“…Particles with an effective diameter of 1 nm to 10 μm are considered colloids (Chrysikopoulos & Sim, 1996; Gao, Cao, Dong, Luo, & Ma, 2011; Sirivithayapakorn & Keller, 2003a; Smith et al, 2007; Vasiliadou & Chrysikopoulos, 2011; Wan & Wilson, 1994b; Wang, Schneider, Parlange, Dahlke, & Walter, 2018). In the subsurface environment colloids comprise many naturally occurring substances including clays, metal oxides, mineral precipitates, and organic compounds as well as biological organisms such as pathogenic bacteria (Wang et al, 2017, 2018; Wang, Schneider, et al, 2018), viruses (Torkzaban, Hassanizadeh, Schijven, Bruijn, & Husman, 2006; Torkzaban, Hassanizadeh, Schijven, & van den Berg, 2006; Zhang, Hassanizadeh, Raoof, van Genuchten, & Roels, 2012), and protozoa (Bradford, Wang, Kim, Torkzaban, & Šimůnek, 2014; Sen, 2011). Many known environmental pollutants can attach to and move with colloids, a process referred to as colloid‐facilitated transport, which has been identified as one of the most important mechanisms responsible for the mobilization of reactive heavy metals in soils (Barton & Karathanasis, 2003; Gao et al, 2011; Grolimund & Borkovec, 2005; Kretzschmar & Schafer, 2005).…”