Experimental Study of Horizontal Barrier Formation by Colloidal Silica

Migration of non-aqueous-phase liquids (NAPLs) such as trichloroethylene or gasoline can be controlled by barrier systems. The research presented in this paper aims to form a horizontal layer by injecting gelling liquids and to test the ability of horizontal barriers to isolate the downward migration of NAPLs in subsurface environments. We developed a methodology in the laboratory to form a contiguous horizontal barrier by injecting gelling liquids through horizontal and vertical wells. A series of batch, column, and 2D sandbox experiments were conducted to investigate the gel development and horizontal barrier formation. Prior to 2D barrier formation experiments, two different methods were employed to measure pre- and postinjection hydraulic conductivity in 1D columns to quantify hydraulic conductivity reductions. After an impervious grout formed, the durability of the grouted porous media in the presence of two NAPL contaminants was investigated. The hydraulic conductivity of filter sand treated with colloidal silica was reduced by 100% in 1D column experiments. In 2D experiments, a contiguous horizontal layer was formed by horizontal or vertical injection of the gelling liquid. The grouted material in 1D columns worked well in controlling the downward migration of contaminants. Although some penetration of the gelled layer by the contaminant was observed, the integrity of the horizontal layer was preserved. Finally, based on scaled capillary pressure versus saturation relationships, it was determined that capillary pressures can reduce as much as 50% with gelling of the colloidal silica solutions.