Ground water

What is ground water and why should we care about it?

Ground water is...

Held in pore space of sediments

Found in fractures and joints of rocks

Easy to pollute but hard to clean up

VERY important fresh water source

How is water held in pores of rocks and soils?

Thin film of water molecules on soil particles (hygroscopic water)

Created by polar nature of water and polar nature of clays

Water that is held in pore space due to surface tension of water

What is surface tension?

What causes high surface tension?

Surface tension video

Gravitational water

Associated with large particles such as sand and gravel

Moves down due to gravity

Responsible for recharging ground water system

What is meant by the groundwater system?

Aquifer

Zone of aeration

Zone of saturation

Water table

Aquitard or Aquiclude

Ground water flows

Follows Surface topography

water flows down hill

Ground water is always in motion

How well does groundwater move?

Two primary variables

Porosity

The amount of pore spaces in a rock or sediment

Determines how much water a rock or sediment can hold

Which type of material would have the highest % of porosity?

Permeability

How connected pore spaces are

determines how water flows through a system

best permeability--> sand and gravel well sorted and rounded

worst permeability--> clay an silt

Porosity and permeability control how water moves through a system

What material would make a good aquifer?

What material would make a good aquitard?

Good aquifers

Sand, gravel or other permeable and porous material

Highly fracture rocks

Good aquitards

Clay or solid rock

Lines aquifers and acts as a pipe

Recharging?

How do water tables recharge?

Infiltration

Influent streams

How long (in general) would it take for ground water to recharge?

What are some factors that would control the time it takes for the table to recharge?

How quickly can water be removed from a ground water system?

What is a well?

Man made

created to harvest ground water

uses some type of mechanical pump

Natural well (artesian)

water rises above the aquifer through hydraulic pressure

1 aquifer and 2 aquitards

reaches surface by fracture or pipe

no pump is required

Other ground water exits

Springs

Effluent streams

Geysers

Society and water

Water use in USA (2005)

surface water 328 billion gal/day

ground water 82 billion gal/day

410 billion gal/day

What sectors of society use water

How are water supplies affected by the population

overuse

pollution

climate

Overuse

1950 ~ 2.6 billion people

2000 ~ 6 billion people

2050 projected to be ~9 billion people

(Numbers from the United Nations)

Competition between wells

Over pumping

draw down and cone of depression

dry wells

Subsidence due to ground water removal

Arizona

1948-1895: 15 ft of subsidence over a 675 mi2 area

Many other areas in state have subsided 3-6 ft

California

Mining of water tables

Short term non-renewable

Must apply safe yield concepts

establish hydrologic equilibrium for each basin

how much water is available

long term monitoring

limit wells

drawdown < recharge

Homework
http://www.npr.org/templates/story/story.php?storyId=18409513

Listen to the three audio links (NPR#1 you heard in class) on the class web page and then do the homework assignment associated with them.

Due 4/15/10 at the beginning of class. NO LATE ASSIGNMENTS ACCEPTED!!