.6842

Selenium transport and transformation modelling in soilcolumns and ground water contamination prediction

Seyed Ahmad Mirbagheri,1 Kenneth K. Tanji2 and Taher Rajaee3*1 Department of Civil Engineering, Khajeh Nasir University of Technology, Tehran, Iran

2 Department of Land, Air and Water Resources, University of California at Davis, Davis CA 95616, USA3 Water and Environmental Civil Engineering, Khajeh Nasir University of Technology, Tehran, Iran

Abstract:

Selenium transport and transformation were simulated in a soil column. A one-dimensional dynamic mathematical and computermodel is formulated to simulate, selenate, selenite, selenomethionine, organic selenium, and gaseous selenium. This computermodel is based on the mass balance equation, including convective transport, dispersive transport, surface adsorption, oxidationand reduction, volatilization, chemical and biological transformation. The mathematical solution is obtained by the finitedifference implicit method. The model was verified by comparison of model results with experimental measurements and alsousing mass balance calculations in each time step of calculation. For example after 4 days of simulation, the simulated value ofadsorbed selenate for depth of 20 cm is 0Ð2 µmol kg�1 and the measured value is 0Ð25 µmol kg�1. Therefore simulated resultsare in good agreement with measured values. With this study and its results the distribution of various forms of selenium insoil column to ground water table can be predicted. Copyright 2008 John Wiley & Sons, Ltd.

KEY WORDS selenium transport; ground water; transformation modelling; prediction; contamination

Received 23 July 2006; Accepted 6 June 2007

INTRODUCTION

Selenium (Se) is considered one of the least plentifulbut most toxic elements in the earth’s crust. Se is mobi-lized in the environment through natural processes ofweathering, disposal of human, animal, and plant wastes,emission of volcanic ashes and fossil fuel burning. Seoccurs in the environment in the following inorganicforms: Se(�2), Se(0), Se(C4), and Se(C6) and as organiccompounds such as seleno-amino acids, seleno-aminoacid derivatives, methyl selenic esters, methyl selenonos,and methyl selenium ions (Thompson-Eagle and Franken-berger, 1990).

High level of Se in soils and shallow aquifers of thewest side of the San Joaquin Valley, California, havebeen questioned as a potential hazard to wildlife andhuman health (Burau, 1985). Cooke and Bruland (1987)identified various soluble Se species in surface waters atKesterson Reservoir and San Joaquin River. Dischargeof drainage water with high Se concentration has beenidentified as the primary source of Se to the KestersonNational Wildlife Refuge (Deverel et al., 1984).

The following studies showed Se is expected to beretained near the soil surface, even under extreme leach-ing conditions. Se transformation in surface and subsur-face soil from Se-contaminated Kesterson Reservoir wasstudied (Zawislanski and Zavarin, 1996). Also, features

* Correspondence to: Taher Rajaee, Water and Environmental CivilEngineering, Khajeh Nasir University of Technology, Tehran, Iran.E-mail: taher [email protected]

of the sediment contamination process that occurred dur-ing disposal of seleniferous agricultural drainage watersat Kestesron Reservoir were simulated in the laboratory(Tokunaga et al., 1996). The contamination of KestersonReservoir at Merced County, California, with seleniferousagricultural drainage waters, and resulting wildlife mor-talities illustrates the need for reliable speciation informa-tion to understand Se transport, immobilization, bioavail-ability, and toxicity (Benson et al., 1991; Tokunaga et al.,1991; White et al., 1991; Tokunaga et al., 1994; Zawis-lanski and Zavarin, 1996; Guo et al., 1999).

Transport of selenate and selenite was studied byseveral researchers (Ahlruchs and Hossner, 1987; Alemiet al., 1988; Fun, 1991). The studies were conducted indifferent kinds of soils under different conditions. Theyreported that selenate was completely leached from thesoil columns whereas selenite was rapidly adsorbed bythe material within the few centimetres of the columninlet. The transport modelling of Se has been reportedby (Alemi et al., 1991; Fio et al., 1990). The modellingeffort was for Se transport in steady state, unsaturated soilcolumns, under sterilized conditions with small residencetimes where microbial activity is negligible.

The Se transport and transformation processes in asoil column under transient flow conditions are complex.Several complicating factors control the transport of dif-ferent Se species: (a) pore water velocity, (b) evaporationand transpiration fluxes, (c) concentration gradient and(d) seasonal rise and fall of the water table. In general,Se is transported in the soil profile by ways ofconvection and dispersion which are the result of

Copyright 2008 John Wiley & Sons, Ltd.

2476 S. A. MIRBAGHERI, K. K. TANJI AND T. RAJAEE

mass flow and concentration gradient. Se transformationprocesses in soil systems are oxidation/reduction, adsorp-tion/desorption, plant uptake, mineralization/immobili-zation and volatilization.

The objectives of this work were to evaluate the Sesources and sinks for the transport and transformationprocesses in the soil column. Also, to develop a dynamicsimulation model which approximates the Se concentra-tion where microbial activities and plant growth werepresent. The prediction of ground water contaminationby different Se species can be achieved by the appli-cation of the model to the real soil and ground watersystem.

MODEL FORMULATION

A numerical model of water flow and Se movementwas developed in this study. This model considers avariety of processes that occur in the plant root zone aswell as leaching to the ground water, including transientfluxes of water and Se species, alternating period ofrainfall, irrigation and evapotranspiration, and variablesoil conditions with depth. Figure 1 shows a simple blockdiagram of the water flow and Se transport model.

Water flow model

Water flow is calculated using a finite-difference solu-tion to the soil-water flow equation.

∂h

∂tc�� D ∂

∂z

[K��

∂H

∂z

]� u�z, t� �1�

where h is the soil water pressure head (in mm), � is thevolumetric water content (in m3 m�3, t time (in days),H is the hydraulic head (h � z), z is the soil depth,K is the hydraulic conductivity (in cm day�1), c�� D∂�∂h is the differential water capacity, and u is a sinkterm representing water lost by transpiration (adsorptionof water by plant). Functions which characterized the

Figure 1. Block diagram of the water flow and Se transport model

relationship between K � � � h describes in LEACHM(Hutson and Wagenet, 1989) are used. There is a two-part function that describes the general shape of ��h�relationships (Hutson and Cass, 1987),

h D [1 � ��/�s�]1/2��i/�s�

�b

[1 � ��i/�s�1/2]

for hi < h < 0 �2a�

h D a��/�s��b for hi > h > �1 �2b�

where hi D a[2b/91 C 2b]�b and �i D 2b�s/�1 C 2b�b isthe point hi, qi of intersection of the two curves, �s iswater content at saturation, a and b are constants. Thetwo curves are exponential and parabolic for dry andsaturated soil respectively. Similarly the equations forhydraulic conductivity can be derived as a function ofsoil water pressure head. When soil water pressure head isgreater than hi the following equation is used to calculatehydraulic conductivity:

K�� D Ks��/�s�2bC2Cp �3�

where Ks is the hydraulic conductivity at saturationwater content (�s), and p is the pore water interactionparameter. When soil water pressure head is less that hi

the equation for the calculation of hydraulic conductivityis:

K D Ks�a/hs�2C�2Cp�/b �4�

Solving Equation (1) using finite difference techniquesprovides estimated values of h at each depth node used inthe differencing equations. Water contents are calculatedusing Equation (2). Water flux densities (q) are calcu-lated over each depth interval using Darcy’s equation�q D �K��H/Z�. Finally, the values of q are thenused to estimate Se transport in the soil profile. The finitedifference solution of Equation (1) described in detail canbe found in LEACHM (Hutson and Wagenet, 1989).

Selenium transport model

The transport of Se through the soil system can beaccomplished by the following processes:

ž Se diffusion in the liquid phase in response to anaqueous concentration gradient.

ž Convection (mass flow) of Se can occur as the result ofmovement of water in response to pore water velocitygradient.

ž Transformation of Se as sources or sinks may effect Setransport.

Se transport in soil systems occurs under steady andunsteady (transient) water flow conditions. The watercontent (�) and water flux (q) both vary with depth andtime.

Continuity relationships of mass over space and timegive a general one-dimensional transport equation for Setransport:

∂C

∂t�� C �Ks� D ∂

∂z

[�D��, q�

∂C

∂z� qC

]š �5�

Copyright 2008 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2475–2483 (2008)DOI: 10.1002/hyp

SELENIUM TRANSPORT AND TRANSFORMATION MODELLING 2477

where C is concentration of all Se species in the soilsolution, and indicates all possible sources or sinksterm. The sources and sinks of Se in the soil system underfield conditions result from the following processes:

ž Transformation of selenate to selenite and selenite toelemental selinium.

ž Volatilization of selenate, selenite and selenomethionineto dimethyl selenite �CH3�2Se.

ž Immobilization and mineralization of selenate andselenite to organic selenium.

ž Decomposition of organic selenium to selenomethion-ine.

ž Plant uptake of selenate, selenite, and selenomethion-ine.

Figure 2 shows the Se sources and sinks, also themechanisms and processes taking place in Se transportin the well aerated agricultural soil system. Assumingthat all transformations of Se obey first order kinetics(Alemi et al., 1988) and that selenate, selenite, andselenomethionine are the only mobile solute species, aseries of differential equations describing the rate changeof all Se species and a simultaneous transport of selenate,

selenite, and selenomethionine are given:

R1∂C1

∂tD ∂

∂z

[D1��, q�

∂C1

∂z

]� v

∂C1

∂z� ˛1C1U�z, t�

�

� [K1 C K3]C1 C �

�K5S0 �6�

R2∂C2

∂tD ∂

∂z

[D2��, q�

∂C2

∂z

]� v

∂C2

∂z� ˛2C2U�z, t�

�

� [K2 C K4]C2 C K1C1 C �

�K6S0 �7�

R3∂C3

∂tD ∂

∂z

[D3��, q�

∂C3

∂z

]� v

∂C3

∂z� C3U�z, t�

�

� KVC3 C �

�K7S0 �8�

∂S0

∂tD �

�K3C1 C �

�K4C2 � K5S0 � K6S0 � K7S0

�9�

∂G

∂tD �

pKVC3 �10�

∂S1

∂tD KS1

∂C1

∂t�11�

Figure 2. Mechanism and processes in the Se transport model



∂S2

∂tD Ka

Kd

∂C2

∂t�12�

∂S3

∂tD KS3

∂C3

∂t�13�

∂S4

∂tD K2

�

�C2 �14�

where C1 is the concentration of solution selenate (inµg l�1), C2 is the concentration of solution selenite (inµg l�1), C3 is the concentration of solution selenomethio-nine (in µg l�1), S0 is organic selenium concentration (inµg kg�1), G is gaseous Se concentration (in µg kg�1).S1 is adsorbed selenate concentration (in µg kg�1), S2

is adsorbed selenite concentration (in µg kg�1), S3, isadsorbed selenomethionine (in µg kg�1), KS1, KS3 areadsorption coefficient for selenate and selenomethion-ine (in l kg�1), Ka is adsorption coefficient for selenite(in l kg�1), Kd is desorption coefficient for selenite (inl kg�1), � is pore water velocity �� D q/��, ˛1, ˛2 arecoefficient for SO2�

4 antagonism effect (dimensionless),U is root absorption coefficient (dimensionless), z is soildepth (in cm), t is time (day), � is solid bulk density(g cm�3), D1��, q�, D2��, q� and D3��, q� are apparentdiffusion coefficient for selenate, selenite, selenomethio-nine respectively (in cm3 day�1), K1 (day�1) is rate con-stant for reduction of selenate, K2 is rate constant forreduction of selenite, K3 is rate constant for immobiliza-tion of selenate, K4 is rate constant for immobilizationof selenite, K5 is rate constant for mineralization of sele-nate, K6 is rate constant for mineralization of selenite,K7 is rate constant for decomposition of organic selenite.R1, R2 and R3 are retardation factor for selenate, selen-ite and selenomethionine, respectively. The equations forR1, R2 and R3 are as follows:

R1 D 1 C(�

�

)[KS1 C nCn�1

1 ] �15�

R2 D 1 C(�

�

)[KS2 C wCw�1

2 ] �16�

R3 D 1 C(�

�

)[KS3C�1

3 ] �17�

where n, w, and are non-equilibrium exponent forselenate, selenite and selenomethionine. Kv (day�1) isthe volatilization rate constant for selenomethionine.

EVALUATION OF SELENIUM SOURCES ANDSINKS

Adsorption and desorption

Adsorption and desorption of selenate, selenite andselenomethionine were studied by (Alemi et al., 1991)and are assumed to be non-linear equilibrium pro-cesses described by S1 D KS1Cn

1 (selenate), S2 D KS2Cw2

(selenite) and S3 D KS3Cm3 (selenornethionine).

Concentration of these species in soil solution and thecoefficients were evaluated according to the following

equation:

Selenate : S1 D 0Ð07C0Ð691 �18�

Selenite : S2 D 22Ð6C1Ð82 �19�

Selenomethionine : S3 D 1Ð7C0Ð93 �20�

Kd D Kwd/waa S�1�wd/wa�

m �21�

The value of KS2 was adjusted to 4Ð50 l kg�1 andKS 3 to 2Ð5 l kg�1 through the model calibration pro-cess. However, adsorption and desorption of seleniteon the soil showed substantial hysteresis (Fio et al.,1990; Fun, 1991). In their studies, they indicated thatadsorption and desorption reactions followed differentisotherms. For adsorption the equation equations areneeded to describe the sorption isotherms. For adsorp-tion the equation is S2 D KaC2

wa and for desorption ofselenite the equation is S2 D KdC2

wd. When adsorptionand desorption isotherms are different, the isotherms willintersect at the relative maximum adsorbed concentra-tion (Sm) before desorption. In this case the desorptionconstant Kd is a function of Sm and the ratio of theslopes of the adsorption and desorption isotherms asEquation (21): According to Fio et al., (1990), the max-imum adsorbed concentration of selenite ranged from0Ð102 to 1Ð764 µg g�1 and the desorption coefficient Kd

was from 0Ð70 to 4Ð05.

Volatilization

Assuming that volatilization of Se obey first-orderkinetics, the volatilization rate model is:

[Se]t D [Se]0e�kvt �22�

where [Se]0 is the initial concentration of Se in solution,[Se]t is the concentration of Se in solution at time t, andKv is the volatilization rate constant: Kv is a function oftemperature, organic matter content, and plant growth.

Thompson-Eagle and Frankenberger (1990) evaluatedthe effect of temperature and organic matter on thevalue of Kv. It was indicated that the Kv value for soilalone is about 0Ð0005 µg l�1 day�1 at soil temperatureof 25 °C. Therefore, the overall value of Kv, as affectedby temperature, organic matter, and plant growth is Kv DKvr C KvT C Kvo � Kvp. Where Kvr is the volatilizationrate constant for soil alone as a reference level, KvT

is the volatilization rate constant as affected by soiltemperature, Kvo is the volatilization rate constant asaffected by soil organic matter content, and Kvp isthe volatilization rate constant as affected by plantgrowth. The rate constant at different temperatures canbe obtained by:

KvT D K25e��T�25� �23�

where � is the temperature coefficient �q D 0Ð074�, KvT

is a volatilization rate constant at a given temperature,T. The value of KvT is valid for the temperature ranges



from 15 to 40 °C. The Se-volatilization rate constant atdifferent organic matter content can be calculated by:

Kvo D K25eQ�OM�25� �24�

where Q is the organic matter character, Q D 0Ð15 foraddition of 5 g of carbon per litre of solution at atemperature of 25 °C.

The volatilization rate constant for plant growthdepends on the type of plant. According to Biggar andJayaweera (1990) barley volatilization Se is about 19Ð6times and salt grass about 6Ð9 times more than the soilalone. Therefore, the Kvp, for soil and barley is about0Ð01 µg l�1 day�1.

Plant uptake

Se uptake by plant depends on water uptake and the Seconcentration at the root zone available for plant uptake.The rate of water uptake by plant root is by the Nimahand Hanks (1973) equation as:

U D [Hroot C zi�1 C Rc� � h � S] ð [RDF.K/xz]�25�

where U is the transpiration sink term (day�1), HrootHroot

is an effective water potential in the root zone, I C Rc isthe root resistance term, Rc is the flow coefficient in theplant root system, h is the soil-water matrix potential (inmm). S is the osmotic potential (in mm), RDF is thefraction of total active roots present in depth increment,K is the hydraulic conductivity (in mm day�1), z isdepth increment, x is the distance interval. The osmoticpotential is calculated using the relationship S D �102Ð2MRT, where M is the sum of the molar concentration ofall solute species �mol l�1�, R is the gas constant (J0,K�1 mol�1), T is the temperature (in Kelvins), the unitof S is hydraulic head (in mm). According to Hutsonand Wagenet (l989) the value of S can be calculatedusing S D �36EC by measuring soil solution electricalconductivity (EC).

Since SO2�4 has strong antagonistic effect on selenate

and selenite uptake by plant (Westerman and Robbins,1974), ˛1 and ˛2 are used as coefficients for antagonismeffect. The values of ˛1 and ˛2 can be determined fromthe regression curves for selenate and selenite versusSO2�

4 in soil solution. Also, they can be adjusted throughthe process of model calibration.

SOLUTION PROCEDURE

Prediction of the concentration of selenite and selenome-thionine in all phases (liquid, sorbed, gas) as wellas leaching losses at any depth for all time levelsrequires simultaneous solution of Equations (6)–(14).The equations are solved numerically using an implicitfinite difference scheme and Crank–Nickolson approxi-mation.

Using Figure 3 for the nodes and segments as well astime interval; the first term in Equation (6) is evaluatedat node i and time tjC1/2 and is differenced as: C D C1

The second term in Equation (6) is a diffusion anddispersion term.

R1∂C

∂tD �CjC1

i � Cji �/t �26�

R1 D 1 C �

�.N.KS1.CN�1

1

D��, q� for the interval between nodes i � 1 and i isdifferenced as:

DjC1/2i�1/2 D qjC1/2

i�1/2 /�jC1/2i�1/2

C DOLa[exp��b�jC1/2i�1/2 �]/�jC1/2

i�1/2 �27�

where:

�jC1/2i�1/2 D ��jC1

i�1 C �ji�1 � �jC1

i � �ji �/2 �28�

∂

∂z

[D��, q�

∂C

∂z

]

D [DjC1/2

i�1/2 �CjC1/2i�1/2 C Cj

i�1 � CjC1i � Cj

i �/z1

�DjC1/2iC1/2 �CjC1

i C Cji � CjC1

iC1 � CjiC1�/z2

]/z3

�29�

The convection term in Equation (6) is differenced as:

v∂C

∂zD vi � 1

2�Cj

i�1 C CjC1i�1 �/z3 � vi

C 1

2�Cj

i C CjC1i �/z3 � vi

C 1

2�Cj

iC1 C CjC1iC1 �/z3 š �30�

Multiplying out and collecting the unknown CjC1i terms

on the left-hand side the know Cji terms on the right-hand

side, the general form of equation as:

AiCjC1i�1 C BiC

jC1i C CiC

jC1iC1 D Di �30a�

where Di considers all the sources and sinks in Equation(6). For example the sources and sink terms forEquation (6) are:

D ˛1C1U�z, t�

�� [K1 C Kv1 C K3]C1 C �

�K5S0

�31�The finite difference forms are written similarly for allother equations for each node from 2 to K � 1 where Kis the lowest node in the profile. This set of equations,then is solved for defined boundary conditions using theThomas three diagonal matrix algorithms. Note that theretardation factor (Equations (15)–(17)), a function ofSe concentration in soil solution, is computed at eachiteration. The time step and the space increment wereadjusted based on water flux (q) and the acceptable massbalance error was 2%.

UPPER AND LOWER BOUNDARY CONDITION

The boundary conditions for solute and water flux are notalways the same algebraic sign within each time interval



Figure 3. Definition of nodes and segments

as water can evaporate from the soil surface while saltaccumulates.

The upper boundary condition for Se needs tobe defined to be representing zero flux, infiltrationand evaporation. For zero flux, Cj

i D 0, qjC1/2iC1/2 D 0 and

DjC1/2iC1/2 D 0. During infiltration the value of Cj

i D Cw

and DjC1/2iC1/2 D 0, where Cw, is the concentration of

Se in applied water and Se enter the profile is equalto qjC1/2

iC1/2 �t��Cw�. During surface evaporation, Cji D

0, qjC1/2iC1/2 D 0 and DjC1/2

iC1/2 D 0.The lower boundary condition for Se needs to be

defined for zero flux, water table and unit hydraulicgradient. For zero flux, qjC1/2

k�1 D 0, DjC1/2k�1/2 D 0 and Ck D

0. If water table is present, the value of Ck D Cgw andDjC1/2

k�1/2 D 0. Finally, for unit hydraulic gradient, Ck Dconstant and DjC1/2

k�1/2 D 0, where Cgw is concentration ofSe in ground water.

SIMULATION RESULT AND DISCUSSION

The model was applied to simulate the transport andtransformation of selenate, selenite and selenomethioninein soil column under steady-state and transient water flow

conditions. The soil column was assumed to be unsatu-rated under both conditions. For the validation of themodel under a steady state, the data collected by Alemiet al. (1991) was used in their column experiments, ver-tical soil columns were uniformly packed (6Ð9 cm thick,15 cm long). The density of soil was 1Ð41 kg l�1. Thesoil columns, initially wetted with 0Ð005 M CaCl2 fromthe bottom were leached with this solution until the con-centration of Ca in the effluent solution remained constantwith time and equal to that of the inflow solution. Thecomposition of the soil atmosphere was controlled by themethod of Wagenet and Starr (1977). The pH of the efflu-ent was monitored and maintained at 7Ð5 by adding smallquantities of Ca(OH)2 solution to the influent.

Under steady-state water flow conditions, 150 to210 ml of influent solution containing 19Ð23 mol l�1 ofSe in the form of sodium selenate, sodium selenite andselenomethionine were applied to the column. The waterflow through the soil column was 1Ð44 cm day�1 in cen-timetres of soil column. The experiment was run for2Ð77 days. At the end of each run the concentration of allSe species were measured in the soil profile. The relativeconcentrations of selenate, selenite and selenomethioninein the soil solution were calculated. The data from theresults of the experiment were used to run the model.



The results indicate that the transport model adequatelysimulates the measured quantities as shown in Figure 4.The simulation results for total time from 0Ð68 day to2Ð77 days indicate that as the time increases the effluentconcentrations approaches the inflow which is compa-rable with measured values as shown in Figure 5. Thesensitivity analysis of the model to some parameter atadsoption coefficient, KS, such that by increasing the KSfrom 0Ð05 to 0Ð07 l kg�1, the simulation results get closerto measured values as shown in Figure 6 for selenate dis-tribution the same results for selenomethionine is alsoobtained.

The model also simulates the concentration of Sespecies under transient water flow condition in unsatu-rated soil column. LEACHM was used for simulatingwater content and water flux. The model developed byHuston and Wagenet (1989). Total time for the simula-tion of water flow and Se transport model was 20 days.The texture of the soil profile was assumed to be uni-form and composed of 28% clay, 30% silt and 42%sand. The hydraulic conductivity of soil was assumedto be 11Ð40 m day�1 with 55% saturation condition. Thedepth of the soil profile taken as 120 cm. The flow rate

Simulation results selenate distribution

0102030405060708090

100

0 10 20 30 40 50 60 70 80 90 100 110 120

Depth from soil surface (cm)

Rel

ativ

e co

ncen

trat

ion:

C/C

0 (%

)

T=2 Days T=4 Days T=6 DaysT=8 Days T=10 Days Measured

Figure 4. Variation of selenate concentration with time steps in the soilcolumn under steady-state conditions

Selenate distribution column experiment

100

0102030405060708090

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Distance from column inlet (cm)

Rel

ativ

e co

ncen

trat

ion

C/C

0(%

)

T= 0.68 Day T= 1.36 Day T= 2.04 DayT= 2.77 Day Measured

Figure 5. Comparison of simulated and measured concentration of sele-nate for different time steps under transient conditions

Selenate distribution column experiment

0

10

20

30

40

50

60

70

80

90

100

0 9 10 11 12 13 14 15 16 17

Distance from column inlet (cm)

Rel

ativ

e co

ncen

trat

ion

C/C

0 (%

)

Simulated (ks=0.07) Simulated (ks=0.06)Simulated (ks=0.05) Measured points

1 2 3 4 5 6 7 8

Figure 6. Effect of adsorption coefficient on selenate concentration in thesoil column

to soil column was assumed to be 30 mm day�1 for thefirst 7 days, for next 6 days; there is no flow (zero flex) inthe last 5 days. There is 5 mm day�1 of evaporation fromthe soil surface. The simulation results for the water flowmodel are shown in Figure 7. The two curves in Figure 7show an exponential and parabolic relation exists in thesoil profile under saturation and unsaturation soil condi-tions. The simulation results for the water flow modelare shown in Figure 8. The curves in Figure 8 showan exponential and parabolic relations exist in the soilprofile under unsaturation and saturation soil conditions.The simulation results for selenate, selenite, selenomen-thionine, adsorbed selenite, adsorbed selenomethionine,organic Se, gaseous Se and elemental Se are in goodagreement with measured values. Figures 9 and 10 showsthat the distribution of different Se species in the soilprofile from the surface to 120 cm depth follow the samepattern. As the wetted front of soil proceed to the bottomof the column, the concentration of different Se speciesdecreases with time. If the water flow to the soil columnis less or about 30 mm day�1 for 7 days and the depth ofthe water table is located below 120 cm of the soil col-umn. There would be no contamination of ground waterwith different Se species.

The simulation results of the model under transientflow was also verified by using mass balance calculationsin each time step calculations. There is about 1% error insimulation results in comparison with mass balance cal-culations. Volatilization rate constant, Kv, mineralizationand immobilization rate constants, K5, K6, K3 and K4,and decomposition rate constant, K7 were determinedthrough calibration. Further experiments are needed toevaluate the exact values of transformation rate constants.Table I shows parameter values used in the model.

SUMMARY AND CONCLUSIONS

This study was conducted to describe different processesaffecting transport and transformation of Se species



Simulated flow filed

0.30.320.340.360.380.4

0.420.440.460.480.5

0.520.540.56

1000 11009008007006005004003002001000 1200


Wat

er c

onte

nt (

m3/

m3)

T= 20 Days T= 14 Days

T= 8 to 13 Days T= 1 to 7 Days

Figure 7. Variation of soil water content with different time steps in thesoil column

Simulation flow field water flux variations

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 150 300 450 600 750 900 1050 1200


Wat

er fl

ux (

cm/d

ay)

T=19.6 Days T=14.6 DaysT=0.5 Day T=7.1 Days

T=11,12,13 Days

Figure 8. Variation of water flux with time steps in the soil column

through an unsaturated soil column under unsteadyand steady state flow conditions. Various mechanismsof solute transport in porous media including convec-tive transport, diffusive and dispersive transport, surface

Simulation results adsorbed selenate distribution

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 100908070605040302010


Con

cent

ratio

n (u

Mol

e/K

g)

T=1 Days T=2 Days T=3 DaysT=4 Days T=5 Days Measured

Figure 9. Variation of adsorbed selenate in the soil column under transientconditions

adsorption and adsorption, oxidation and reduction, min-eralization and volatilization were determined. Basic gov-erning equations of Se transport and transportation suchas convective dispersive equation including sources andsinks were derived using mass balance criteria.

For the solution of transport and transformationequations, flow field data including water flux and watercontent in the soil column were calculated using suitableflow models. LEACHW, a model for predicting unsteady-unsaturated flow field variables was used.

To solve the equations of different Se species, animplicit Crank–Nicholson scheme of finite differencemethod was used and a computer program was developedfor this reason.

The model was verified by comparison of the resultswith experimental measurements and also using massbalance calculations in each time step calculation. Thesimulated results are in good agreement with measuredvalues. The model is a useful tool for appraising pollu-tion problems and also for the prediction of ground watercontamination. The model and its users manual can beobtained through the Department of Civil Engineering

Table I. Description of parameters used in the model

Parameters Description Values andunits

Reference

KS 1 Adsorption coefficient for selenate 0Ð07 l kg�1 Alemi et al. (1991)KS 2 Adsorption coefficient for selenite 4Ð5 l kg�1 Model calibrationKS 3 Adsorption coefficient for selenomethionin 2Ð5 l kg�1 Model calibrationD Diffusion coefficient in pure liquid 1Ð2 cm2 day�1 Alemi et al. (1991)K1 Rate constant for reduction of selenate 0Ð01 day�1 Alemi et al. (1991)K2 Rate constant for reduction of selenite 0Ð01 day�1 Alemi et al. (1991)K3 Rate constant for immobilization of selenate 0Ð01 day�1 Model calibrationK4 Rate constant for immobilization of selenite 0Ð01 day�1 Model calibrationK5 Rate constant for mineralization of selenate 0Ð01 day�1 Model calibrationK6 Rate constant for mineralization of selenite 0Ð01 day�1 Model calibrationK7 Rate constant for decomposition of organic selenite 0Ð01 day�1 Model calibrationKv Volatilization rate constant for selenomethionin 0Ð02 day�1 Model calibration



0

10

20

30

40

50

60

70

80

90

100

0 10 15 20 25 30 35


Rel

ativ

e co

ncen

trat

ion:

C/C

0 %

Simulation results selenomrthionine distribution

T=2, 4, 6 Days T=8, 10, 12 DaysMeasured

5

Figure 10. Variation of selenomethionine concentration with time stepsin the soil column under transient conditions

at Shiraz University, Shiraz-Iran, or through the Depart-ment of Land, Air and Water Resources, University ofCalifornia, at Davis.

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Alemi MH, Goldhamer DA, Nielsen DR. 1991. Modeling seleniumtransport in steady state, unsaturated soil columns. Journal ofEnvironmental Quality 20: 89–95.

Benson SM, White A, Halfman S, Flexser S, Alavi M. 1991. Ground-water contamination at the Kesterson Reservoir, California—Part I:Hydrogeologic setting and conservative chemical transport. WaterResources Research 27(6): 1071–1084.

Biggar JW, Jayaweera RG. 1990. Soil–Water–Vegetation ManagementStudies of Selenium in Kesterson Restoraration and ContaminantProject , University of California, Davis. December 1990.

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Cooke TD, Bruland KW. 1987. Aquatic chemistry of selenium: evidenceof biomethylation. Environmental Science (Technical) 21: 1214–1219.

Deverel SJ, Gilliom RJ, Fujii R, Izbickii JA, Fields JC. 1984. A RealDistribution of Selenium and Other Inorganic Constituents in ShallowGround Water of the San Luis Drain Service Area, San Joaquin Valley,California: A Preliminary Study , Water Research Investigation Report84, 4319. US Geological Survey: Reston, VA; 67 pp.

Fio JL, Fujii R, Deverel SJ. 1991. Evaluation of selenium mobility anddistribution in irrigated and non-irrigated alluvial soils. Soil ScienceSociety of America submitted for publication.

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Nimah MN, Hanks RJ. 1973. Model far estimation of soil water, plantatmospheric interrelations: I. Description and sensitivity. Soil ScienceSociety of America 37: 522–527.

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Tokunaga TK, Zawislanski PT, Johannis PW, Benson S, Lipton DS.1994. Field investigations of selenium speciation, transformation, andtransport in soils from Kesterson Reservoir and Lahontan Valley.In Selenium in the Environment , Frankenberger WT, Benson S (eds).Marcel Dekker: New York; 119–138.

Tokunaga TK, Lipton DS, Benson SM, Yee AW, Oldfather JO, DuckartEC, Johanis PW, Halvorsen KE. 1991. Soil selenium fractionation,depth profiles and time trends in a vegetated upland at KestersonReservoir. Water Air Soil Pollution 57: 31–41.

Zawislanski PT, Zavarin M. 1996. Nature and rates of seleniumtransformations; a laboratory study of Kesterson Reservoir soils. SoilScience Society of America 60: 791–800.

Wagent RJ, Starr JL. 1977. A method for the simultaneous control of thewater regime and gaseous atomosphere in soil columns. Soil ScienceSociety of America 41: 658–659.

Westerman DT, Robbins CW. 1974. Effect of SO4� S fertilization on Seconcentration of alfalfa (Madicago sativa L.). Agronomy Journal 66:207–208 (Cited from Mikkelsen, et al., 1989).

White AF, Benson SM, Yee AW, Wollenberg HA, Flexser S. 1991.Groundwater contamination at Kesterson Reservoir, California.2. Geochemical parameters influencing selenium mobility. WaterResources Research 27: 1085–1098.


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