GROUNDWATER
The source is ultimately precipitation. The amount of precipitation that ends up as groundwater and the rate of percolation depend on the regolith (esp soil) texture (porosity and permeability).
Of the amount that soaks in
some held in soil
rest percolates down under force of gravity and flows down slope under force of gravity
at depth rocks are water saturated = zone of saturation. Here all openings are water filled.
The upper, not saturated zone = zone of aeration
The two are separated by the water table.
elevation of the water table depends on:
Where the water table intersects the surface water must flow out.
happens in springs
wells = artificial surface intersections
Rock layers with high porosity and permeability have much water
- these are aquifers - sought out as water source
If virtually no water or no permeability
- aquitards or aquicludes.
Geological effects of groundwater
Groundwater doesn't move fast enough to erode. So effects confined to weathering, solution, deposition
- weathering and solution mostly in zone of aeration where there is water and also air.
- solution - effects most spectacular in carbonate rocks - get caves and other solution effects
May dissolve so much get sinks - vertical passages through which surface drainage goes underground. There may be short streams which soon disappear underground in depressions.
- leads to an unmistakable type of topography - irregular and rugged without surface drainage called karst.
Another depositional feature - caliche or hardpan
In arid climate. Dissolve carbonate at surface but due to aridity the water evaporates and deposits dissolved material close to surface.
Hydrothermal (hot) waters - are due to progressive rise of temperature with depth. Esp if igneous rocks near surface, usually in areas of recent volcanism such as Yellowstone, Iceland
May get hot water springs with large channels direct, free of constrictions unhindered circulation
geysers - large capacity reservoirs, outlet channel very narrow in some sections.
For more information about groundwater in general and groundwater in Pennsylvania in particular see:http://www.dcnr.state.pa.us/topogeo/education/es3.pdf