Physical Geology
Groundwater
- movement of groundwater controlled by porosity and permeability.
Porosity:
- the percentage of the total volume of a rock consisting of voids.
- pore spaces: spaces between mineral gains
fractures
- solution cavities
- vesicles
- for example:
Permeability:
- the capacity of a rock to transmit fluids
- depends on the size of the voids, and whether the voids are interconnected.
- a rock can have high porosity but low permeability if the voids are not connected so as to allow water to flow.
- velocity of gw averages 1 m/day to 1 m/year.
The Water Table
- zone of aeration
- below this is the zone of saturation, where pore space is filled with water only
- the upper level of the zone of saturation is called the water table.
- water table is flat in flat country, but rises and falls with the ground surface in hilly country
- because the water table rises during precip from infiltration.
- gw flows from high points to low points, and the difference in elevation between the high and low points is called the hydraulic head.
- gw seeps out in lakes and stream cuz these are areas of least pressure, and gw always flows in the direction of least pressure.
- Areas where the water table intersects the surface are called natural springs.
- Wells are drilled to remove the water artifically
Artesian water
- water confined to an aquifer between impermeable layers, under pressure
- can produce flowing wells when drilled into.
- water will rise in a well to the level of the artesian-pressure surface, which slopes downward from the recharge area.
- water enters the aquifer at the recharge area only.
- this system usually only encountered in areas of flat-lying sedimentary rx.
Geysers:
- areas near hot magma beneath the surface
- gw circulates close the magma, gets heated up, and rises to the surface to erupt as a geyser.
- the amount of time between eruptions is the amount of time needed for gw to perculate downward through fractures and get heated to the point of eruption.
Erosion by groundwater
- in some areas, gw dissolves large areas of rock and carries it away.
- leaves behind karst topography
- usually in areas where limestone is present cuz air reacts with groundwater to form carbonic acid (H2CO3), and this acid easily dissolves limestone.
- water seeps between bedding planes and fractures in limestone to eventually for long, continuous cave systems
- Kentucky & West Virginia have many of these.
- sinkholes form where rock has been removed below and the roof collapses
- solution valleys are areas where the roof has not collapsed yet, but has instead sunk down somewhat.
- small streams often flow on the surface, then disappear into a sinkhole. They may reappear again downstream. These are called disappearing streams.
- within these cave systems, minerals in the gw often precipitate on the roof and floor of the caves.
- these originate from dripping water which contains calcite dissolved from the limestone.
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