“…Thus, the hydraulic conductivity can vary several orders of magnitude even in relatively mild heterogeneous aquifers [Gelhar, 1993;Rubin, 2003]. This typically leads to geological structures with preferential channels [Sanchez-Vila et al, 1996;Gomez-Hernandez and Wen, 1998;Trinchero et al, 2008] and low permeability areas where contaminants can be trapped and slowly released in time [Stroo et al, 2012;de Barros et al, 2013]. Moreover, degradation rates of contaminants in aquifers can vary substantially in space [Allen-King et al, 2006] due to, for instance, changes in the bacteria activity responsible for biodegradation [e.g., Fennell et al, 2001;Sandrin et al, 2004].…”