Mass transport in geologic systems often involves chemical transformations, rock-mechanical effects, and heat transfer. Subsurface flow processes occur under a vast range of thermodynamic conditions and chemical constituents, from vadose zones just beneath the land surface to deep crustal environments. Hydrogeology Department researchers are developing a quantitative understanding of the main physical and chemical processes that operate in these systems, and providing practically useful engineering tools for recovering, storing, and protecting subsurface fluid resources. Areas of specific interest include geologic disposal of heat-generating nuclear wastes; recovery of oil, gas, and geothermal energy; subsurface storage of greenhouse gases; protection of groundwater resources; remediation of environmental contamination problems; techniques for recovering natural gas from hydrates; and management of microbial interactions in landfills. Current research trends are towards describing subsurface processes in a more comprehensive manner, with consideration of coupled processes, and across multiple interacting scales. The Earth and Environmental Sciences Area (EESA) is home to the TOUGH family of numerical simulation codes, which are widely used to analyze multiphase processes that involve heat transfer effects and rock-fluid interactions. The EESA modeling group offers symposia and training courses as a service to the worldwide TOUGH user community.