2007 Ground Water Summit

Tuesday, May 1, 2007 : 2:50 p.m.

Feedback of Coupled Thermal-Hydrological-Chemical Processes on Flow in Unsaturated Fractured Rock: Application in Seepage Modeling Studies

Sumit Mukhopadhyay, Ph.D., Eric L. Sonnenthal, Ph.D. and Nicolas Spycher, Ph.D., Lawrence Berkeley National Laboratory

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.

 

Sumit Mukhopadhyay, Ph.D., Lawrence Berkeley National Laboratory Sumit Mukhopadhyay (B.E., chemical engineering, Jadavpur University, India, 1987; Ph.D., chemical engineering, University of Southern California, Los Angeles, USA, 1995) is a scientist in the Earth Sciences Division of the Ernest Orlando Lawrence Berkeley National Laboratory (LBNL). Sumit’s main expertise is in subsurface hydrology, with emphasis on fluid, gas, solute and heat transport in complex subsurface systems such as saturated and unsaturated porous or fractured media. His current research interests include coupled thermal, hydrological, and chemical processes accompanying the disposal of radioactive wastes, reactive flow and transport, and microbial-stimulated metal reduction processes in the subsurface.

Eric L. Sonnenthal, Ph.D., Lawrence Berkeley National Laboratory Dr. Eric Sonnenthal is a Staff Scientist, Deputy Department Head for Geochemistry, and Group leader in Geochemical Transport in the Earth Sciences Division at the Lawrence Berkeley National Laboratory. He has been a principal investigator and contributor on numerous projects evaluating the coupled effects of heat and fluid flow on geochemistry. He is a co-developer of the reactive transport code Toughreact, and has also developed reactive transport codes for sedimentary basin diagenesis and crystallization of magmas. Sonnenthal is a co-P.I. on several multi-year projects funded by the DOE Office of Science and Technology and International

Nicolas Spycher, Ph.D., Lawrence Berkeley National Laboratory Nicolas Spycher received his Ph.D. from the University of Oregon in 1987, and M.S. from the University of Geneva, Switzerland, in 1980. A geochemist by training, Nic has dedicated his research career to problems involving water/rock interactions, including the development of geochemical and reactive transport models to study complex and dynamic geochemical environments. His current research interests include coupled thermal, hydrological, and chemical processes accompanying the geologic disposal of nuclear waste, metal biogeochemistry and transport in lake sediments, CO2 geologic sequestration, and hydrothermal systems. Nic also worked many years as a hydrogeologist dedicated to the remediation of contaminated sites.


The 2007 Ground Water Summit