GROUND WATER Chapter 13

 

A.     Introduction

1.      Definitions:

a.      Ground Water = all water stored beneath the earth’s surface (poor definition)

b.      Aquifer = a body of rock or sediment through which GW flows

c.       Ground Water Table (GWT):  page 297-298, see Fig 13.3, p 298

i.                    Vadose zone = zone of aeration (air) = unsaturated zone= zone above the ground water table that may contain soil moisture but is not totally saturated

ii.                  Capillary fringe—area marking the ground water table

iii.                Saturation Zone = zone below the GWT where every crack or space between grains is filled with GW

iv.                In humid climates the GWT generally follows the surface topography and the depth to the GWT is variable; the GWT intersects the bottom of major streams

v.                  The top of the Saturation Zone is a relatively sharp boundary (GWT); however, the “bottom” depends on how far down there are still open spaces to hold any water

2.      Porosity page 296-297,  see Fog 13.2 and Table 13.1 (ignore exact values given in this table for % porosity, but know relative porosities (hi or low) of materials

a.       Definition: a measure of how much GW an aquifer can hold (expressed as a percentage)

                                                            = (volume of pore spaces)/(total volume) x 100

b.      Unconsolidated gravel, sand, silt have high porosity; poor cemented sandstone or conglomerate have moderate porosity; igneous and metamorphic rocks have low porosity (only spaces are fractures); carbonates are highly variable depending on the extent of solution.

3.      Permeability page 297---a measure of the extent to which water can move through the aquifer; i.e., how extensively are these fractures and pore spaces connected in order to permit water to move.  

4.      Movement of GW:  p. 298

a.      Requires both permeability and a (hydraulic) gradient (i.e., a “down hill” slope)

b.      The rate of movement of GW varies greatly from fairly rapid (but slower than surface stream flow) to extremely slow movement---time measured in centuries-----most GW rates are somewhere between these extremes

B.     Types of Aquifers see page 298-302----but rely on notes, not the book for the three types

1.      Unconfined Aquifer (most common) ; cone of depression see Fig/ 13.6; p. 300

2.      Confined Aquifer:  only type that may result in an ARTESIAN system see p. 300 & 302;  see Fig. 13.7 on p. 302

3.      Perched Aquifer = Perched Water Table

C.     Geologic Work of Ground Water

1.      Erosion pp. 302-304

a.       Most landscapes show no influence from GW erosion, but there are many areas that do;

For example:  S. Indiana, N. Kentucky, Florida, and parts of former Yugoslavia, China & Puerto Rica

b.      Ground Water erosion proceeds almost entirely by solution of soluble rocks, mostly carbonates (limestone & dolostone

c.       Cave Formation p. 303-304

i.                    Almost all caves form in limestone or dolostone

ii.                  Caves are formed directly below the GWT, even those now above the GWT are the ones familiar to most people

d.      Karst Topography—named after a region in the former Yugoslavia  see p 305, Fig 13.10; see p 306, Fig 13.11

i.                    Early stages:  cave formation (see above)

ii.                  Middle stages: sink holes begin to form, may have sinking creeks = disappearing streams

iii.                Late stages: most of the land surface has now collapsed leaving only a few erosional remnants (i.e., areas not eroded) called Karst Towers or “haystack” hills

2.  Transportation---Important, but extremely little solid sediment transported; almost all material is transported as dissolved mineral matter.

3.      Deposition:  Not too important

a.  Cave deposits—dripstone, forms ABOVE the GWT see p. 303; see Fig. 13.8-b, p. 303

i.                    Stalactites (grow from the ceiling of caves)

ii.                  Stalagmites (grow from the floor of caves)

b.  Petrified Wood/Geodes/Concretions  

D.    Ground Water as a valuable resource; two threats: contamination and overuse  See pages 307-311

1.      Overuse---many areas are withdrawing GW accumulated over 1000’s of years in areas with slow recharge

2.      Overdrawing a single well:  Cone of depression see Fig/ 13.6; p. 300

a.      The Great Plains:  Ogallala Aquifer ---over ˝ of available water used in past 70 years see Fig 13.13, p. 307

b.      San Joaquin Valley, Calif. ----extensive withdrawal for irrigation.  see p. 309, Fig. 13.16

The water table has dropped over 100 feet since pumping began in the 1930’s.  Because air cannot support weight as well as water, there has been accompanying subsidence of the land surface of about 25 feet during this period resulting in a permanent loss of porosity in the aquifer

c.       Incursion of salt water occurs if GW is overdrawn near the coast see p. 307

2.    Contamination—GW pollution is more difficult to detect that surface water pollution and is much, much more difficult to remediate (clean up) than streams. 

 

NOTE:  Geysers and Geothermal Energy (pages 311-316) will NOT be covered on Test III.