Transcript Groundwater - Ms. Martel

Chapter 14 Groundwater
Ms. Martel
14.1 Water in the Ground
• Although some rain and precipitation becomes runoff, water
also enters the ground where it is stored as groundwater.
• Factors that affect the amount of water that seeps into the
ground are:
• Type of rock or soil on which the water falls.
• Climate, topography, vegetation, and land use.
Porosity
• The amount of water that soil or rock can hold
depends upon the amount of space, or pore space,
between the grains of the material.
• Porosity is the percent of a material’s volume that is
pore space.
• Factors that affect porosity are:
• Particle shape. Rounded particles have more pore space.
Flat materials or angular, have less pore space.
• Sorting. Materials that are made up of particles that are
the same size are well sorted. They are more porous.
• Materials that are poorly sorted, ie. Are made up of a
mixture of materials fill the porous spaces, reducing the
porosity.
Permeability
• Permeability is the rate at which water or other
liquids pass through the pore spaces of a rock.
• Typically, permeability increases with grain size
because large grained materials have larger pore
spaces.
• A material that water cannot pass through is called
impermeable.
• Some of the water that passes through a sediment
or rock sticks to particles, forming a film of water.
• This film is called capillary water.
Measuring Porosity Mini-Lab
The Water Table
• When rain falls, it enters the pores in the soil, the
water will then continue downward until it reaches
impermeable material.
• As more rain falls, the part of the ground where all
pore spaces are filled is called the zone of
saturation.
• The upper surface of this zone is called the zone of
saturation.
• The zone just above the zone of saturation, is
called the zone of aeration, because air can enter
this region.
• In between these two zones is the capillary fringe.
• The interaction between the water and the soil is
called capillary action.
• The water table’s distance from the surface depends
upon many factors:
• Rainfall, amount of time that passes between rains, the
season, the slope of the ground surface, the thickness of soil,
and the climate.
• The water table is important in several ways:
• Seepage of water from the water from the water table, keeps
the streams flowing between rains and maintains the water
levels of swamps and lakes.
• It also supplies drinking water to springs and human made
wells.
Ordinary Wells and Springs
• Humans can reach groundwater that does not reach
the surface by digging or driving wells into the ground.
This is called an ordinary well.
• On a hillside where the water table meets the surface,
ground water may flow out as a spring.
Artesian Formations
• Permeable layers of rock and sediment that store and
carry groundwater in enough quantity to supply wells
are called aquifers.
• An artesian formation is when an aquifer dips
underground between impermeable rock layers.
• Great quantities of water may enter the aquifers of
artesian formations. This can create a lot of pressure.
• Artesian formations may be broken by cracks in the
cap rock called fissures.
• Artesian springs rise through these cracks and
sometimes form a desert oasis.
Hot Springs, Geysers, and
Fumaroles
• Ground water can become heated if its is at a
great depth.
• Subsurface temperatures increase with depth,
water from deep artesian wells or springs may
be much warmer than normal.
• In many recent regions of recent volcanic
activity, igneous rocks near the surface are hot
enough to boil water.
• Sometimes volcanic gases make hot
groundwater acidic.
• A geyser is a hot spring that
intermittently shoots columns
of hot water and steam into
the air.
• It consists of long, vertical
irregularly shaped tube that
may extend hundreds of
meters into the ground.
• Because it is so far down into
the Earth, water is heated by
surrounding igneous rock etc.
• This creates pressure,
eventually the heat will rise to
the top and steam is finally
able to be released.
• Where fairly recent volcanic eruptions have
occurred, groundwater is released as steam, along
with other gases, from fissures in the ground called
fumaroles.
• There are fumarole fields in Iceland and Japan.
14.2 Conserving Groundwater
• About 50% of drinking water comes from ground
water, humans must conserve and protect
ground water supplies.
Water Budgets
• A water budget describes the income and spending
of water for a region.
• In a water budget, the income is rain or snow.
• The spending includes water lost by use, by runoff,
and by evapotranspiration.
• If it rains during a time when the plants need little
moisture, the extra moisture soaks in the soil, where
it is stored between the grains of soil.
• This is a time of soil recharge.
• If the rain continues so that the soil becomes
saturated the surplus water raises the water table or
becomes part of a stream runoff.
• If the need for moisture is greater than the rainfall, the
plants can draw water from the soil water supply.
• This is a time of soil water usage.
• If the need for moisture continues to be greater than the
rainfall, all of the water available in the soil may be used
up.
• When the water storage is gone, a water deficit
occurs.
Groundwater Conservation
• In many regions, humans are using groundwater
faster than natural processes can replenish supplies.
• In addition, pollution threatens groundwater
supplies.
Overuse of Ground Water
• When ground water supplies are depleted, the water
table drops, thus lowering the water level in wells
and springs.
• This can cause them to go dry.
• In coastal regions, this can cause the groundwater
levels to fall below sea level, causing salt water to
contaminate drinking water. This renders them
unusable.
• Subsidence occurs when the drop in groundwater
causes the group levels to drop.
• This compromises foundations of houses buildings,
and underground pipes.
Groundwater Pollution
• Groundwater is recharged by rain seeping down through
the soil.
• Any pollution agent in the soil becomes part of the
groundwater.
• This includes oil from roads, nitrated from fertilizers,
pesticides from plants, farm wastes, sewage from septic
tanks etc.
• Hazardous wastes are poisonous by products of some
industrial processes.
• Toxic chemicals from accidental spills, careless disposal,
or rotting underground storage containers pollute the
sol around them, and the groundwater its in contact
with.
• For years, toxic wastes were dumped with little care, and their
locations were never recorded.
• In some cases houses were built on these sites.
• When the toxic waste was later discovered, families had to
be moved.
• As yet, no simple or inexpensive way to purify polluted
groundwater is known.
• The best measure we have available to us is prevention of
pollution in the first place.
14.3 Groundwater and Geology
Minerals in Groundwater
• When water evaporates to become water vapor, it
leaves impurities behind.
• As groundwater passes through the lower soil layers
or bedrock, it dissolves minerals.
• Much of this dissolved mineral matter remains in
the groundwater.
• When ground water contains large amounts of
ions from dissolved minerals, it is called hard
water.
• Calcium ions are the most commonly found ions in
hard water.
• The ions of hard water interfere with the use of
water.
• In hot water pipes, groundwater containing
dissolved minerals leaves behind deposits
called boiler scale.
• Artesian water is usually harder than ordinary
groundwater. It usually is able to dissolve more
minerals because of the heat.
• By contrast, ordinary groundwater is almost
always harder than river water.
Mineral Deposits by
Groundwater
• When groundwater that contains dissolved mineral cools
or evaporates, mineral deposits are left behind.
• Hot groundwater often leaves deposits of minerals in
bedrock cracks and fissures.
• Such mineral veins may contain quartz, calcite, gold,
and silver.
• Petrified wood is formed when minerals dissolved in
groundwater replace the decaying wood of buried trees.
• Perhaps the most important groundwater deposit is the
cement that binds together sand grains and pebbles to
form sedimentary rocks.
Mineral Springs
• A spring with a high concentration of mineral matter is called a
mineral spring. The high mineral content of the water may be
due to any of the factors below:
• Water passes through rock containing easily-dissolved
minerals.
• Water contains large quantities of gases, such as CO2 and
hydrogen sulfide, that form acids when mixed with water.
• Water dissolves minerals more easily because its very hot.
Caverns
• Limestone is a common bedrock t hat dissolves more
easily than some other types of rock.
• Limestone is dissolved by the carbonic acid found in
groundwater.
• Frequently, limestone formations are split by fissures that
run down from the surface and by cracks that run
horizontally between the beds.
• After thousands of years, the cracks between beds
become so large that they form networks of underground
tunnels.
• These tunnels are called caverns, or caves.