Notes from 10.3 Groundwater Supply
Wells – holes dug or drilled into the ground to reach an aquifer
Ordinary wells – as water is drawn out of the well, it is replaced by surrounding water in the aquifer.
Overpumping is when water is drawn out faster than it can be replaced.
Drawdown – the difference in the original water table and the water level in the pumped well.
Recharge – when water from precipitation replenishes the water in an aquifer. If overpumping exceeds
recharge, wells become dry.
Cone of depression – the over pumping of a well and
surrounding wells become dry.
Changes to groundwater supplies can lead to environmental issues such as a lowered water table,
subsidence, and pollution. The largest aquifer in the US is the Ogallala Aquifer in the Great Plains. The
recharge sources for this are in the Rocky Mountains and the Black Hills in South Dakota.
U.S. daily water use
Subsidence – the sinking of land. The water
underground helps support the land above it.
When the water is over pumped faster than it
can be recharged, a sinking of the land can
occur.
Pollution in Groundwater - Sources of pollution in groundwater include sewage from faulty septic tanks
and farms, landfills, and other waste disposal sites. Pollutants enter the
ground above the water table, but eventually infiltrate into the ground water.
Chemicals – can travel through the smallest pores and can contaminate any type of aquifer.
A pollution plume is a mass of contaminates that spread through the aquifer.
Once chemicals enter the groundwater, they cannot be easily removed.
Salt – a natural pollutant in the groundwater can pollute it. Over pumping of wells,
especially near coastal water, can cause the underlying salt water to rise into the
wells.
Radon – a natural pollutant that gets into groundwater through the radioactive decay of
uranium in rocks and sediment. It usually occurs in very low concentrations of all
groundwater. Too much can cause cancer.
Groundwater Pollution Sources
Infiltration from fertilizers
Leaks from storage tanks
Drainage of acid from mines
Seepage from faulty septic tanks
Saltwater intrusion into aquifers near shorelines
Leaks from waste disposal sites
Radon
Because pollution plumes spread slowly, there is adequate time for alternate water supplies to be found.
Sometimes the pollution can be stopped by building impermeable underground barriers around the
polluted area. Sometimes the water can be pumped out for treatment on the surface.