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-Ovilla Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA article info Article history: Received 8 December 2010 Received in revised form 5 February 2011 Accepted 7 March 2011 Available online 14 March 2011 This manuscript was handled by P. Baveye, Editor-in-Chief, with the assistance of Magnus Persson, Associate Editor Keywords: Colloids Solute Transport Surface runoff Drainage Tracer summary 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 many studies 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 laboratory experiments to examine the transport behavior of colloids in a surface runoff system, made of a soil box packed with quartz sand with four soil drainage outlets and one surface flow outlet. A natural clay colloid (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 conditions tested, our results showed that surface runoff dominated the transport processes. As a result, kaolinite and 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 bromide in 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 solutes may behave similarly in overland flow on bare soils with limited drainage when surface runoff dominates the transport processes. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Colloids (i.e., particles with diameter in the range of 1 nm to 10 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 may pose risks to surface water and groundwater quality not only be- cause many biocolloids are pathogenic, but also because abiotic colloids 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 and modeling investigations have been conducted to explore the reten- tion and transport mechanisms of colloids and colloid- contaminant complexes in soils under both water-saturated and unsaturated conditions (McCarthy and McKay, 2004; Ryan and Elimelech, 1996). Findings from laboratory experiments indicate that the transport of colloids in soils is controlled by multiple reten- tion/release mechanisms, such as grain-surface deposition, pore straining, 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 models to predict their fate and transport in the subsurface environment (Flury and Qiu, 2008; Simunek et al., 2006). Most of these models are based on the advection–dispersion equations coupled with reac- tions, which are similar to the models developed for solute transport in soils. However, the transport behavior of colloids in soils may dif- fer from that of chemical solute because of the size exclusion effect and the distinct retention mechanisms (Chrysikopoulos and AbdelSalam, 1997; Simunek et al., 2006). The different breakthrough behavior 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 colloids in 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 often treated as dissolved phase if they can pass through a 0.45 lm filter (Lead and Wilkinson, 2006). Ren and Packman (2002, 2005), 0022-1694/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2011.03.011 ⇑ Corresponding authors. Tel.: +1 352 392 1864; fax: +1 352 392 4092. E-mail addresses: bg55@ufl.edu (B. Gao), carpena@ufl.edu (R. Muñoz-Carpena). Journal of Hydrology 402 (2011) 159–164 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol