ABSTRACT
This chapter introduces the fundamental concept relates to ground-water flow. Water moves through the pore-space and/or discontinuity passageways in earth materials from points of high water-table elevation to points of lower water-table elevation, seeking the pathways of least resistance. Engineered waste-disposal facilities retain the essential water-balance components of the global hydrologic cycle. Residence time is the amount of time required for a molecule of water to travel through a component of the hydrologic cycle before being discharged to another part of the system. Evaporation from the ocean and other surface-water bodies results in most dissolved constituents being "left behind," so that water which enters the atmosphere is effectively clean. Pressure tunnels make use of the flow concept introduced in relation to ground-water flow, namely that water in a confined state will locally flow upwards, provided that, in the larger picture, an overall downward gradient is maintained.
