Journal of Hydrology 402 (2011) 159–164

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Journal of Hydrology

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Technical note

A laboratory study of colloid and solute transport in surface runoff on saturated soil

Congrong Yu, Bin Gao ⇑, Rafael Muñoz-Carpena ⇑, Yuan Tian, Lei Wu, Oscar Perez-OvillaDepartment of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA

a r t i c l e i n f o s u m m a r y

Article history:Received 8 December 2010Received in revised form 5 February 2011Accepted 7 March 2011Available online 14 March 2011This manuscript was handled by P. Baveye,Editor-in-Chief, with the assistance ofMagnus Persson, Associate Editor

Keywords:ColloidsSoluteTransportSurface runoffDrainageTracer

0022-1694/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jhydrol.2011.03.011

⇑ Corresponding authors. Tel.: +1 352 392 1864; faE-mail addresses: bg55@ufl.edu (B. Gao), carpena@

Colloids in surface runoff may pose risks to the ecosystems not only because some of them (e.g., patho-gens) are toxic, but also because they may facilitate the transport of other contaminants. Although manystudies have been conducted to explore colloid fate and transport in the environment, current under-standing of colloids in surface runoff is still limited. In this study, we conducted a range of laboratoryexperiments to examine the transport behavior of colloids in a surface runoff system, made of a soilbox packed with quartz sand with four soil drainage outlets and one surface flow outlet. A natural claycolloid (kaolinite) and a conservative chemical tracer (bromide) were applied to the system under a sim-ulated rainfall event (64 mm/h). Effluent soil drainage and surface flow samples were collected to deter-mine the breakthrough concentrations of bromide and kaolinite. Under the experimental conditionstested, our results showed that surface runoff dominated the transport processes. As a result, kaoliniteand bromide were found more in surface flow than in soil drainage. Comparisons between the break-through concentrations of bromide and kaolinite showed that kaolinite had lower mobility than bromidein the subsurface flow (i.e., soil drainage), but behaved almost identical to bromide in the surface runoff.Student’s t-test confirmed the difference between kaolinite and bromide in subsurface flow (p = 0.02).Spearman’s test and linear regression analysis, however, showed a strong 1:1 correlation between kao-linite and bromide in surface runoff (p < 0.0001). Our result indicate that colloids and chemical solutesmay behave similarly in overland flow on bare soils with limited drainage when surface runoff dominatesthe transport processes.

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1. Introduction

Colloids (i.e., particles with diameter in the range of 1 nm to10 lm) are widely distributed in the environment (Stumm,1977). There are mainly two categories of natural colloids: (1) bio-colloids including viruses, bacteria, and some of the protozoa, and(2) abiotic colloids including all kinds of colloidal minerals and nat-ural organic matters. Once mobilized by water flow, colloids maypose risks to surface water and groundwater quality not only be-cause many biocolloids are pathogenic, but also because abioticcolloids are effective ‘‘carriers’’ of a variety of common contami-nants found in soils and water (Flury and Qiu, 2008; Gao et al.,2011). It is therefore important to study the transmission of col-loids and their consequent fate in the hydrological pathways.

In the literature, research of colloids in water resources mainly fo-cuses on their fate and transport in the subsurface, such as in ground-water and in soil vadose zone. A number of experimental andmodeling investigations have been conducted to explore the reten-tion and transport mechanisms of colloids and colloid-contaminant complexes in soils under both water-saturated andunsaturated conditions (McCarthy and McKay, 2004; Ryan and

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x: +1 352 392 4092.ufl.edu (R. Muñoz-Carpena).

Elimelech, 1996). Findings from laboratory experiments indicatethat the transport of colloids in soils is controlled by multiple reten-tion/release mechanisms, such as grain-surface deposition, porestraining, air–water interface deposition, film straining, and immo-bile-water trapping (Gao et al., 2006; Ryan and Elimelech, 1996).The improved understanding of colloid transport mechanisms in-formed the construction and refinement of mathematical modelsto predict their fate and transport in the subsurface environment(Flury and Qiu, 2008; Simunek et al., 2006). Most of these modelsare based on the advection–dispersion equations coupled with reac-tions, which are similar to the models developed for solute transportin soils. However, the transport behavior of colloids in soils may dif-fer from that of chemical solute because of the size exclusion effectand the distinct retention mechanisms (Chrysikopoulos andAbdelSalam, 1997; Simunek et al., 2006). The different breakthroughbehavior between colloids and solute in soils has been well-docu-mented (Bradford et al., 2005; Keller et al., 2004).

Although colloid-facilitated transport is also an important con-tamination process to surface water, fate and transport of colloidsin overland flow has received relatively less research attention(Haygarth et al., 2006; Leguedois et al., 2008). Colloidal contami-nants (e.g. colloid–metal complexes) in surface runoff are oftentreated as dissolved phase if they can pass through a 0.45 lm filter(Lead and Wilkinson, 2006). Ren and Packman (2002, 2005),

160 C. Yu et al. / Journal of Hydrology 402 (2011) 159–164

however, demonstrated that colloidal particles with sizes equal orsmaller than 0.45 lm played significant roles in facilitating tracemetal transport in surface stream water. Similarly, Heathwaiteet al. (2005) showed that the release of phosphorus from agricul-tural soils to surface runoff was mainly controlled by soil colloidswith size between 0.001 and 2 lm. Several laboratory and fieldstudies have also been conducted to examine the fate and trans-port of biocolloids in surface runoff (Kay et al., 2007; Oliver et al.,2005). Mathematical models of biocolloid transport in overlandflow have been proposed and model simulations have been testedagainst experimental data (Kouznetsov et al., 2007; Pachepskyet al., 2006). Nevertheless, there is still a debate in the literatureon how colloidal particles are transported in surface runoff. Whilesome suggested that colloid transport in surface runoff is similar tothat of chemical solute (Edwards et al., 1996; Roodsari et al., 2005);others showed that colloids may behave substantially differentfrom chemical solutes in surface runoff (Crane et al., 1983; Dosskeyet al., 2007). Further investigations are therefore needed to im-prove current understanding of the fate and transport of colloidsin surface runoff (Kay et al., 2007).

This study used a series of laboratory experiments to examinethe transport behavior of colloids in surface runoff. A soil boxpacked with sand was placed under a laboratory rainfall simulatorto compare the transport behavior of colloids and solutes. A naturalclay colloid (kaolinite) and a conservative chemical solute (bro-mide) were applied to one end of the soil box as inflow during asimulated rainfall event. Effluent samples were collected fromthe other end of the soil box and from four drainage pipes to deter-mine colloid and solute breakthrough concentrations. Multiplerunoff experiments were conducted and statistical analysis wasconducted to aid in the data interpretation. Our objectives wereto: (1) identify similarities and differences between colloids andsolutes in surface flow as well as in subsurface flow; and (2) deter-mine the governing processes that control the fate and transport ofcolloids in surface runoff.

2. Materials and methods

2.1. Materials

Kaolinite powder (Thiele Kaolin Company) was used to make acolloidal kaolinite suspension; about 10 g of dried kaolinite pow-der (at 100 �C for 2 h) was suspended in 700 mL of deionized (DI)

Fig. 1. Schematic of the laborator

water. The kaolinite suspension was shaken, placed in an ultra-sonic bath for 30 min, and then let stand for 24 h. The fraction ofkaolinite remaining in suspension after 24 h was siphoned into aflask. The concentration of kaolinite in an aliquot of this stock sus-pension was determined gravimetrically before diluting the stockto colloid suspensions used in the experiments. The mean sizesof the kaolinite colloids, as determined by dynamic light scattering(DLS), did not vary significantly during the experiments and wereabout 0.4 lm.

Quartz sand (Standard Sand & Silica Co.) was used as experi-mental soil. The sand had a size range between 0.5 and 0.6 mmand was used as received. The bulk density of the sand was1.54 g/cm3 and the saturated hydraulic conductivity was0.12 cm/s. Sodium bromide (certified, Fisher Scientific) was usedin the experiment as the conservative chemical solute.

2.2. Surface runoff system

A stainless steel box with dimensions of 153.1 cm long, 40.2 cmwide, and 10 cm deep was used to hold the experimental soil(Fig. 1). The bottom of the box was separated into four shallow com-partments, which were 5 cm deep and each equipped with a drain-age outlet to partition infiltration along the flow path (numbered as1–4 in Fig. 1). The quartz sand was wet packed in the compartmentsas saturated soil with a depth of 5 cm, which would prevent any lat-eral subsurface flow among the compartments. About 12 kg sandwas used to fill each compartment. The packed soil box was thenplaced on an adjustable metal shelf at a slope of 2.2 degree. A sprea-der consisting of a PTFE tube with uniformly distributed small holeswas placed at the up end of the soil box to distribute inflow (Fig. 1).A peristaltic pump was connected to each side of the PTFE tube toapply a constant inflow at 0.31 L/min. A rain-shielded trough wasmounted at the lower end of the box to collect outflow. Four PTFEfunnels are mounted below at the drainage outlets to collect infil-trating water. Cumulative flow from the surface runoff and drain-age outlets were collected and continuously monitored during theexperiment with ECH2O Dielectric Aquameters (Decagon Devices,Inc.) that record the water level in the collection containers belowthe box. The realtime data were recorded with a CR 10 data logger(Campbell Sci., Logan, WA) every 30 s.

The surface runoff system included a rain simulator located in theWater Resources Lab at the University of Florida. The rain simulatorused a shallow well pump (Central Mechinery) and a pressure gauge

y runoff experimental setup.

C. Yu et al. / Journal of Hydrology 402 (2011) 159–164 161

to supply a constant flow to a tee jet 1/2 HH SS 50 WSQ nozzle (Spray-ing Systems Co., Wheaton, IL) to generate simulated rainfall (Fig. 1).Catch-can uniformity tests indicated that the rain simulator can gen-erate uniform rainfall over the entire soil box with uniformity great-er than 90%. The rainfall intensity was adjusted to a constant rate of64 mm/h for all runoff experiment conducted, which was equivalentto a flow rate of 0.66 L/min (i.e., 1.07 L/min/m2). This rain intensitywas chosen because it can represent a typical storm (10 year returnperiod and duration of 70 min) in Alachua County, Florida. The soilbox was placed 2 ml below the rainfall simulator nozzle and six man-ual rain gauges and one electronic tipping bucket rain gauge (TexasElectronics, Dallas, TX) were used to monitor the intensity during theexperiments.

2.3. Runoff experiments

To initiate a runoff experiment, simulated rainfall and inflowwith colloid-free water were first applied to the soil box for 2 hto establish a steady flow condition for the overland flow and sub-surface drainage. Once the flow was stabilized, the colloid suspen-sion (100 ppm) was introduced to the inflow spreader for 30 min.The inflow was then switched back to water at the same flow ratefor another 50 min. Water samples were collected from the surfacerunoff and the four drainage outlets from the time of colloid injec-tion to the end of the experiment. Colloid concentrations in thesamples were determined by measuring the total extinction oflight at a wavelength of 350 nm with UV–visible spectrophotome-try. To insure the data quality, transport experiments of colloidswere repeated six times.

Bromide was applied to the surface runoff system as a con-servative solute for the transport studies. The experimental proce-dures were the same as those used for the colloid and an ionchromatograph (Dionex Inc. ICS90) was used to determine bromideconcentrations in water samples. Similarly, bromide transportexperiments were repeated eight times to insure the data quality.

2.4. Statistical analysis

Average breakthrough concentrations for all the bromide andkaolinite experiments were reported in this study with standard er-ror of the mean (SEM). Student’s t-test with Welch’s correction wasused to compare the concentration distributions of bromide withkaolinite in surface runoff and drainage. In addition, Spearman’srank correlation coefficient was used to evaluate the statisticaldependence between bromide and kaolinite in the effluents.

3. Results and discussion

3.1. Flow distribution

Measurements of flow distribution in the runoff systemindicated that surface flow (0.60 L/min) was much higher thandrainage flow (0.073, 0.094, 0.079, and 0.081 L/min for drainage #1–4, respectively). Although the sand in the soil box was pre-saturated, the drainage rates in the experiment were much lowerthan the saturated conductivity because the drainage outlets onthe soil box were small, which limited the drainage flows. The re-corded cumulative flow in the surface runoff and drainage flowwas used to determine the water balance of the runoff system.For all the experiments, the total flow recovered from the surfaceand drainage was close to the total water input with an error smal-ler than 5%. This result indicated that the surface runoff system wassuitable for the transport experiments. Because the rainfall inten-sity used in this study was relatively high, surface runoff accountedfor about 2/3 of the total outflow, indicating that surface runoffdominated the transport process in the experimental system.

3.2. Subsurface transport of bromide and kaolinite

Subsurface transport of bromide in the soil box showed typicalbreakthrough behavior (Fig. 2). After application, bromide was firstdetected at drainage outlet #1 and the breakthrough responses in-creased for all the four drainage outlets with further bromide injec-tion. Only about 29% of the total bromide was recovered from thefour drainage outlets, confirming the dominance of surface runoffon the transport process in the system. Drainage #1 had the high-est breakthrough concentrations because bromide concentrationsin overland flow were higher in the first segment. Because of rain-fall dilution effect, the other three drainage outlets showed muchlower breakthrough concentrations than the drainage #1. Thebreakthrough concentrations of drainage #4, however, wereslightly higher than those of drainage #3, probably due to experi-mental uncertainties.

The breakthrough responses of kaolinite in the four drainageoutlets followed similar patterns to those of bromide (Fig. 3). Thenormalized breakthrough concentrations (i.e. C/C0), however, werelower for kaolinite than bromide. This was most obvious at drain-age #1 where the average peak breakthrough concentration of bro-mide reached about 0.4, while that of kaolinite was about 0.3. Onlyabout 23% of the total kaolinite was recovered from the four drain-age outlets. The lower breakthrough of kaolinite in the subsurfacerunoff was due to the filtration of colloids by the sand (Chen et al.,2005; Gao et al., 2004a). The breakthrough responses were slightlyquicker for kaolinite than bromide, which can be attributed to thesize exclusion effect of colloids in porous media (Bradford et al.,2005; Keller et al., 2004). Because the sampling interval used inthe experiment was not small enough, kaolinite breakthrough atall drainage outlets appeared to be almost instantaneous afterthe pulse was applied (Fig. 3).

The t-test analysis of the kaolinite and bromide concentrationsshowed that their concentration distributions were statistically dif-ferent in the subsurface flow (p = 0.02). Spearman’s test suggestedthat they were positively correlated with q equals to 0.70 in drain-age (p < 0.0001). Linear regression of kaolinite and bromide concen-tration in drainage (Fig. 4) showed that the slope is smaller than one(0.74), confirming that kaolinite had a lower mobility in the subsur-face flow than bromide under limited drainage conditions.

3.3. Surface transport of bromide and kaolinite

Bromide showed very fast breakthrough in surface runoff after ithad been applied, corresponding to the high surface flow rate(Fig. 5a). The bromide concentrations remained relatively high (C/C0 > 0.2) for almost the entirely bromide injection period and thendropped dramatically when inflow was switched to water. At theend of the experiment, the bromide concentration demonstrateda long tail, indicating the slow releasing of chemicals from the soilinto the surface runoff. The release of chemicals from soil to surfacerunoff may be attributed to several mechanisms including film dif-fusion (Wallach et al., 1988; Wallach and Vangenuchten, 1990),raindrop-induced exchange (Gao et al., 2004b, 2005; Walter et al.,2007), and pumping exchange (Packman et al., 2000; Ren andPackman, 2002). Because surface runoff dominated the transportprocesses, about 50% of the total bromide was recovered from thesurface runoff, which was much higher than that from subsurfaceflow. Although the bromide tracer is nonreactive, its total recoveryrate (i.e., sum of the surface and subsurface recovery rates) at theend of the experiment was only close to 80%. This incomplete recov-ery could be attributed to two reasons: (1) there was still a certainamount of bromide in the soil at the end of the experiment, asevidenced in tails of the breakthrough curves of both drainageand surface flow; and (2) raindrops could splash certain amount

Fig. 2. Transport of bromide in subsurface flow.

Fig. 3. Transport of kaolinte in subsurface flow.

162 C. Yu et al. / Journal of Hydrology 402 (2011) 159–164

Fig. 4. Correlation between kaolinite and bromide in subsurface flow.

(a)

(b)

Fig. 5. Transport of bromide (a) and kaolinite (b) in surface flow.

Fig. 6. Correlation between kaolinite and bromide in surface flow.

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of bromide out of the runoff system, particularly in the area close tothe edges of the soil box.

Responses of kaolinite in the surface runoff resembled those ofbromide and the peak concentrations also covered almost the en-tire kaolinite injection period (Fig. 5b). Similarly, about 51% ofthe total kaolinite was recovered from the surface runoff, whichwas also much higher than that from subsurface flow. The kaoliniteconcentration in the runoff also had a long tail, but it was slightlylower than that of bromide. This is probably because the colloids

had a lower concentration in soil pore water compared to bromide,which had no interactions with the sand grains.

The t-test with Welch’s correction analysis of the kaolinite andbromide concentrations in runoff could not prove that their distri-butions were statistically different in the surface runoff (p = 0.70).The Spearman’s test suggested they were strongly positively corre-lated with q equals to 0.99 (p < 0.0001). Linear regression results(Fig. 6) showed that the slope is close to one (0.93), suggesting thatkaolinite and bromide transport in surface runoff was almost iden-tical under the experimental conditions tested. The results pre-sented here indicated that colloids in overland flow on bare soilswith limited drainage may behave similar to chemical solutes,when the surface runoff dominates the transport processes. Undersuch conditions, factors such as dispersion/diffusion, and exchang-ing/pumping may have little effect on the concentration profile ofcolloids in surface flow. However, these factors could play impor-tant roles in controlling the mass transfer processes within theoverland flow and between the overland flow and the soil under-neath under other conditions. For example, if the overland flowwas much slower, colloid and bromide might not behave the samein the surface runoff because of the differences in their dispersion/diffusion rates and release rates from soil to the over land flow. Thefate and transport of colloids in surface runoff may also be affectedby scale factors, such as travel distance, plot length, soil depth, andsource loading, which are also governing factors of hillslope runoffand erosion (Parsons et al., 2006).

4. Conclusions

Laboratory runoff experiments were conducted to examine thetransport dynamics of kaolinite and bromide in overland flow andsoil drainage. We found that the transport of kaolinite in drainageflow was lower than that of bromide, which is in agreement to col-loid filtration theory. The transport of kaolinite in surface runoff al-most resembled that of bromide and statistical analysis confirmedtheir strong positive correlation with a slope close to one. The sim-ilarity between kaolinite and bromide transport in overland flowcan be attributed to the dominance of surface runoff in the trans-port processes under the experimental condition tested wheredrainage was limited.

References

Bradford, S.A., Simunek, J., Bettahar, M., Tadassa, Y.F., van Genuchten, M.T., Yates,S.R., 2005. Straining of colloids at textural interfaces. Water Resour. Res. 41 (10),W10404. doi:10.1029/2004WR003675.

164 C. Yu et al. / Journal of Hydrology 402 (2011) 159–164

Chen, G., Flury, M., Harsh, J.B., Lichtner, P.C., 2005. Colloid-facilitated transport ofcesium in variably saturated Hanford sediments. Environ. Sci. Technol. 39 (10),3435–3442.

Chrysikopoulos, C.V., AbdelSalam, A., 1997. Modeling colloid transport anddeposition in saturated fractures. Colloid Surf. A 121 (2–3), 189–202.

Crane, S.R., Moore, J.A., Grismer, M.E., Miner, J.R., 1983. Bacterial pollution fromagricultural sources – a review. T. ASAE 26 (3), 858–866.

Dosskey, M.G., Hoagland, K.D., Brandle, J.R., 2007. Change in filter strip performanceover ten years. J. Soil Water Conserv. 62 (1), 21–32.

Edwards, D.R., Moore, P.A., Daniel, T.C., Srivastava, P., 1996. Poultry litter-treatedlength effects on quality of runoff from fescue plots. T. ASAE 39 (1), 105–110.

Flury, M., Qiu, H.X., 2008. Modeling colloid-facilitated contaminant transport in thevadose zone. Vadose Zone J. 7 (2), 682–697.

Gao, B., Saiers, J.E., Ryan, J.N., 2004a. Deposition and mobilization of clay colloids inunsaturated porous media. Water Resour. Res. 40 (8), W08602. doi:10.1029/2004WR003189.

Gao, B., Walter, M.T., Steenhuis, T.S., Hogarth, W.L., Parlange, J.Y., 2004b. Rainfallinduced chemical transport from soil to runoff: theory and experiments. J.Hydrol. 295 (1–4), 291–304.

Gao, B., Walter, M.T., Steenhuis, T.S., Parlange, J.Y., Richards, B.K., Hogarth, W.L.,Rose, C.W., 2005. Investigating raindrop effects on transport of sediment andnon-sorbed chemicals from soil to surface runoff. J. Hydrol. 308 (1–4), 313–320.

Gao, B., Saiers, J.E., Ryan, J., 2006. Pore-scale mechanisms of colloid deposition andmobilization during steady and transient flow through unsaturated granularmedia. Water Resour. Res. 42 (1), W01410. doi:10.1029/2005WR004233.

Gao, B., Cao, X., Dong, Y., Luo, Y., Ma, L.Q., 2011. Colloid deposition and release insoils and their association with heavy metals. Crit. Rev. Envion. Sci. Technol. 41(4), 336–372.

Haygarth, P.M., Bilotta, G.S., Bol, R., Brazier, R.E., Butler, P.J., Freer, J., Gimbert, L.J.,Granger, S.J., Krueger, T., Macleod, C.J.A., Naden, P., Old, G., Quinton, J.N., Smith,B., Worsfold, P., 2006. Processes affecting transfer of sediment and colloids, withassociated phosphorus, from intensively farmed grasslands: an overview of keyissues. Hydrol. Process. 20 (20), 4407–4413.

Heathwaite, L., Haygarth, P., Matthews, R., Preedy, N., Butler, P., 2005. Evaluatingcolloidal phosphorus delivery to surface waters from diffuse agriculturalsources. J. Environ. Qual. 34 (1), 287–298.

Kay, D., Edwards, A.C., Ferrier, R.C., Francis, C., Kay, C., Rushby, L., Watkins, J.,McDonald, A.T., Wyer, M., Crowther, J., Wilkinson, J., 2007. Catchment microbialdynamics: the emergence of a research agenda. Prog. Phys. Geogr. 31 (1), 59–76.

Keller, A.A., Sirivithayapakorn, S., Chrysikopoulos, C.V., 2004. Early breakthrough ofcolloids and bacteriophage MS2 in a water-saturated sand column. WaterResour. Res. 40 (8), W08304.

Kouznetsov, M.Y., Roodsari, R., Pachepsky, Y.A., Sheltonc, D.R., Sadeghi, A.M.,Shirmohammadi, A., Starr, J.L., 2007. Modeling manure-borne bromide and fecal

coliform transport with runoff and infiltration at a hillslope. J. Environ. Manage.84 (3), 336–346.

Lead, J.R., Wilkinson, K.J., 2006. Aquatic colloids and nanoparticles: currentknowledge and future trends. Environ. Chem. 3 (3), 159–171.

Leguedois, S., Ellis, T.W., Hairsine, P.B., Tongway, D.J., 2008. Sediment trapping by atree belt: processes and consequences for sediment delivery. Hydrol. Process.22 (17), 3523–3534.

McCarthy, J.F., McKay, L.D., 2004. Colloid transport in the subsurface. Past, present,and future challenges. Vadose Zone J. 3, 326–337.

Oliver, D.M., Clegg, C.D., Haygarth, P.M., Heathwaite, A.L., 2005. Assessing thepotential for pathogen transfer from grassland soils to surface waters. Adv.Agronomy 85, 125–180.

Pachepsky, Y.A., Sadeghi, A.M., Bradford, S.A., Shelton, D.R., Guber, A.K., Dao, T.,2006. Transport and fate of manure-borne pathogens: modeling perspective.Agr. Water Manage. 86 (1–2), 81–92.

Packman, A.I., Brooks, N.H., Morgan, J.J., 2000. A physicochemical model for colloidexchange between a stream and a sand streambed with bed forms. WaterResour. Res. 36 (8), 2351–2361.

Parsons, A.J., Brazier, R.E., Wainwright, J., Powell, D.M., 2006. Scale relationships inhillslope runoff and erosion. Earth Surf. Proc. Land. 31 (11), 1384–1393.

Ren, J.H., Packman, A.I., 2002. Effects of background water composition on stream-subsurface exchange of submicron colloids. J. Environ. Eng. – ASCE 128 (7), 624–634.

Ren, J.H., Packman, A.I., 2005. Coupled stream-subsurface exchange of colloidalhematite and dissolved zinc, copper, and phosphate. Environ. Sci. Technol. 39(17), 6387–6394.

Roodsari, R.M., Shelton, D.R., Shirmohammadi, A., Pachepsky, Y.A., Sadeghi, A.M.,Starr, J.L., 2005. Fecal coliform transport as affected by surface condition. T.ASAE 48 (3), 1055–1061.

Ryan, J.N., Elimelech, M., 1996. Colloid mobilization and transport in groundwater.Colloid Surf. A 107, 1–56.

Simunek, J., He, C.M., Pang, L.P., Bradford, S.A., 2006. Colloid-facilitated solutetransport in variably saturated porous media: numerical model andexperimental verification. Vadose Zone J. 5 (3), 1035–1047.

Stumm, W., 1977. Chemical interaction in particle separation. Environ. Sci. Technol.11 (2), 1066–1070.

Wallach, R., Vangenuchten, M.T., 1990. A physically based model for predictingsolute transfer from soil solution to rainfall-induced runoff water. WaterResour. Res. 26 (9), 2119–2126.

Wallach, R., Jury, W.A., Spencer, W.F., 1988. Transfer of chemicals from soil solutionto surface runoff – a diffusion-based soil model. Soil Sci. Soc. Am. J. 52 (3), 612–618.

Walter, M.T., Gao, B., Parlange, J.Y., 2007. Modeling soil solute release into runoffwith infiltration. J. Hydrol. 347 (3–4), 430–437.