Colloid-Facilitated Solute Transport in Variably Saturated Porous Media: Numerical Model and Experimental Verification

Strongly sorbing chemicals (e.g., heavy metals, radionuclides, pharmaceuticals, and explosives) in porous media are associated predominantly with the solid phase, which is commonly assumed to be stationary. However, recent fieldand laboratory-scale observations have shown that in the presence of mobile colloidal particles (e.g., microbes, humic substances, clays, and metal oxides), colloids can act as pollutant carriers and thus provide a rapid transport pathway for strongly sorbing contaminants. To address this problem, we developed a one-dimensional numerical model based on the HYDRUS-1D software package that incorporates mechanisms associated with colloid and colloid-facilitated solute transport in variably saturated porous media. The model accounts for transient variably saturated water f low, and for both colloid and solute movement due to advection, diffusion, and dispersion, as well as for solute movement facilitated by colloid transport. The colloid transport module additionally considers the processes of attachment/detachment to/from the solid phase and/or the air–water interface, straining, and/or size exclusion. Various blocking and depth dependent functions can be used to modify the attachment and straining coefficients. The solute transport module uses the concept of two-site sorption to describe nonequilibrium adsorption–desorption reactions to the solid phase. The module further assumes that contaminants can be sorbed onto surfaces of both deposited and mobile colloids, fully accounting for the dynamics of colloid movement between different phases. Application of the model is demonstrated using selected experimental data from published saturated column experiments, conducted to investigate the transport of Cd in the presence of Bacillus subtilis spores in alluvial gravel aquifer media. Numerical results simulating bacteria transport, as well as the bacteria-facilitated Cd transport, are compared with experimental results. A sensitivity analysis of the model to various parameters is also presented. COLLOIDAL PARTICLES (e.g., humic substances, clays, metal oxides, and microorganisms) are commonly found in subsurface environments (Kretzschmar et al., 1999). Many contaminants can sorb onto colloids in suspension, thereby increasing their concentrations in solution beyond thermodynamic solubilities (Kim et al., 1992). Experimental evidence now exists that many contaminants are transported not only in a dissolved state bywater, but also sorbed tomoving colloids. This colloidfacilitated transport has been illustrated in the literature for numerous contaminants, including heavy metals (Grolimund et al., 1996), radionuclides (Von Gunten et al., 1988; Noell et al., 1998), pesticides (Vinten et al., 1983; Kan and Tomson, 1990; Lindqvist and Enfield, 1992), pharmaceuticals (Tolls, 2001; Thiele-Bruhn, 2003), hormones (Hanselman et al., 2003), and other contaminants (Magee et al., 1991; Mansfeldt et al., 2004). Since mobile colloids often move at rates similar or faster as nonsorbing tracers, the potential of enhanced transport of colloid-associated contaminants can be very significant (e.g., McCarthy and Zachara, 1989). Failure to account for colloid-facilitated solute transport can severely underestimate the transport potential and risk assessment for these contaminants. Models that can accurately describe the various mechanisms controlling colloid and solute transport, and their mutual interactions and interactions with the solid phase, are essential for improving predictions of colloid-facilitated transport of solutes in variably saturated porous media. The transport behavior of dissolved contaminant species has been studied for many years. By comparison, colloid transport and the mutual interactions among contaminants, colloids, and porous media are less well understood. While colloids are subject to similar subsurface fate and transport processes as chemical compounds, they are also subject to their own unique complexities (van Genuchten and Šimůnek, 2004). Since many colloids and microbes are negatively charged, they are electrostatically repelled by negatively-charged solid surfaces. This will lead to an anion exclusion process that can cause slightly enhanced transport relative to fluid flow. The advective transport of colloids may similarly be enhanced by size exclusion, which limits their presence to the larger pores (Bradford et al., 2003, 2006). In addition to being subject to adsorption–desorption process at solid surfaces, colloids are also affected by straining in the porous matrix (Bradford et al., 2003, 2006) and may accumulate at air– water interfaces (Wan and Wilson, 1994; Thompson and Yates, 1999; Wan and Tokunaga, 2002). All of these additional complexities require colloid transport models to be more flexible than regular solute transport models. Models that consider colloid-facilitated solute transport are based on mass balance equations for all colloid and contaminant species. The various colloid-facilitated solute transport models that have appeared in the literature differ primarily in the manner which colloid transport and contaminant interactions are handled. For example, Mills et al. (1991) and Dunnivant et al. (1992) assume that colloids are nonreactive with the solid phase, Corapcioglu and Jiang (1993) and Jiang and Corapcioglu (1993) consider a first-order kinetic attachment of colloids, Saiers and Hornberger (1996) consider an irreversible nonlinear kinetic attachment of colloids, and van de Weerd and Leijnse (1997) describe colloid attachment kinetics using the Langmuir equation. All colloidfacilitated transport models account for interactions beJ. Šimůnek and C. He, Dep. of Environmental Sciences, Univ. of California Riverside, CA 92521; L. Pang, Institute of Environmental Science&Research,Christchurch,NZ; S.A.Bradford,GeorgeE.Brown Jr. Salinity Laboratory, USDA, ARS, Riverside, CA 92521. Received 22 Dec. 2005. *Corresponding author ([email protected]). Published in Vadose Zone Journal 5:1035–1047 (2006). Original Research doi:10.2136/vzj2005.0151 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA R e p ro d u c e d fr o m V a d o s e Z o n e J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1035 Published online August 24, 2006