Mechanism – The injection of large volumes of amendment solution can potentially displace contaminated water to regions outside the treatment zone (Payne et al., 2008 and Huling and Pivetz, 2006). Displacement of contaminants can potentially occur when amendments are injected in large volumes or reactions of amendments produce large volumes of gas in the subsurface, (e.g., Fenton reagents for ISCO). In addition, increasing subsurface temperature during in situ Fenton oxidation will increase contaminant mobility and cause groundwater to volatilize and expand potentially displacing contaminants. Contaminants may move downgradient or be displaced along preferential pathways of groundwater flow.