Seepage refers to dripping of water into an underground tunnel. When hot radioactive waste is placed in tunnels situated in unsaturated fractured rock, groundwater undergoes vaporization and boiling. Subsequently, vapor migrates out of the matrix into fractures, moving away through the permeable fracture network by buoyancy, by the increased vapor pressure caused by heating and boiling, and through local convection. In cooler regions, the vapor condenses on fracture walls, where it drains through the fracture network. Slow imbibition of water thereafter leads to gradual rewetting of the rock matrix.

The chemical evolution of waters, gases, and minerals is coupled to the thermal-hydrological processes above. Amorphous silica precipitates from boiling and evaporation, and calcite from heating and CO₂ volatilization.  The precipitation of amorphous silica, and to a lesser extent calcite, results in long-term permeability reduction.  Evaporative concentration also results in the precipitation of gypsum (or anhydrite), halite, fluorite and other salts.  These minerals eventually redissolve when the boiling front collapses, however, their precipitation results in a significant temporary decrease in permeability. Reduction in permeability also causes changes in fracture capillary characteristics. Summarizing, the coupled thermal-hydrological-chemical (THC) processes dynamically alter the hydrological properties of the rock, and influence groundwater flow.

A model based on the TOUGHREACT reactive transport software (Xu et al., 2006) is used to investigate the impact of THC processes on groundwater flow near an emplacement tunnel at Yucca Mountain, Nevada. We show how transient changes in hydrological properties caused by THC processes often lead to local flow channeling and saturation increases above the tunnel.  For models that include only permeability changes to fractures, such local flow channeling may lead to seepage relative to models where THC effects are ignored, however, coupled THC seepage models that include both permeability and capillary changes to fractures may not show this additional seepage.