Groundwater

Where is Earth’s water found?

• Oceans = 97%• Glaciers/ice caps = 2%• Groundwater = 0.5%• Lakes, rivers, soil, living things,

atmosphere, etc. = < 0.3%

What is groundwater?• Water that lies beneath the surface• Small portion of Earth’s total water (0.5%)• Significant source of water for humans• Source is rain and snow• Water not taken up by plants and soil

flows into ground through infiltration

How does groundwater move?

• Water flows slowly downward (gravity) through soils and rock

• Once saturation is reached, water flows sideways, following landscape patterns

What is porosity?• As water flows downward, it fills the

spaces between the soil and rocks– The amount of space in a rock or soil

describes its porosity– The more porous the material, the more

water it can hold– e.g., sand is more porous than clay

What is permeability?• As groundwater flows slowly underground,

it has to squeeze through the pores of the material – The ability of a material to let water pass is

called its permeability– e.g., clay has very small unconnected

pores, which makes it impermeable

What are the zone of saturation and water table?Zone of saturation: the depth below the

surface at which the groundwater completely fills all the pores in the soil

Water table: the top of the zone of saturationshape/height depends onsurrounding elevation

What is an aquifer?• An aquifer is a permeable layer of

material full of groundwater, which can bubble up at a spring or be extracted through a well– Too much pumping from a well can lower

the water table

How does groundwater interact with surface

water?• At the base of a slope or at a surface

depression that is lower than the water table, the groundwater may come out as surface flow– e.g., stream valleys, lakes, springs

How does groundwater interact with surface

water?

Springs and caves• Spring: a place where water flows

naturally from rock onto the land surface• Caves can form when groundwater flows

through limestone and erodes them into underground openings

Water moves along fractures in crystalline rock and forms springs where the fractures intersect the land surface

Water enters caves along joints in limestone and exits as springs at the mouths of caves

Springs form at the contact between a permeable rock such as sandstone and an underlying less permeable rock such as shale

Springs can form along faults when permeable rock has been moved against less permeable rock. Arrows show relative motion along fault

Sinkholes!Jacksonville, NC 2011

Daisetta, TX

What might pollute groundwater?

• Chemicals applied to agricultural crops can seep into groundwater with rain and irrigation water

• Rain can draw pollutants (heavy metals, household poisons) from city dumps into groundwater supplies

• Urban and agricultural sewage may contain bacteria, viruses, and parasites that contaminate groundwater

• It’s almost impossible to clean it up!

Pollution of groundwater• Pumping wells can cause or aggravate

ground-water pollution

Water table steepens near a dump, increasing the velocity of ground-water flow and drawing pollutants into a well

Water-table slope is reversed by pumping, changing direction of the ground-water flow, and polluting the well

What happens when too much water is pumped?

• Local of groundwater can last indefinitely if pumped out at an equal or lower rate than the rate of recharge to the aquifer

• Nationwide, groundwater is being pumped faster than it is being recharged, and many areas are facing serious declines

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