GROUNDWATER

GROUNDWATER
Groundwater begins with precipitation that seeps into the ground. The amount of water that seeps into
the ground will vary widely from place to place, depending on the slope of the land, amount and
intensity of rainfall, and type of land surface. Porous, or permeable, land containing lots of sand or
gravel will allow as much as 50 percent of precipitation to seep into the ground and become
groundwater. In less permeable areas, as little as five percent may seep in. The rest becomes
runoff or evaporates. Over half of the fresh water on Earth is stored as groundwater.
As water seeps through permeable ground, it continues downward until it reaches a depth where
water has filled all the porous areas in the soil or rock. This is known as the saturated zone. The top
of the saturated zone is called the water table. The water table can rise or fall according to the
season of the year and the amount of precipitation that occurs. The water table is typically higher in
early spring and lower in late summer. The porous area between the land surface and the water table
is known as the unsaturated zone.
AQUIFERS
Water-bearing rock, sand, gravel, or soil that is capable of yielding usable amounts of groundwater is
called an aquifer. The water yield from an aquifer depends greatly on the materials that make it up.
Mixtures of clay, sand, and fine particles yield small amounts of water because the spaces between
the particles don t allow water absorption and flow. Materials sorted into distinct layers will yield high
amounts of water from coarse-grained materials like large sand grains and gravel, but low amounts
from fine-grained sand, silt, or clay. Bedrock aquifers will yield substantial amounts of water if there
are large openings or cracks in the rock. The capacity of soil or rock to hold water is called its
porosity; the capacity for water to move through the aquifer is called permeability.
There are two types of aquifers: confined, or artesian aquifers, and unconfined, or water table
aquifers. Artesian aquifers contain groundwater that is trapped under impermeable soil or rock and
may be under pressure. Artesian wells are wells that pierce artesian aquifers. The water in these wells
usually rises toward the surface under its own pressure. If the water level in the well is higher than the
land surface, it may be a flowing artesian well. A well in an unconfined aquifer has the same water
level as the water table around it.
GROUNDWATER RECHARGE
Water that seeps into an aquifer is known as recharge. Recharge comes from a variety of sources,
including seepage from rain and snow melt, streams, and groundwater flow from other areas.
Recharge occurs where permeable soil allows water to seep into the ground. Areas in which this
occurs are called recharge areas. They may be small or quite large. A small recharge area may
supply all the water to a large aquifer. Streams that recharge groundwater are called losing streams
because they lose water to the surrounding soil or rock.
GROUNDWATER DISCHARGE
Groundwater can leave the ground at discharge points. Discharge happens continuously as long as
enough water is present above the discharge point. Discharge points include springs, stream and lake
beds, wells, ocean shorelines, and wetlands. Streams that receive groundwater are called gaining
streams because they gain water from the surrounding soil or rock. In times of drought, most of the
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surface water flow can come from groundwater. Plants can also contribute to groundwater discharge,
because if the water table is close enough to the ground, groundwater can be discharged by plants
through transpiration.
GROUNDWATER MOVEMENT
Groundwater usually moves slowly from recharge areas to discharge points. Flow rates within most
aquifers can be measured in feet per day, though in karst bedrock the rate of flow can be measured in
miles per hour. Flow rates are faster when cracks in rocks or very loose soil allow water to move
freely. However, in dense soil, groundwater may move very slowly or not at all.
Groundwater typically moves in parallel paths, or layers. Since groundwater movement is slow, it
doesn??t create enough turbulence to cause mixing the way surface waters mix when a river or stream
empties into another waterbody. That is, layers of groundwater remain relatively intact. This can be
an important factor in locating and determining the movements of contaminants that might enter the
groundwater supply. But eventually contaminants will disperse through part or all of an aquifer.
Wells affect groundwater flow by taking water out of an aquifer and lowering the nearby water table.
Removed water is recharged from the water table, and the lowered water table caused by the well is
called a cone of depression. The cone of depression from a well may extend to nearby lakes and
streams, causing the stream to lose water to the aquifer. This is known as induced recharge.
Streams and wetlands have been completely