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

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<ul><li><p>Biosystems Engineering (2002) 83 (3), 255273t</p><p>U</p><p>ll</p><p>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</p><p>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</p><p>1. Introduction</p><p>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</p><p>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.</p><p>2. Three-phase representation of contaminant transport</p><p>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</p><p>REVIEW</p><p>Transport of Particulate and Colloid-sorGeneral P</p><p>M.B. McGecha</p><p>Environment Division, SAC, Bush Estate, Penicuik EH26 0PH,</p><p>(Received 14 September 2001; accep</p><p>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,</p><p>1537-5110/02/$35.00 255tp://www.idealibrary.com on</p><p>PAPER</p><p>bed Contaminants through Soil, Part 1:rinciples</p><p>n; D.R. Lewis</p><p>K; e-mail of corresponding author: m.mcgechan@ed.sac.ac.uk</p><p>ted in revised form 1 August 2002)</p><p>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</p><p># 2002 Silsoe Research Institute. Published byElsevier Science Ltd. All rights reserved</p></li><li><p>Dp particle diffusivity in Brownian motion, m s T absolute temperature, K1</p><p>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 &amp; 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</p><p>phases for both colloids and chemical contaminants(Fig. 1).The mobile solid phase consists of colloidal (or</p><p>particulate) particles in the size range from 1 nm to1 mm (Buddemeier &amp; 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,</p><p>f collector surface area per unit volume of packedbed, m2 m3</p><p>F ltration sink term, g m3 h1</p><p>g acceleration due to gravity, m s1</p><p>I rate of colloid capture per grain, s1</p><p>k Boltzman constant, J K1</p><p>kd colloid deposition rate coefcient, s1</p><p>kps equilibrium solidliquid partition coefcientbetween dissolved solute and solute sorbedonto immobile soil matrix</p><p>kpcs equilibrium solidliquid partition coefcientbetween dissolved solute and solute sorbedonto colloids</p><p>kp biochemical transformation rate for dissolvedsolute, s1</p><p>kr colloid release rate coefcient, s1</p><p>ks biochemical transformation rate for solutesorbed onto immobile soil matrix, s1</p><p>kcs biochemical transformation rate for dissolvedsolute sorbed onto colloids, s1</p><p>U uid velocity, m sv pore water velocity, m h1</p><p>vref pore water velocity at which fref is measured,m h1</p><p>vp velocity of colloidal particles in porous mediam s1</p><p>vs particle settling velocity, m s1</p><p>z distance, ma volume of collector grains in array, m3</p><p>ae particle attachment efciencyb excluded area parameter, kg kg1</p><p>y soil porosity, m3 m3</p><p>lf ltration coefcient, m1</p><p>lref reference ltration coefcient, m1</p><p>l0 clean bed ltration coefcient, m1</p><p>m uid dynamic viscosity, N s m2</p><p>rb solid matrix bulk density, kg m3</p><p>rf, rp uid and particle densities, kg m3A Hamaker constant in Londonvan der Waalsattractive force equations, J</p><p>As Parameter correcting particle capture by asingle isolated collector for capture within apacked porous medium</p><p>B(s) Dynamic blocking functionc Mass concentration of colloids in solution,</p><p>kg kg1</p><p>cn Suspended colloid number concentrationper unit volume of packed bed, m3</p><p>dm diameter of mobile particle, mdp diameter of collector grain, mDh hydrodynamic dispersion coefcient for colloi-</p><p>dal particles, m2 s12 1</p><p>Notakt particle transfer coefcient, m s1</p><p>mc average mass of colloidal particles, kgnf empirical exponentp0 clean bed porosity, fractionP Parameter related to media porositys Fraction of collector surface covered by</p><p>particlessm Deposited colloidal particles per unit mass</p><p>of the porous media, kg kg1</p><p>sj jamming limit slopeSmax maximum fractional coverage by deposited</p><p>colloidal particless1 hard sphere jamming limitt time, s</p><p>tion</p></li><li><p>TRANSPORT OF CONTAMINANTS THROUGH SOIL 257Chemical</p><p>Dissolvedsolute</p><p>Adsorbed</p><p>Adsorbed</p><p>solute</p><p>solute</p><p>(immobile soil matrix)</p><p>(colloidal solids)</p><p>Colloids</p><p>Mobilecolloids</p><p>Generation</p><p>kp</p><p>kps</p><p>kpcs</p><p>kcs</p><p>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.</p><p>3. Role of macropores and fractures in particulatecontaminant transport</p><p>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</p><p>Immobilisation Mobilisation</p><p>Immobile colloids</p><p>Immobilesoil</p><p>matrix</p><p>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;</p><p>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</p><p>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</p><p>restrictions</p></li><li><p>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.</p><p>4. Nature and size of particulate and colloidallytransported contaminants</p><p>4.1. Chemical pollutants</p><p>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.</p><p>4.2. Carrier particles and colloids</p><p>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.</p><p>4.3. Biological pollutants</p><p>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 diamete...</p></li></ul>