Chapter 16

 

GROUND WATER

 

Groundwater

Water beneath the ground surface stored within pores and fractures.

 

Groundwater and the Hydrologic Cycle

††††† Groundwater represents ~22% of the world supply of freshwater.

Source of groundwater: 1) precipitation that enters soil or rock, 2) water infiltrating from lakes, streams, etc.).

 

Material Properties

Porosity.Amount of open space in rock.Percentage of materialís total volume that is pore space.

††††† Sedimentary rocks (and sediments)- have highest porosity; depends on size and sorting of particles (fig. 16-2); typically ~30-50% before consolidation; ~10-30% after

†††††† compaction; ~5% after cementation (Table 16-1).

††††† Igneous and metamorphic (crystalline) rocks - have little if any pore space because of interlocking grains; groundwater moves through fractures.

 

Permeability- ability of water to flow through a rock; depends on: porosity, size of pores, and interconnections of pores.

 

Aquifer- a permeable layer transporting groundwater; well sorted sand and gravel or any highly fractured rock is best.

 

Aquiclude- rock layers that prevent movement of groundwater; shales and most igneous and metamorphic rocks.

 

Water Table (fig. 16-3)

Zone of aeration (unsaturated zone)- has both air and water in pore spaces.

Zone of saturation- all pore space filled with groundwater.

Water table- surface separating zone of aeration from zone of saturation.In humid areas, shape of water table is a subdued replica of the topography and is at the same

level as streams and lakes.In arid areas, the water table is flat and is below the level of streams.

 

Groundwater movement

Groundwater moves as a result of gravity; moves through unsaturated zone to saturated zone; in saturated zone moves from areas of high pressure (under hills) to areas of

low pressure (valleys) (fig. 16-4).

 

Springs, Water Wells, and Artesian System

Springs.Water flows or seeps out naturally from rock/soil to surface; anywhere where laterally moving groundwater intersects the earth's surface (figs. 16-6, 7).

Well. A hole drilled or dug into ground to the water table (fig. 16-9); water pumped to surface; if pumped too much, a cone of depression forms (fig. 16-9).

Artesian Water.Confined groundwater that flows out of the ground under its own pressure; rises above aquifer if pressure is released by drilling through confining layer

(figs. 16-11, 12).Requirements: 1) aquifer confined above and below by aquicludes, 2) aquifer exposed at surface, 3) enough precipitation to keep aquifer filled.

 

Groundwater erosion

Because groundwater moves so slowly, cannot erode by abrasion and hydraulic action like streams; erodes by solution; limestone very soluble in slightly acidic water.

Sinkhole- depression in ground formed by 1) solution of underlying limestone, 2) collapse of a cave roof (fig. 16-15).

Solution valley- merging of several sinkholes.

Karst topography- solution formed topography with many sinkholes, caves, springs, solution valleys, and disappearing streams (figs. 16-15).

Disappearing stream- flow for short distance at surface, then disappears into a sinkhole.††††††††††††††

Cave and Cave Deposits

Cave.Subsurface opening large enough for a person to enter; most formed by solution.

1) cement between rock particles.

2) cave deposits: dripstone- icicle-shaped structures formed in caves by precipitation of calcite from solution; stalactites (grow from roof of cave), stalagmites

(from floor), and columns (extends from roof to floor) (figs. 16-17, 18).

 

Humans Impact

1-     Lowering of water table (fig. 16-19)

2-     Loss of hydrostatic pressure

3-     Saltwater incursion (fig. 16-21)

4-     Subsidence (figs. 16-22, 23, 24, 25; table 16-2)

5-     Groundwater contamination (fig. 16-25)