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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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