SW—Soil and Water: Transport of Particulate and Colloid-sorbed Contaminants through Soil, Part 1: General Principles

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  • Biosystems Engineering (2002) 83 (3), 255273t

    U

    ll

    otherwise harmless mobile colloidal clay particles or soil organic matter. Potential impediments to movementof colloids through soil can be subdivided into straining and ltration, depending on whether a particle has a

    dimension similar to pores (leading to physical trapping) or much smaller. Filtration mechanisms, includinginterception, diffusion and sedimentation, have been compared to those described in the extensive engineeringliterature on deep bed ltration. Sorption processes are discussed, both those to static components of the soilmatrix and onto mobile colloids. The chemical inuences of ionic strength and pH to colloid transport arereviewed, as well as the double diffusion layer as a mechanism linking particles to surfaces.Numerous reported studies using column experiments to measure colloid or contaminant transport throughsoil have been reviewed. Many indicate the importance of macropore ow which allows rapid unrestrictedtransport of contaminant carrying colloids. Some experiments determine a ltration coefcient for a simpleltration equation representing straining and ltration processes. The few existing models (incorporating thisltration equation), both for column experiments and for the eld situation, are reviewed as background tofurther development of a eld-scale model representing colloid-facilitated transport of a range of chemical andmicrobiological contaminants. # 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved

    1. Introduction

    Many contaminants move through the soil inparticulate or colloidal form, either because they areinherently particulate (sometimes due to low solubility),or because they are sorbed onto otherwise harmlessmobile particles which are present in the soil. However,unlike soluble contaminants which can move freely withsoil water movements, there are various restrictions onthe movement of particulate contaminants, includingboth capture mechanisms and sorption by staticcomponents of the soil. This paper discusses themovement and restrictions on movement of suchparticulate and colloidal contaminants, and the mechan-isms which control them. There appear to be two main

    which covers a range of mechanisms by which particlesare captured in pores with dimensions larger than theparticles. Other restrictions on movement includesorption and various electrostatic charge mechanismsincluding the diffuse double layer (DDL). An expandeddiscussion of capture mechanisms relating particle sizesto pore sizes is presented in Part 2 (McGechan, 2002a),making use of various theoretical and mathematicalconcepts.

    2. Three-phase representation of contaminant transport

    In many previous studies of contaminant transport,only a two-phase approach has been used, in whichdoi:10.1016/S1537-5110(02)00192-7, available online at hSW}Soil and Water

    REVIEW

    Transport of Particulate and Colloid-sorGeneral P

    M.B. McGecha

    Environment Division, SAC, Bush Estate, Penicuik EH26 0PH,

    (Received 14 September 2001; accep

    Literature is reviewed describing through soil colphosphorus, pesticides and other agrochemicals, pmicroorganism (viruses and bacteria) are transported mtypes of physical capture mechanisms restricting colloidmovement in soil and other porous media: straining,where the physical size of the pore is smaller than theparticle so the particle is unable to pass; and ltration,

    1537-5110/02/$35.00 255tp://www.idealibrary.com on

    PAPER

    bed Contaminants through Soil, Part 1:rinciples

    n; D.R. Lewis

    K; e-mail of corresponding author: m.mcgechan@ed.sac.ac.uk

    ted in revised form 1 August 2002)

    oid-facilitated transport of contaminants such asus a range of biological microorganisms. Smallerainly (like chemical contaminants) by adsorption ontocontaminants are partitioned between an immobile solidphase and a mobile aqueous phase. In such a two-phasesystem which is most appropriate to soluble contami-nants, the rate of overall migration can be signicantly

    # 2002 Silsoe Research Institute. Published byElsevier Science Ltd. All rights reserved

  • Dp particle diffusivity in Brownian motion, m s T absolute temperature, K1

    M.B. MCGECHAN; D.R. LEWIS256slower than the rate at which the water ows, becausereactive contaminants tend to be absorbed onto theimmobile solid phase. However, colloids in water canact as a third mobile solid phase (McCarthy & Zachara,1989), which can sorb contaminants in a similar fashionto the immobile solid phase, and can migrate at ratessimilar to or even greater than the mobile aqueousphase. A three-phase approach is therefore necessary toaccurately simulate the migration of sorbing contami-nants. McDowell-Boyer et al. (1986) also discuss theimplications of this mobile colloid third phase, referringto experiments by Vinten et al. (1983c) and Jury et al.(1986) which rst challenged the then accepted view thatstrongly sorbing compounds would be completelyimmobilised by sorption onto the static soil componentalone. Mills et al. (1991) describe mobile and immobile

    phases for both colloids and chemical contaminants(Fig. 1).The mobile solid phase consists of colloidal (or

    particulate) particles in the size range from 1 nm to1 mm (Buddemeier & Hunt, 1988), although otherauthors state the upper size limit as 10 mm. Theseparticles move mainly under the inuence of Brownianmotion (i.e. due to bombardment by uid moleculesmoving with a random thermal nature). Ibaraki andSudicky (1995a) have introduced the distinction betweentrue colloids and pseudo-colloids. True colloids aregenerated from contaminants such as radionuclideswhich are precipitated when their concentration exceedstheir solubility. Pseudo-colloids originate from non-contaminant sources, such as clay particles, which dueto their small particle size and large specic surface area,

    f collector surface area per unit volume of packedbed, m2 m3

    F ltration sink term, g m3 h1

    g acceleration due to gravity, m s1

    I rate of colloid capture per grain, s1

    k Boltzman constant, J K1

    kd colloid deposition rate coefcient, s1

    kps equilibrium solidliquid partition coefcientbetween dissolved solute and solute sorbedonto immobile soil matrix

    kpcs equilibrium solidliquid partition coefcientbetween dissolved solute and solute sorbedonto colloids

    kp biochemical transformation rate for dissolvedsolute, s1

    kr colloid release rate coefcient, s1

    ks biochemical transformation rate for solutesorbed onto immobile soil matrix, s1

    kcs biochemical transformation rate for dissolvedsolute sorbed onto colloids, s1

    U uid velocity, m sv pore water velocity, m h1

    vref pore water velocity at which fref is measured,m h1

    vp velocity of colloidal particles in porous mediam s1

    vs particle settling velocity, m s1

    z distance, ma volume of collector grains in array, m3

    ae particle attachment efciencyb excluded area parameter, kg kg1

    y soil porosity, m3 m3

    lf ltration coefcient, m1

    lref reference ltration coefcient, m1

    l0 clean bed ltration coefcient, m1

    m uid dynamic viscosity, N s m2

    rb solid matrix bulk density, kg m3

    rf, rp uid and particle densities, kg m3A Hamaker constant in Londonvan der Waalsattractive force equations, J

    As Parameter correcting particle capture by asingle isolated collector for capture within apacked porous medium

    B(s) Dynamic blocking functionc Mass concentration of colloids in solution,

    kg kg1

    cn Suspended colloid number concentrationper unit volume of packed bed, m3

    dm diameter of mobile particle, mdp diameter of collector grain, mDh hydrodynamic dispersion coefcient for colloi-

    dal particles, m2 s12 1

    Notakt particle transfer coefcient, m s1

    mc average mass of colloidal particles, kgnf empirical exponentp0 clean bed porosity, fractionP Parameter related to media porositys Fraction of collector surface covered by

    particlessm Deposited colloidal particles per unit mass

    of the porous media, kg kg1

    sj jamming limit slopeSmax maximum fractional coverage by deposited

    colloidal particless1 hard sphere jamming limitt time, s

    tion

  • TRANSPORT OF CONTAMINANTS THROUGH SOIL 257Chemical

    Dissolvedsolute

    Adsorbed

    Adsorbed

    solute

    solute

    (immobile soil matrix)

    (colloidal solids)

    Colloids

    Mobilecolloids

    Generation

    kp

    kps

    kpcs

    kcs

    ksbecome contaminants due to sorption of something elseonto their surface. In the former case the process ofcolloid transport is the main focus, whereas in the lattercase (which is most common in agricultural applica-tions) both colloid transport and the sorption of thecontaminants onto the colloids are important.

    3. Role of macropores and fractures in particulatecontaminant transport

    A number of researchers have discussed the impor-tance of macropores and fractures in subsurface layersfor fast relatively unrestricted transport of colloidalcontaminants; e.g. Rahe et al. (1978) observed very fastmovements of E. coli organisms through some soilhorizons which could only be attributed to macroporeow. Macropores arise in agricultural soils for a numberof reasons, including worm holes, channels created byplant roots which have since died and withered away,cracks in dry clay soil, and inter-aggregate spaces which

    Immobilisation Mobilisation

    Immobile colloids

    Immobilesoil

    matrix

    Fig. 1. Framework of phases for colloid facilitated contaminanttransport (based on Mills et al., 1991); lines with single arrowsdenote kinetic processes; lines with two arrows denote equili-brium processes; ks, kcs, kp, biochemical transformation rates;

    kps, kpcs, equilibrium solid-liquid partition coefficientsbecome water-lled in wet soil conditions (Fig. 2). Someauthors such as Ibaraki and Sudicky (1995a, 1995b) andGwo et al. (1998) use the term fractures rather thanmacropores, particularly with reference to contaminantsreaching groundwater aquifers after passing throughdeep subsurface layers as well as the soil prole.However, whether described as macropores or fractures,the mathematics of water and contaminant transportthrough them are the same. Jarvis et al. (1999) quoteMcDowell-Boyer et al. (1986) to suggest that macro-pores are the only pathways by which suspended mattercan pass through the unsaturated zone, since suchparticles are efciently retained by physical ltrationprocesses when moving through the more tortuous soilmatrix pores (Fig. 2). A similar conclusion was reachedby Kretzschmar et al. (1994). Macropore or fractureow generally needs to be studied experimentally under

    Fig. 2. Water and contaminant movement in soil matrix poresand macropores; soil matrix flow leads to straining or filtrationof particulate (colloid-sorbed) contaminants; macropore flowoccurs when inter-aggregate pore spaces are water-filled,particulate contaminants pass rapidly through soil without

    restrictions

  • M.B. MCGECHAN; D.R. LEWIS258eld conditions, since any attempt to extract, transportor reconstitute soil cores for laboratory experimentstends to alter or destroy the macropore structure.However, Abu-Ashour et al. (1998) managed to workin the laboratory with reconstituted soil columns inwhich macropores had been created articially, demon-strating fast movement of bacteria out the bottomthrough the macropores compared to nothing comingthrough columns without the macropores.

    4. Nature and size of particulate and colloidallytransported contaminants

    4.1. Chemical pollutants

    The main agriculturally derived chemical substanceswhich pollute water and are transported through the soilin particulate or colloidal form are phosphorus arisingfrom mineral fertiliser or manure applications, and somepesticides. The important mechanism by which thesesubstances are transported is by molecules beingadsorbed onto small but otherwise harmless particlesor colloids, such as clay components of soil or organicmaterial from the soil or from manure. This mechanismappears to be of far greater importance than anymovement of precipitated insoluble phosphorus com-pounds or pesticides. The physical size and othercharacteristics of the carrier particles or colloid aretherefore the important determinants for the movementof such chemical pollutants.

    4.2. Carrier particles and colloids

    Soil particles are generally categorised as clay (ratherthan silt or sand) if they have a diameter of less than2 mm. McCarthy and Zachara (1989) suggest a size rangeof 0.110 mm for various soil mineral derived colloids,including clays, iron (hydr)oxides, silica and lime.Pilgrim and Huff (1983) measured sediments in a largermean size range 48 mm diameter in subsurface owsfollowing storms of low to moderate intensity, whichthey attribute to detachment on the surface andtransport through the soil via macropores. Soil organicmatter and organic particles from manure and slurryhave a wide range of physical sizes, but particles at thesmall end of the range have the highest surface area tovolume ratio, so provide the largest number of sites forsorption of pollutants. Matthews et al. (1998) haverecently analysed the quantity of phosphorus sorbedonto colloidal particles in various size ranges beingtransported in water owing either by combined over-land and inter-ow or through mole drains. This showedthat a high proportion of the phosphorus was sorbedonto particles much smaller than 2 mm. Voice et al.(1983) and Gschwend and Wu (1985) have described animportant role played by non-settling microparticles andorganic macromolecules as carriers for hydrophobicorganic pollutants. Thurman et al. (1982) analysedaquatic humic substances, concluding that some hadhigh molecular weights within the colloidal size range.Atteia and Kozeel (1997) measured size distributions forparticles and colloids in Karstic aquifers. They con-sidered particles up to 4 mm to be contaminant carryingcolloids, which although accounting for a small propor-tion of the mass of suspended material, had a very highproportion of the area of sorbing surfaces. McCarthyand Shevenell (1998) carried out a similar analysis ofcolloids in aquifers, also examining chemical composi-tions of colloids and carried contaminants.

    4.3. Biological pollutants

    Information in the literature about biological pollu-tants, particularly pathogens in livestock wastes (includ-ing faeces deposited by grazing animals), and theirpotential for movement through the soil to causeenvironmental pollution, have been reviewed byMawdsley et al. (1995). This review categorises micro-organisms in ascending size order as viruses, bacteria(including E. coli and salmonella) and protozoa(including Cryptosporidium and Giardia), but no sizesare stipulated for viruses or bacteria. A size range of 20200 nm for viruses is stated by Bitton (1975), althoughviruses can form aggregates under certain conditions asdescribed by Floyd and Sharp (1978a, 1978b) and Floyd(1979). McCarthy and Zachara (1989) suggest a sizerange of 1-100 nm for viruses and 0.53 mm for bacteria.Kretzschmar et al. (1999) describe graphically the sizeranges for viruses and bacteria, alongside those forcarrier colloids (Fig. 3). Mawdsley et al. (1995) describecryptosporidium as consisting of transmissive oocystswith a diameter of about 46 mm (although they maycoagulate to an unknown degree to form larger units),and the most commonly studied out of three species ofGiardia having infective cysts with diameter 714 mm(also shown in Fig. 3). Similar size ranges for the maintypes of microorganisms and colloids are described byStumm (1977), who also discusses how such sizes can bechanged by coagulation processes.

    5. Sources of colloidal material

    Carrier particles and colloids are either added to thesoil surface along with other substances in land spreadwastes, or are created within the soil by particles being

  • TRANSPORT OF CONTAMINANTS THROUGH SOIL 259detached from the static soil matrix. Slurry, manure andother organic wastes contain large quantities of parti-

    Fig. 3. Size ranges of colloids and microorganisms (adaptedfrom Kretzschmar et al., 1999)culate and colloidal material with a wide range ofparticle sizes, of which the smaller particles will alreadybe carrying a high load of sorbed pollutants (such asphosphorus and microorganisms) when applied to thesoil. In contrast, clay and other colloidal mineralparticles plus some organic colloids are created bydetachment. A number of authors have suggested thatsome detachment of such particles takes place as waterpasses through the macropores and soil matrix pores ofthe soil prole. Kaplan et al. (1993), Govindaraju et al.(1995) and Jacobsen et al. (1997) all measured thisprocess in laboratory soil columns or lysimeters, andGovindaraju et al. (1995) used their results to calibrate amodel of detachment, transport and deposition of clayparticles. Ryan and Gschwend (1990) analysed colloidsin groundwater aquifers attributable to detachment inwater passing through subsurface layers. However,Jarvis et al. (1999) have expressed the opinion thatdetachment within the pores will be dwarfed by a muchmore extensive detachment occurring when raindropsimpinge on the soil surface, the classical erosion process,and eroded particles then enter pores in the soil prole(as described also by Pilgrim and Huff, 1983). There isextensive literature on erosion, some authors relating theextent of erosion to kinetic energy of raindrops(e.g. Rose et al., 1983), giving higher levels of erosionfor a given rainfall quantity in short-lasting, heavy,thundery rain than more continuous gentle rain. Otherauthors including Miller and Baharuddin (1986),Shainberg et al. (1992), Brubaker et al. (1992), Curtinet al. (1994) and Lieffering and McLay (1996) measuredvariations in dispersivity (a measure of erodability)amongst different soils in relation to their differingcomposition and chemistry. Quirk and Schoeld (1955),Rowell et al. (1969), Frenkel et al. (1978) and Moutieret al. (1998) investigated an effect where an electrolytesolution in soil water causes some clay soils to hydrateand swell (reducing the size of large soil pores and henceconductivity), and this results in an increase indispersivity of small clay particles.

    6. Interactions with surfaces

    6.1. Sorption

    6.1.1. General principles of sorption mechanisms andprocesses

    Sorption is the process by which reactive substancesincluding contaminants attach themselves to surfaces ofotherwise harmless solids. A nely divided solid such asclay has a high sorption capacity due to its small particlesize and large specic surface area. Sorption sites in soiloccur both in the static soil matrix and on mobile soil(mainly clay) particles. Sorption is the mechanism bywhich a reactive solute combines with an otherwiseharmless colloid to become a pseudo-colloid contami-nant.The extent of sorption varies between different

    reactive solutes and between solid materials, and formobile colloids in relation to the particle size distribu-tion. The term isotherm is commonly used for theequation relating the concentration of a substance onsorption sites to its concentration in water. Isothermstend to be non-linear, as energy levels vary betweendifferent binding sites on the solid surfaces, high-energysites becoming occupied before low-energy sites. Somecommonly used sorption isotherm equations, withalternatives for instantaneous equilibrium and for akinematic, time-dependent approach to equilibrium,have been described by McGechan and Lewis (2002).

    6.1.2. Sorption of contaminants in soilThe extensive literature on sorption of phosphorus

    onto soil particles (mainly in relation to the static soilmatrix), e.g. Barrow (1973,1974) and Barrow and Ellis(1986), has been reviewed in detail by McGechan andLewis (2002) and McGechan (2002b). An example ofexperimentally derived isotherms for phosphorus from

  • M.B. MCGECHAN; D.R. LEWIS260Hooda et al. (1999) is shown in Fig. 4, based on theFreundlich equation with different coefcients for lowand high phosphorus concentrations below and above abreak-point. There are many studies of sorption ofcontaminants onto static soil components (e.g. Knight &Tomlinson, 1967; Faust & Zarins, 1969; Gonzalez &Ukrainczyk, 1999), a process which locks up thecontaminants and reduces water pollution in the shortterm. The kinetics of time-dependent desorption (thereverse of sorption) of sorbed uranium from the staticcomponent of soils is described by Braithwaite et al.(1997).

    6.1.3. Sorption of contaminants onto colloidsIn contrast to work on sorption onto static soil

    components, there is very little published informationon sorption to mobile colloids, the process whichaggravates pollution. Karickhoff et al. (1979) studiedsorption of various hydrophobic pollutants (mainlyorganic liquids) onto natural sediment particles, ttinglinear or logarithmic isotherms to their data. Voice et al.(1983) measured sorption of hydrophobic pollutants,

    Fig. 4. Freundlich isotherms for phosphorus sorption in silty clayloam soil for a range of soil depths at Crichton Royal Farm,Dumfries, logarithmic plot (above) and linear plot (below).From Hooda et al. (1999); , 0-0.1 m; , 0.1-0.2 m;

    , 0.2-0.3 m; , 0.3-0.45 m; , 0.45-0.6 mbut concluded that results were complicated by thepresence of microparticles at the small end of the colloidsize range. Gschwend and Wu (1985) also experimentedwith sorption of hydrophobic organic pollutants ontosediments, concluding that the process could bedescribed by a simple partition coefcient, equivalentto a linear isotherm with complete and instantaneousreversibility. In contrast, Penrose et al. (1990) concludedthat radionucleides are almost irreversibly sorbed ontocolloidal material in the size range 0025045 106 m.OConnor and Connolly (1980) studied the effect oforganic matter content and clay content on the partitionof contaminants between the dissolved phase and thesorbed phase for a number of pesticides and heavy metalcompounds.

    6.2. Colloidal particles interacting with soil surfaces

    6.2.1. Sorption of microorganismsMarshall (1971) and M .uuller and Hickisch (1970) have

    reviewed the subject of sorption of microorganisms onsoil particles, and further information on sorption ofbacteria by clay minerals is presented by Filip (1973).Filip (1973) also observed that the growth of somemicroorganisms could be enhanced where other sub-stances which inhibit growth are removed from solutionby sorption onto clay particles. However, Bitton (1975)reviewing the subject of sorption of viruses indicated alack of information about sorption for these organisms.Burge and Enkiri (1978) measured virus sorption by vedifferent soils. Goyal and Gerba (1979) observed largevariations in the extent of adsorption between differenttypes and strains of viruses under the same conditions.Moore et al. (1982) describe the mechanisms by whichviruses are sorbed in soil, and measured variations insorption capacity of a range of soil minerals. Murrayand Parks (1980) measured sorption of poliovirus onoxide surfaces in soil, showing that the ionic strength,pH and composition of the soil solution had a majoreffect. Vilker and Burge (1980) and Vilker (1981) havedescribed a mass transfer model of transport of virusesthrough soil which includes sorption according to theFreundlich isotherm equation, but for sorption by thestatic soil matrix only.

    6.2.2. Surface charge effectsCharged colloids within soils originate from a multi-

    tude of sources, ranging from small mineral clays,organic breakdown products, to small living organismssuch as coliforms or viruses. These colloids can often beconsidered as behaving like charged particles, interact-ing strongly with soils through a variety of surface

  • TRANSPORT OF CONTAMINANTS THROUGH SOIL 261reactions. An important feature of colloid transport isthus the nature and occurrence of surface charge in soils(Brady & Weil, 1996).A negative soil surface charge in inorganic matter is

    the result of isomorphic substitution in the matrix orbonds broken at the surface of oxides, etc. withdissociation of ions passing into solution, while fororganic matter a negative charge is produced when e.g.hydroxyl groups become attached to polymer chains.Apart from the rst case of isomorphic substitution,these reactions are usually pH dependent.

    6.2.3. Diffuse double layerPhysiochemical surface reactions affecting colloids

    include the ltration processes discussed later andthe concept of the diffuse double layer (DLL). Thislayer arises from the nite spatial arrangement of ionsdissociated from a charged surface. Such ions aresubjected to two opposing forces: an adsorptionforce from the electric eld generated by the chargedsurface, and a diffusive force from concentrationgradients acting to equilibrate the ion concentrationsthroughout the solution (Brady & Weil, 1996). Ions in the bulk solution, outside of the DLL, arenot inuenced by the charges of the soil solids.Consideration of the DDL facilitates our under-standing of the behaviour of colloidal material withinsoils.The original GouyChapman theory of DDL, men-

    tioned in Shaw (1999), assumes that the distribution ofions around a charged surface follows the Boltzmann-Maxwell distribution and the resulting electric potentialis determined by the Poisson equation. According to thistheory, there will be an exponential accumulation ofcations within the layer when there is a negative chargeon the surface, and the anionic concentration willdecrease from the bulk solution to the solid surface.There are several more complex theories for DDLbehaviour involving different ion and solid surfacedistributions (Shaw, 1999), but the essentials areencapsulated by the GouyChapman theory.An important characteristic of a DDL is the thick-

    ness, which can be determined by the three variables,logarithm of the bulk soil solution concentration,valence of the cations and the soil water content. Thelayer thickness is large (10 mm) for monovalent cationscombined with low electrolyte concentrations undersaturated conditions. Correspondingly this layer is thin(0.1 mm) for multi-valent ions, high electrolyte concen-trations and unsaturated conditions. Within the DDL,charged colloids, nutrients, etc. can be held, and releasedslowly when needed by cation exchange to the bulksolution.7. Transport, straining and ltration of colloidalcontaminants in soil

    Following their description of types and sizes ofmicroorganisms (Section 4.3), the review by Mawdsleyet al. (1995) continues to discuss factors inuencingmicrobial movement (both horizontal and vertical)through the soil, including soil type, soil water content,soil water ow, intensity and type of rainfall, surfaceproperties of microorganisms and presence of plantroots. They quote literature sources which suggest thatsoil pore size will signicantly affect movement ofbacteria and protozoa through the soil, but not virusesbecause of their smaller size, and that movements will begreater in coarse-textured than in ne-textured soils.They quote a paper by Grifn and Quail (1968) thatmovement of rod-shaped bacteria will be severelyrestricted in pores of less than 115 mm diameter. Theyalso discuss adsorption of microbes onto soil material(in a similar manner sorption of phosphorus orpesticides), particularly the higher sorptive capacity ofclay and organic material compared to other soilparticles. Sorption is particularly important for viruses,being the main mechanism for their retention in soil(Goyal & Gerba, 1979; Bitton, 1975). However,Mawdsley et al. (1995) regard high contents of clayand organic material in soil as a factor retardingmovement of microbes, ignoring the possibility ofsorption onto mobile colloids which move with the soilwater. They also mention the importance of macroporesfor transport of protozoal cysts in particular because oftheir largest size. Further studies reviewed by Mawdsleyet al. (1995) indicate increased movements of bacteriaand viruses in soils with high water contents, includingdetection of bacteria in water owing out through elddrains when animal manure is spread at times of highrainfall. Both Evans and Owens (1972) and Patni et al.(1984) observed that rainfall increased concentrations ofbacteria in drains from elds with grazing animals orafter slurry spreading, the higher mobility of bacteria inwetter soil more than offsetting any dilution effect of therainfall.Vinten and Nye (1985) discuss transport and deposi-

    tion of colloidal particles in soil by a number ofmechanisms (Fig. 5), while other authors includingMcDowell-Boyer et al. (1986), Hunt et al. (1987),McCarthy and Zachara (1989) and Ibaraki and Sudick-ey (1995a) discuss transport through porous media inrelation to contaminants reaching groundwater viavarious subsurface layers. Corapcioglu and Haridas(1984) discuss these processes theoretically for micro-organisms as the contaminant. All these authors quotethe extensive literature on ltration (in relation toindustrial rather than agricultural applications and

  • M.B. MCGECHAN; D.R. LEWIS262covering ltration of air as well as liquids), such as Ives(1975) and Tien and Payatakes (1979).

    8. Colloid capture processes

    8.1. Transport equation

    Assuming steady-state saturated ow conditions, thetransport of colloidal particles through granular porousmedia can be described by an advectiondispersiontransport equation with terms for colloid deposition andrelease (Kretzschmar et al. 1999):

    @c

    @t Dh

    @2c

    @z2 vp

    @c

    @zrby

    @sm@t

    1

    and

    rby

    @sm@t

    kdc rby

    krsm 2

    These equation describe the evolution in time t of thecolloidal mass or number concentration c in suspension,

    Fig. 5. Particle capture mechanisms. From Vinten and Nye,(1985); F, flow streamline; D, diffusion; I, interception; ST,

    straining; SD, sedimentationand the colloidal amount deposited per unit mass of theporous matrix sm. Here, Dh is the hydrodynamicdispersion coefcient for colloidal particles, rb thematrix bulk density, y the porosity and np the averagevelocity of colloidal particles over a distance z. Thecolloidal deposition and release rate coefcients aregiven by kd and kr respectively.

    8.2. Straining

    Capture by straining, where a colloidal particle isphysically larger than a pore or pore restriction it isattempting to pass through, is discussed in detail in Part2 (McGechan, 2002a).

    8.3. Restriction on colloid movements by filtration

    8.3.1. IntroductionMcDowell-Boyer et al. (1986), Hunt et al. (1987) and

    Ibaraki and Sudickey (1995a) describe a range ofcapture processes as ltration, including retention ofcolloidal particles through attraction to porous materialsurfaces, fracture surfaces and trapping in pores.Referring to the literature on ltration, they describesurface (mechanical) ltration as occurring when colloi-dal particles are too large to ow into a porous mediumand consequently form a surface mat (cake) over theporous material; however, other authors call this processstraining (as discussed above and in Part 2, McGechan,2002a). Deep bed ltration is described as occurringwhen colloids are small enough to enter the porousmedia and are transported and deposited within it. Twotypes of deep bed ltration are also described: mechan-ical ltration of large particles in the matrix (whichagain is in effect straining as discussed in Part 2) andphysicochemical ltration of small particles throughmolecular forces. For industrial purposes, deep bedmechanical ltration is not a practical option as the lterrapidly becomes clogged, but the theory is neverthelessdiscussed by Tien and Payatakes (1979), and bothforms of deep bed ltration are relevant to ltration bysoil. Equations which include the ltration coefcient lfare appropriate to either of the deep bed ltrationprocesses.Ives (1975) describes physicochemical deep bed

    ltration as being the capture of ne particles insuspension by ltration through a porous medium,and divides the process into three steps: transport,attachment and (in some situations only) detachment.Particles tend to be mainly transported with the owstreamlines, but to become attached they must moveacross the streamlines for one or more reasons

  • TRANSPORT OF CONTAMINANTS THROUGH SOIL 263including: interception where the particle passes closerthan one particle radius from the solid surface andelectrostatic forces come into play; inertia where thestreamline takes a curved path around an obstructionbut the particle tends to keep going in a straight line;gravitational forces causing sedimentation; diffusioncaused by bombardment by water molecules undergoingBrownian motion; and hydrodynamic action where oneside of the particle is in a faster moving streamline thanthe other causing the particle to rotate and inducingpressure differences. Attachment can take place whenparticles intercept or come into close proximity with thepore surface due to a combination of electrostatic forcesand Londonvan der Waals forces. Kretzschmar et al.(1997) have described the process of colloid ltration insoil, where particles are much smaller than the pores inwhich they become trapped. Vinten (1981) also studiedthis process, describing details of the forces leading toattachment. The extent of all the ltration mechanismsdepends on a range of factors including the relative sizesof particles and pores. Ives (1975) concluded thatsedimentation is likely to be unimportant since sedi-mentation velocities can be calculated as beingextremely low for clay particles due to their smalldiameter; also, sedimentation will be almost zero forbiological pollutants which have a density near to thatof water.

    8.3.2. Basic filtration equationsThe basic equation used in all ltration theory to

    represent removal of particles (mass concentration c)with distance z was rst empirically observed by Iwasaki(1937) to be a rst-order relationship with suspendedparticle concentration:

    @c

    @z

    lf c 3

    Subsequent studies have shown that the ltrationcoefcient lf is a function of the deposited particles(Hunt et al., 1987) and various other factors. Vinten andNye (1985) suggest that these factors include packinggeometry, particle size, ow rate, electrolyte composi-tion and surface potentials of both particles and poresurfaces (as reviewed by Spielman, 1977). Assuming thattransport by dispersion can be neglected, then in termsof the deposition rate coefcient, kd, the ltrationcoefcient lf can also be expressed as lf= kd/vp.Classical particle capture theories can be used to

    identify some of the above dependencies. These theoriesfocus on collection by a representative element such asspherical porous medium grains. Interpretation ofltration experiments then requires relating capture bya representative element to the assemblage that com-prises the porous matrix (Spielman, 1977). If weconsider a bed of collecting grains perpendicular to themean ow (of velocity U), the change in suspendedparticle number concentration dcn over depth dz is givenby:

    U dcn I6apd3p

    !dz 4

    where: 6a=pd3p is the ratio of the volume fraction ofgrains in the array to the grain volume (diameter dp),and the rate of particle capture per grain is given by I.The dimensionless grain capture efciency is conven-tionally dened as Z 4I=pd2p Ucn and the clean bedltration coefcient by l0 6aI=pd3p Ucn: A clean bedltration coefcient assumes that particle retentionoccurs on a clean medium without deposited particlesaltering the medium surface or uid ow.Both McDowell-Boyar et al. (1986) and Hunt et al.

    (1987) quote a semi-empirical equation, again based onclassical particle capture theory, from Tien and Paya-takes (1979) for the clean bed ltration coefcient l0 fora lter consisting of stationary spherical collectors.This has three terms, taking account (respectively) ofparticle capture by Brownian motion, interception andgravitational sedimentation, with size dependence in allthree terms:

    l0 3

    2

    1 p0 dm

    4A1=3sUdm

    Dp

    23056As

    A

    md2p U

    !1=824

    dp

    dm

    15=824 103As

    vs

    U

    12 dpdm

    04#5

    where: p0 is the clean bed porosity, U is the uidvelocity, m is the uid dynamic viscosity, and dm and dpare the collector and mobile particle diameters. Theparameter correcting particle capture As by a singleisolated collector for capture within a packed porousmedium is given by

    As 1 P5

    1 32P 3

    2P5 P6

    6

    and

    P 1 p01=3 7

    The rst term (Brownian motion) includes the particlediffusivity Dp given by

    Dp kT=3p mdp 8

    where: k is the Boltzman constant (for waterk=2 1020 J) and T is the absolute temperature. Inthe second term (interception), A is the Hamakerconstant from Londonvan der Waals attractive forceequations. The third term (sedimentation) includes the

  • M.B. MCGECHAN; D.R. LEWIS264particle settling velocity vs given by:

    vs g

    18mrp rf

    d2p 9

    where: g is the acceleration due to gravity and rp and rfare the particle and uid densities in kg m3. Wan et al.(1995) describe the mathematics of sedimentation withan equation similar to Eqn (9).Johnston (1982) combined Poiseuilles Law with

    ltration theory as a method of determining thespecication of lters to achieve a specied degree ofltration efciency. However, it should be noted thatltration theory such as presented by these authorsapplies to a much less dense porous medium than soil.Changes in hydraulic conductivities of soils due to wasteapplications are discussed in Section 9.

    8.3.3. Adaptation of filtration equations for soilsBasic ltration equations have been applied to soils,

    particularly in relation to macropores, by Chen andWagenet (1992a, 1992b), but in this case relating tosolute rather than particulate transport. Jarvis et al.(1999) have described an adaptation of the MACROtwo-domain contaminant transport model (Jarvis, 1994)to represent colloid-facilitated contaminant transport.Filtration is treated differently in the macropore andmicropore (soil matrix) zones. In the micropores, thestandard ltration equation [Eqn (3)] is applied, forwhich a value of the ltration coefcient lf must bespecied. In the macropores, the ltration equation isadapted to take account of the ow velocity, asfollows:

    F lref nnfref v

    1nf cy 10

    where: F is the ltration sink term in g m3 h1, lref is areference ltration coefcient in m1, nf is an empiricalexponent, v is the pore water velocity in m h1 and vref isthe pore water velocity in m h1 at which lref ismeasured. Determination of parameters for this equa-tion has been described by Villholth et al. (2000) andMcGechan et al. (2002).

    8.4. Colloid motility effects

    Camesano and Logan (1998) showed that motile andnon-motile status has an important inuence onbacterial transport. At low uid velocities, colloidltration theory predicts the number of collisions willincrease, leading to greater retention. However formotile cells, the retention was observed to decrease.They concluded that swimming cells are able to avoidsticking to soil grains at low velocities, but at highvelocities the attachment process is unaffected.9. Changes in soil characteristics over time due to colloiddeposition

    9.1. Clogging of pores due to colloid capture

    Deep bed ltration theory (Section 8.2.2) such aspresented by Sakthivadivel et al. (1972) and Tien andPayatakes (1979) takes account of a gradual increase inresistance to uid ow over time due to particledeposition (clogging up of the lter). Similarly, cloggingof soil pores, leading to reductions in hydraulicconductivity, is an issue discussed by a number ofresearchers, e.g. Daniel and Bouma (1974). Jenny andSmith (1935) describe the formation of a low-conduc-tivity clay pan in subsoil due to deposition of colloidalclay particles mobilised from surface layers. A problemof reduced conductivities in soil used for disposal (andpurication) of water containing contaminants in wasteshas been investigated by Rice (1974), Daniel and Bouma(1974) and Vinten et al. (1983a, 1983b, 1983c). The workof Rowell et al. (1969) and Frenkel et al. (1978) ondispersivity of clay soil particles (as discussed inSection 5), also showed reduced hydraulic conductivitiesof soils due to clogging of pores by these dispersedparticles.

    9.2. Colloid deposition dynamics

    In determining the dynamics of colloid transport, anaccurate description is required of the transient rate ofcolloid deposition onto the grains (collectors) making upthe porous material. The initial stage of ltration ischaracterised by a constant deposition rate whichbecomes transient as colloidal particles accumulate onthe collectors. This transient rate has been observed toincrease or decrease depending upon the solutionchemistry and chemical characteristics of the colloidsand collector grains (Ryan & Elimelech, 1996).The dynamics of colloid deposition onto granular

    porous media has been described successfully byrandom sequential adsorption (RSA) mechanics(Adamczyk et al., 1992; Johnson & Elimelech, 1995).Such models are applicable to irreversible particledeposition onto stationary surfaces, with monolayercoverage maintained by prohibiting particle contact,surface diffusion and surface detachment. The kineticrate equation describing the deposition process has beengiven by Johnson and Elimelech (1995) as

    @s

    @t ae kt p d2p cn B s 11

    where: s is the fraction of collector surface covered byparticles, ae is the particle attachment efciency, kt is theparticle transfer coefcient, dp is the spherical particle

  • TRANSPORT OF CONTAMINANTS THROUGH SOIL 265diameter and cn is the number concentration of colloidalparticles in suspension. The colloid quantity depositedper unit mass sm can be related geometrically to s suchthat

    sm f ymcpd2p pb

    s 12

    where: f represents the specic surface area in m2 m3 ofthe porous medium and mc an average mass for thecolloidal particles in kg. The dynamic blocking functionB s account for excluded area effects of depositedparticles hindering subsequent attachment of particles.For initially bare collector surfaces B s 1; with thefunction declining until the jamming limit occurs wherethe maximum coverage smax is attained and B s 0:This function can be based on either the Langmuir

    adsorption model or RSA mechanics. For a linearLangmuiran blocking function this gives

    Bs smax s

    smax 1 bs 13

    where b 1=smax; the excluded area parameter which isa measure of the collector surface area blocked fromsubsequent deposition by a deposited particle. For theRSA dynamic blocking function, expressions can bedetermined for a hard sphere captured onto a atcollector surface, with a jamming limit s1: A generalexpression for any collector geometry and a soft particledeposition is given by Johnson and Elimelech (1995):

    B s

    1 4s1bs 3:308 s1bs 21:4069 s1bs

    3

    for s40:8smax

    smax s 2s2j

    3

    for 0:8smax5s4smax

    8>>>>>>>>>>>>>:

    14

    where sj is the jamming limit slope. This has a value of0236 for hard spheres depositing on at surfaces, and isdependent upon solution chemistry and hydrodynamicconditions (Johnson & Elimelech, 1995).As shown in Eqns (13) and (14), a declining colloid

    deposition rate is generally attributed to the reducedavailability of deposition sites on the collector grains asdeposited colloids exclude others from their immediatevicinity. Experimental investigations (Johnson & Elim-elech, 1995; Jewett et al., 1995; Johnson et al., 1996)demonstrate that the excluded area parameter increasesas the ionic strength of the electrolyte decreaseslogarithmically, thus indicating that the area blockedis inuenced strongly by the thickness of the DDL.These studies show the very large changes in ionicstrengths needed to signicantly inuence transportconditions. For the case of an increasing depositionrate, multi-layer coverage of the collectors by depositedcolloids is assumed to occur. This phenomenon istermed ripening, and arises because previously depos-ited colloids provide a more favourable surface foradhesion than the original soil (Liu et al., 1995;Camesano & Logan, 1998).

    9.3. Shear strength effect

    Hunter and Alexander (1963) considered that theshear strength of a sol (a colloid suspension in water) ofkaolinite (very ne clay soil particles) had a signicanteffect on the ability of the sol to pass through a soilcolumn, which was not attributable to clogging of soilby colloidal particles.

    10. Experimental investigations into colloid transport andcapture

    There are a number of reported experiments in whichthe passage of colloids or colloidally attached contami-nants through soil has been measured. These havegenerally taken the form of laboratory column experi-ments with water containing colloids and contaminantsbeing passed through samples of soil (or sometimesother porous media). In most instances, materialemerging from the bottom of the column has beenanalysed to produce breakthrough curves for the colloidor contaminant, e.g. Lahav and Tropp (1980), Kretzsch-mar et al. (1997) and Hesketh et al. (1998).

    10.1. Column experiments with colloids

    Lahav and Tropp (1980) passed suspensions ofsynthetic latex microspheres with diameter 012 or021 mm through real soil samples observing materialemerging from the bottom of the column. They alsoexamined the soil columns after the experiment, noting ahigh concentration of trapped spheres in surface layers.Seta and Karathanasis (1997) compared characteristicsof suspended soil-derived colloids in water emergingfrom the bottom of a column with those in water appliedat the top, for several soils. They observed more colloidtransport in soils with higher macroporosity, and alsoionic charge effects dependent on the pH values of thecolloid suspension. Toran and Palumbo (1992) com-pared transport of colloids through columns both withand without articially created fractures (similar tomacropores). They concluded that chemical effectsaccounted for a high proportion of colloid capture,since particles were substantially smaller than the pores.Coutts et al. (1986a, 1986b) studied colloid generation at

  • M.B. MCGECHAN; D.R. LEWIS266the surface by rainfall impact, using a radioactive tracerto follow colloid movements.Hu and Brusseau (1995) carried out experiments

    passing organic solutions through porous media (notsoil). Solutions with different sizes of molecules wereused, the larger being of similar size to some colloids.

    10.2. Column experiments with radioactive and chemicalcontaminants

    Buddemeier and Hunt (1988) carried out columnexperiments with radionucleides, concluding that trans-port generally took the form of attachment to otherwiseharmless soil-derived colloidal material. Champlin andEichholz (1968) had concluded in similar experimentsthat colloid attachment accounted for most of thetransport even when a radionucleide was very soluble.Jury et al. (1986) experimented with the moderatelysorbed herbicide naprapamide, concluding that colloid-facilitated transport and macropore ow were importantprocesses. De Jonge et al. (1998) carried out columnexperiments with the pesticide prochloraz, concludingthat some but not all was transported attached tocolloids, and also that macropores had an importantrole in transport of both solute and colloids. Gonzalezand Ukrainczyk (1999) concluded that sorption ontostatic soil components would cause herbicide to betrapped rather than transported. Van der Lee et al.(1994) tested their model (Section 11.1) in columnexperiments, commenting on the lack of informationabout colloid sorption parameters.

    10.3. Column experiments with microorganisms

    A number of soil column experiments have beenreported for biological contaminants. White (1985a) andSmith et al. (1985) describe adding suspensions of E. colibacteria in column experiments with undisturbedsamples of a range of soils, while White (1985b)discusses the importance of macropores for the resultsobtained. Abu-Ashour et al. (1998) also worked with E.coli bacteria in column experiments, concluding that soilwater content and the presence of macropores had themost pronounced effect on microorganism transport.Other experiments on transport of bacteria through soilare described by Gannon et al. (1991) and Tan et al.(1992), while Wan et al. (1995) describe bacterialsedimentation experiments using soil columns. McMur-ray et al. (1998) carried out laboratory experiments withpoultry manure spread over intact soil samples subjectedto articial rain. Water emerging from the bottom of theblocks was analysed for faecal coliforms, and afterwardsthe soil blocks were analysed destructively to determinethe distribution of deposited faecal coliforms. Othercolumn experiments with viruses have been reported byBitton et al. (1974), Dizer et al. (1984) and Lance et al.(1982), while Jin et al. (2000) and Chu et al. (2000)respectively demonstrated the importance of mineralcolloids and buffer solutions for virus transport. Similarexperiments with bacteria have been reported byWollum and Cassel (1978), McCoy and Hagedorn(1979), Trevors et al. (1990), Van Elsas et al. (1991)and Paterson et al. (1993). Column experiments withcryptosporidium parvum oocysts have been reported byMawdsley et al. (1996a, 1996b) and Brush et al. (1999).Bitton et al. (1979) reviewed a number of studies of thetransport of viruses through soil, comparing resultsfrom column experiments with those obtained in theeld. Vilker (1981) has similarly listed a number ofearlier experimental studies of microorganism transport,both with laboratory columns and eld soil beds.

    10.4. Enhanced colloid migration and other processesaffecting transport

    Seta and Karathanasis (1996) evaluated the transportof metolachlor through intact soil columns. Colloidrecovery in the drainage efuent ranged from 54 to 90%,and enhanced metolachlor transport by 2270%,depending upon colloid type and mobility. Johnsonand Logan (1996) measured the enhanced transport ofbacteria in porous media. Dissolved and sedimentorganic matter were independently added to quartz inmini-columns and retention efciencies measured. Thelargest decrease in bacterial retention (60%) wasassociated with sediment organic matter in the absenceof dissolved organic matter. Jacobsen et al. (1997)carried out inltration experiments on intact sandy loamsoil columns, with incident colloid suspensions. Asignicant transport of particles through macroporeswas observed at two soil depths. The particle size in thecollected efuent generally decreased over time follow-ing an irrigation event, but with a relatively constantconcentration of particles of size510 mm. This variationin particle size of the efuent can be explained by thedifferent absorption capacities and exposure to hydrau-lic forces, and by mechanical entrapment. Karanthana-sis (1999) determined the migration rates of copper andzinc in the presence of various water-dispersible soilcolloids (suspensions of montmorillonitic, illitic andkaolintic, with a range of surface properties). Usingundisturbed soil columns (clay, silty clay), the presenceof colloids typically enhanced metal transport by 550-fold over the control treatment with no colloids. Zincwas consistently more mobile than copper, with highesttransport occurring with colloids of high negative

  • TRANSPORT OF CONTAMINANTS THROUGH SOIL 267surface charge and organic content, and the lowest bycolloids with large particle size, low negative surfacecharge and high iron- and aluminium-hydroxyoxidecontents.Camesano et al. (1999), demonstrated the blocking

    effect (see Section 9.2) for several bacterial colloidalspecies in packed-bed soil column experiments, withexcluded areas ranging from 11 to 24% for Pseudomo-nas putida. As a practical method for enhancing themobility of colloids in soils, Seaman and Bertsch (2000)used soil column leaching experiments to evaluate theuse of a cationic surfactant as a chemical additive toblock negatively charged deposition sites. Leaching thesurfactant through an iron-oxide-rich column of aquifersediment resulted in the selective dispersion and trans-port of iron-oxide colloids with little column pluggingobserved.

    10.5. Hydrophobic and hydrophillic effects

    In unsaturated sand column experiments, Schaferet al. (1998) determined that retention of hydrophobicand hydrophillic bacteria is markedly increased at lowwater contents. Strong retention is attributed to theaccumulation of bacteria at the airwater interface, asthe colloidal particles have a high afnity compared withthe solid surface. Trapping of bacteria by the airwaterinterface is thus an important process controllingmobility in unsaturated soils.For bacteria (Gram-negative rods) moving through

    sandy soils, McCaulou et al. (1994) showed thatattachment takes approximately 1 h, and is usuallyreversible, but detachment can take up to several days.Hydrophillic bacteria has slower attachment and de-tachment rates than hydrophobic bacteria, indicatingthat they could move further before being removed byattachment and would be resuspended at a slower rate.Transport studies of Pseudomonas fluorescens andBacillus subtilis bacteria through laboratory columnspacked with silica beads (Kinoshita et al., 1993) showedthat both species were highly retarded with between 20and 94% reductions in initial concentrations. Lessretardation was evident at lower pH and producedlittle change in the electrostatic charge of the bacteria,with the consequence that it was relatively unimportantin determining bacterial retardation. The transportof phages in sandy soils (Kinoshita et al., 1993)showed the predominant effect of hydrophobicity,with a high removal efciency for hydrophobicphages. Detachment was slow, with an increase inpH having a moderate enhancing effect. Hydrophilicphages tend to act conservatively in low-carbonsandy soils.10.6. Experiments on colloid deposition

    Various experiments have been carried out toinvestigate the interplay between soil water dynamicsand DDL interactions, and their effect on the dynamicsof particle deposition and blocking, as described inSection 9.2. Flow intensity (Camesano & Logan, 1998;Adamczyk & Weronski, 1999), particle sizes (Adamczyket al., 1992; Bohmer et al., 1998) and soil ionic strength(Ryde et al., 1992; Liu et al., 1995; Gross & Logan,1995; Gross et al., 1995) uniquely determine the rate ofblocking and the maximum surface coverage attained oncollectors. Ko and Elimelech (2000) simultaneouslyexamined the above three factors of ow intensity,particle sizes and soil ionic strength, using suspensionsof colloidal latex particles and packed quartz grains incolumn experiments. Their results suggest that the shearcomponent of the uid ow around collector grainscreates a shadow zone on the collector surfaces, with adeclining gradient of deposited particles. In this zonedeposition rates are substantially reduced, by an extentdetermined by the hydrodynamic interactions andelectrostatic DDL repulsion. Increasing the ow velocityand particle size, and decreasing the solution ionicstrength, result in a larger shadow zone and hence areduced maximum attainable surface coverage ofdeposited colloids.

    10.7. Experimental determination of filtration coefficient

    The only known studies in which column experimentshave been used to estimate the ltration coefcient lf arepresented by Vinten and Nye (1985) and Kretzschmaret al. (1994). By this means, Vinten and Nye (1985)estimated lf for clay particles (the most importantmineral carrier colloid) labelled with 14C-paraquat, alabel with a very low tendency to desorb into solution,so colloid ltration could be measured independentlyfrom the sorption process.

    11. Models of colloid-facilitated contaminant transport

    11.1. Models representing transport processes in columnexperiments

    Eneld and Bengtsson (1988) presented equations fora model (simulating a column experiment) of transportof a hydrophobic contaminant assisted by the presenceof sorbing macromolecules of another substance. Littonand Olsen (1993) use an adaptation of the ltrationequation with an attachment efciency factor (similar

  • M.B. MCGECHAN; D.R. LEWIS268to the ltration coefcient) when testing porous mediafor extracting colloids during wastewater treatment. Vander Lee et al. (1994) describe a model of transport of apollutant sorbed onto a sorbing colloid in a homo-geneous non-sorbing porous medium. Corapcioglu andKim (1995) have described a contaminant transportmodel for the situation where bacteria act as the mobilecolloids onto which contaminants are sorbed. Ouyanget al. (1996) reviewed colloid-enhanced transport modelsand included a section on measurements of the transportof attached chemicals. Knabner et al. (1996) havedescribed a model of reactive solute transport withsorption to mobile and immobile sorbents. They test thismodel using breakthrough data for various contami-nants from column experiments (Totsche et al., 1996).Germann and Beven (1981) describe a statistical modelfor separate representation of macropores and micro-pores, as an alternative to the two-domain adaptation ofthe capillary bundle model approach described by Bevenand Germann (1981). Stagnitti et al. (1999) describe amodel where heterogeneity of soil conductivity (large ina soil with extensive macropores) is described by amathematical distribution function.

    11.2. Field scale models

    Mills et al. (1991) describe a model (called COMET)of colloid and contaminant processes in subsurface soillayers, with separate equations for the upper unsatu-rated zone and the lower saturated zone. COMET wasintended for investigating the use of land (without elddrains) for disposal of wastes by surface spreading,where the concern is contamination of the underlyingaquifers. Another model of colloid facilitated transportin a fractured porous medium has been described byIbaraki and Sudicky (1995a). It has been applied to thetransport of contaminants down to deep subsoil layersin clay and sandstone rocks (Ibaraki & Sudicky, 1995b).The most comprehensive eld-scale model incorpor-

    ating most of the important processes relating tocolloid-facilitated contaminant transport through soilis MACRO. This model simulates contaminant trans-port with dual-porosity allowing for different contami-nant concentrations in the soil matrix and themacropores. Pollutant ows both to surface waters viaeld drains and to groundwater (aquifers) by deeppercolation are represented. Earlier versions (Jarvis,1994) simulated solute transport with the option ofsorption onto the static soil matrix only. In the mostrecent version (Jarvis et al., 1999), the option of colloid-facilitated contaminant transport has been added, withthree-phase representation of contaminant transport asdescribed in Section 2 (contaminants in solution, sorbedonto the static soil matrix and sorbed onto mobilecolloidal particles). Two simulation runs must be carriedout for each scenario, the rst representing transport ofthe carrier colloid only, the second representing thecontaminant in each of the three phases. In the rst run,colloids are generated by detachment caused by rainfallimpact (as discussed in Section 5) or added in irrigationwater (a procedure which can also be used to representspreading of animal manure slurry which contains largequantities of contaminated colloidal material). Colloidsare ltered in both macropores and micropores,according to the equations listed in Section 8.2.3.Colloids are excluded from (but not trapped in) aproportion of the soil pore space (the smallest pores), aphenomenon described by Kretzschmar et al. (1997). Inthe second run, the contaminant is applied in irrigationwater (as a solute, or sorbed onto colloids added in therst run). Isotherm equation (Freundlich) parametersare specied for both the static soil matrix and themobile colloid, to give the three contaminant phases asdiscussed in Section 2. MACRO was developed primar-ily for modelling pesticide transport, but it includes mostof the features required for transport of phosphorus andbiological microorganisms. An application for the studyof phosphorus pollutant ows following animal slurryspreading has been described by McGechan et al.(2002). This application represents the only one out ofa number of eld-scale phosphorus dynamics models (asreviewed by Lewis & McGechan, 2002) to includerepresentation of through soil colloid-facilitated trans-port of phosphorus.

    12. Conclusions

    Literature reviewed in this paper indicates that manycontaminants are transported in colloidal form throughthe soil to surface water or groundwater. Thesecontaminants include phosphorus, pesticides and otheragrochemicals, plus a range of biological microorgan-isms. With the exception of the largest category ofmicroorganism, protozoa, most such contaminants aretransported mainly by adsorption onto otherwiseharmless mobile colloidal material such as clay particlesor soil organic matter. There are potential impedimentsto movement of colloids through soil, and these can besubdivided into two main classes: straining (sometimescalled physical ltration or mechanical ltration)where a particle has a dimension larger than the porethrough which it is trying to pass so it becomes trapped,as discussed further in Part 2 (McGechan, 2002a,2002b); true ltration covers a range of mechanismswith the common feature that particle dimensions aremuch smaller than the pores. Such ltration mechanisms

  • TRANSPORT OF CONTAMINANTS THROUGH SOIL 269include interception, diffusion and sedimentation,processes akin to those described in the extensiveengineering literature on deep bed ltration. Sorptiononto static components of the soil matrix plays a partin restricting the movement of reactive contaminantsthrough the soil. However, such contaminantsalso readily become sorbed onto colloidal particles,which if mobile aggravate polluting losses to theenvironment.Capture processes can be represented in models by a

    simple equation with a ltration coefcient, the value ofwhich can be determined experimentally. However, outof numerous reported studies using column experimentsto measure colloid or contaminant transport throughsoil, very few lead to derivation of such a ltrationcoefcient. The importance of macropore ow, whichleads to rapid unrestricted transport of contaminantcarrying colloids, is widely reported in literature oncolumn experiments and other studies. Such transportdepends on the macropores being water lled, requiringnear-saturation soil water contents, conditions commonin Northern Europe in winter.There are a few existing models of colloid-facilitated

    contaminant transport processes, both for columnexperiments and for the eld situation. They generallyinclude the simple ltration equation (or a slightvariation on it) with sorption of contaminants ontocolloids and static soil components, and separateequations (or coefcient values) for macropores andmicropores where macropore ow is considered. Thematerial presented in this review provides backgroundinformation for further development of a eld-scalemodel representing colloid-facilitated transport of arange of chemical and microbiological contaminants.Understanding contaminantmineral interactions at

    the molecular level has also been shown to be important,and extensive experimentation on individual mineralsystems has provided a foundation for knowledge on thesubject. However, investigations on model mineralsystems cannot be readily extrapolated to complexmineral assemblages in natural systems. Recent studieson phyllosilicate and metal oxohydroxide mineral soilshave elucidated the role of less abundant mineral andorganic substrates as surface chemical modiers. In suchsystems the surface chemical modiers control colloidgeneration and transport processes, as well as thetransport of solutes and ionic tracers. Surface chargingmechanisms operating in these complex mineral assem-blages are difcult to predict using only surfacereactivities and other characteristics of their individualmineral components. It is likely that a complete under-standing of the surface chemistry of complex mineralassemblages will be required to assess accurately theenvironmental fate of contaminants.Acknowledgements

    The Scottish Executive Environment and RuralAffairs Department provided funds to carry out thework.

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    Notation1. Introduction2. Three-phase representation of contaminant transportFigure 1Figure 2

    3. Role of macropores and fractures in particulate contaminant transport4. Nature and size of particulate and colloidally transported contaminants5. Sources of colloidal materialFigure 3

    6. Interactions with surfacesFigure 4

    7. Transport,straining and .ltration of colloidal contaminants in soilFigure 5

    8. Colloid capture processes9. Changes in soil characteristics over time due to colloid deposition10. Experimental investigations into colloid transport and capture11. Models of colloid-facilitated contaminant transport12. ConclusionsAcknowledgementsReferences

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