Groundwater

Main topics:• Location of groundwater• Storage of groundwater• Movement of Groundwater• Groundwater erosion and deposition• Groundwater systems and protection

The hydrosphere• Water located on and inside the Earth’s crust

make up the hydrosphere.• 97% of the water in the hydrosphere is contained

in the oceans. (salt water)

3% is contained by landmasses • Most of the fresh water is located in polar ice

caps or glaciers. (2.15%)• Groundwater represents the next largest block of

all the freshwater. (0.31%)• Lakes, streams, rivers and wetland represent the

rest of the freshwater on Earth (0.0091%)

Hydrosphere

Source of Groundwater• Ultimate source of all water is the water

cycle.• Precipitation that falls on land can either

runoff or infiltrate (most will infiltrate).• The process of water seeping into the

ground and becoming ground water is called infiltration.

• Infiltration will also be known as groundwater recharge.

Groundwater Storage

• Groundwater is located in the spaces between soil particles and rock.

• These open spaces betweens materials is called pore space.

• The percentage of pore space in a material is called porosity.

Groundwater Storage• Well sorted material will have similar

porosity, despite grain size.• Large grained material will have larger pore

size, but contain fewer pores…while smaller grained material will have smaller pores but many more pores! (lab results?)

• Poorly sorted material usually has low porosity.

Location of Groundwater• Soil moisture zone – the area of soil directly at

the surface that is moist due to vegetation.• Zone of aeration – the area between the soil

moisture zone and water table, pores are mostly filled with air. (vadose zone, unsaturated zone.)

• Zone of saturation – the location below the water table where all pores are filled with water.

• Water table – the boundary between the zone of aeration and zone of saturation.

Location of Groundwater

Movement of groundwater

• Groundwater will move similar to water on the surface…downhill and from areas of high pressure to areas of lower pressure.

• Groundwater movement is controlled by permeability.

• Permeability is the ability of a material to allow water to pass through it. (transmit water)

• Permeability can also be referred to as how well the pores are connected.

Permeability

Permeability• Coarse grained material usually has high

permeability such as sand and gravel.• Fine grained material usually has low

permeability such as clay.• What type of material would make a good

aquifer?

Gravel• What type of material would make a good

protective liner for a capped landfill?

Clay

Flow of Groundwater• Darcy’s Law helps us determine how well water

will flow in the ground.

Q = K (dh/dl)

Q = discharge velocity (m/s)

K = hydraulic conductivity (how well material transmits water) (m/s)

dh/dl = hydraulic gradient (slope of water table)

(dh is change in rise and dl is change in run)

Flow of Groundwater

Using Darcy’s Law: Q = K (dh/dl)Example: fine sand (K=6m/day) vs. gravel (K=140m/day)

65 m

40 m

150 m

Groundwater Erosion• Dissolution – carbonic acid in groundwater

dissolves limestone (calcium carbonate).

CO2 + H2O H2CO3

carbon dioxide + water carbonic acid

H2CO3 H+ + HCO3-

carbonic acid hydrogen ions + bicarbonate ions

CaCO3 + H+ Ca2+ + HCO3-

calcium carbonate + hydrogen ions calcium ions + bicarbonate ions

Groundwater Erosion• Karst topography – areas of limestone that

have sinkholes, caves, and sinking streams due to dissolution of limestone.

• Sinkholes – an area of land that falls inward into the Earth due to supporting limestone bedrock being eroded away.

Groundwater Deposition• Stalactites – deposits on the TOP of cave.• Stalagmites – deposits on the bottom of cave.• Hardwater – deposits left behind in tap

water. (calcium)

Groundwater Systems

• Emergence of Springs – when the land surface intersects the water table.

- water will leak or seep out on to the surface.

Groundwater Systems• Well – vertical hole dug into the ground, into the

zone of saturation.• Drawdown – sinking of the water table due to

water being pumped out of a well.• Recharge – new water entering into a

groundwater system.

Groundwater Systems• Artesian wells – water that is forced to the

surface because of a confined aquifer, hydrostatic pressure provides force for the water. H:\ES_class\Unit 3 - Surface process\18.swf

Artesian well

Artesian well

Threats to Groundwater Supply

• Overuse – too much water being pumped from a well…discharge is greater than recharge.

• Subsidence – sinking of the land surface because of too much water being pumped from the ground.

• Groundwater pollution (chemicals, salt, radon) – water can carry dissolved pollutants.