Groundwater Lecture Notes

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Physical Geology

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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:

vesicles & fractures in a basalt = 30-40%

solution cavities in limestone = 30%

spaces between grains in cemented sandstone = 5%

spaces between grains in conglomerate = 20%

fractures in granite = <1%

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

- pore space is filled with air and water

- 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.

- usually at bottom of lakes & streams.

- sometimes on steep hillsides and cliffs where the water table intersects the hillside.

- Wells are drilled to remove the water artifically

- drill well into the zone of saturation.

- water fills the well to the level of the water table

- water is then pumped out

- water table around the well is drawn down to form a cone of depression.

- if gw is not replenished fast enough, well will eventaully go dry as the cone deepens over time.

- can cause nearby shallower wells to go dry = water wars.

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

- sinkholes

- solution valleys & underground streams

- disappearing streams

- 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.

- stalactites hang from the ceiling

- stalagmites rise up from the floor

- these originate from dripping water which contains calcite dissolved from the limestone.