Vasey's Paradise in the Grand
Canyon. A streasm is issuing from a small cavern in the Redwall Limestone near
river level.
Groundwater is liquid water that lies in the subsurface in fractures in rocks and in pore space between grains in sedimentary rocks.
Porosity is the percentage of open space in a rock. Porosity can be as high as 50% in loose sand to 5% in cemented, lithified sandstone, to near zero in unfractured igneous rocks.
Permeability is the ability of fluids to flow through rock. It depends on the connectivity of the pore space. Permeable rocks include sandstone and fractured igneous and metamorphic rocks and karst limestone. Impermeable rocks include shales and unfractured igneous and metamorphic rocks.
The water table is the natural level of liquid ground water in an open fracture or well. The water table follows topography.
The unsaturated zone is the region above the water table where pores and fractures are partially filled with water and partly by air. The pressure in the unsaturated zone is atmospheric. Conditions are usually highly oxidizing due to the presence of free oxygen in the air and highly reactive due to the presence of water.
The saturated zone is the region below the water, table where all fractures and pores are filled with water. The pressure in the saturated zone is hydraulic (= that of the overlying water column). It increases at the rate of 1atm per 10m. At very great depths ( > 2km or so) the pores are sealed, and the pressure is that of the overlying rock column (330 atm / km).
A perched water table is isolated from the main water table by a layer of impermeable rock.
Surface springs occur where the water table crops out of meets the surface
An aquifer is a porous and permeable layer through which water can easily flow (e.g. coarse sandstone, unconsolidated sediments).
An aquitard is a body of rock that retards flow (e.g. shale, unfractured igneous or metamorphic rock).
An artesian basin is one that is under pressure due to hydraulic head. The artesian aquifer is usually confined between aquitards. An artesian well is one that will flow without pumping.
Karst topography refers to a surface topography marked by sinkholes, disappearing streams, and small closed drainage basins. It is an indication of underground drainage in caverns in limestone.
Limestone (CaCO3) is dissolved by acids in soil and rainwater to form caverns. Precipitation gives rise to stalactites that hang from the roof, and stalagmites that build up on the floor of the cavern from dripping water saturated in calcite.
Hot springs are caused by the outcrop of hot water heated by local igneous activity or by water that is rising from great depths.
Geysers are caused by hot springs of water at or above the boiling point. Geysers occur in active volcanic regions such as Yellowstone Park, Wyoming, Geysers, California, and Iceland.
The unsaturated zone is a region of active chemical reaction due to the presence of water, air, soil and rock that are out of chemical equilibrium. Where human activity such as mining has dis turbed the water table oxidation and hydration reactions can produce acid mine drainage, concretions (iron sulfate/sulfide reactions), and movement of uranium and thorium oxides.
Ground water is used for municipal and agricultural water supplies through much of the U.S. Pollution of ground water can be a major problem. Ground water pollution is very expensive to clean up. Major sources of ground water pollution include fuel from buried gasoline tanks, indus trial solvents, agricultural chemicals, landfills, defense wastes, and domestic septic systems.