ES.9 The student will investigate and understand how freshwater resources are influenced by
geologic processes and the activities of humans.
a) processes of soil development
Please see the power points on weathering and mass wastage.
Rocks are produced generally at high temperatures and pressures; when exposed at the surface of
the earth, they must reach a new equilibrium under these conditions. The processes that change
rocks into soil at the earth’s surface are collectively known as weathering. There are two major
categories of weathering: (1) physical (mechanical) weathering processes break rock down into
smaller particles thus increasing the surface area and promoting (2) chemical weathering
(decomposition) which change minerals into those stable at surface temperatures and pressures.
The impact of weathering processes can be summarized by the general equation for rock
weathering:
Rock + Air + Water + Heat → quartz + clay + rock fragments + rust + dissolved salts
The exact nature of the soil that is produced is more a function of the amount of water and heat
available; i.e. climate. Quartz is a mineral that is fairly stable under surface conditions, so it only
dissolves under extreme heat and moisture such as occurs in tropical rainforests. Clay is an
alteration product of all other common rock forming silicate minerals such as plagioclase,
orthoclase, & the micas. Rust is a product of the oxidation of iron & is what turns our soil red.
Dissolved salts such as calcium, magnesium, sodium, silicon, and potassium contribute to the salts
in the sea; some are selectively removed (calcium & silicon) by organic and chemical processes to
make non-clastic sedimentary rocks.
Soil components can be divided into mineral and organic (rotted twigs, bugs, worms, etc.)
categories. It is instructive for students to spend some time poking around in fresh topsoil to
discover both components of soil. The darker color of topsoil is due to the presence of weathered
organic material called humus. To see the different sized mineral particles that comprise soil do
the following experiment. Remove organic material from soil and dry it thoroughly. Break up any
lumps of soil, and then put it in a tall, narrow container of water. Shake it up and allow the soil
particles to settle to the bottom. To completely clear the water you will need to wait at least 24
hours. Layers of sand silt and clay will be formed from largest on the bottom to smallest on the top.
b) development of karst topography
Please see the power point on groundwater features.
The most important thing to know about karst topography is that it is associated with limestone
bedrock and caves, and is present in the Great Valley of Virginia. Limestone (calcite) is soluble in
weak acid such as forms from rainwater and carbon dioxide. When this becomes groundwater, it
will enlarge crack systems in limestone into caverns. If the roof of a cave is too thin, the roof will
collapse, leaving a depression on the surface of the earth called a sinkhole, which may or may not
fill with groundwater. Sometimes streams may disappear into a sinkhole and reappear elsewhere.
These landforms are particularly susceptible to pollution as they are very permeable. Cave
formations such as stalactites and stalagmites form when calcite precipitates in a cave from
dripping groundwater.
c) identification of groundwater zones including the water table, zone of saturation, and zone of
aeration.
Please see the power point on groundwater.
When rainwater (or snowmelt) soaks into the ground, it trickles downward. When it is not actually
raining, pores spaces and cracks near the surface are filled with air, creating the zone of aeration.
Water may be held by electrostatic charges between touching soil particles. This is called soil
(vadose) water and supplies plant roots with water. Further down, the pore spaces are entirely
filled with water. This is the zone of saturation. The surface of the zone of saturation is called the
water table. During a rainstorm the zone of saturation increases, thus raising the water table. In
between rainstorms, the water table is lowered. Water can move from the zone of saturation into
the zone of aeration to replenish soil water through the capillary fringe. The water moves upward
between soil particles much as water creeps up from the edge of a paper towel placed in water.
This movement is called capillary action.
d) identification of other sources of fresh water including rivers, springs, and aquifers, with
reference to the hydrologic cycle
In the hydrologic cycle, water moves continually between natural water reservoirs on Earth’s
surface and the atmosphere. Energy to drive this cycle comes from the sun. Each year the top
one meter of the oceans evaporates into the atmosphere where a molecule of water spends on the
average 2 weeks. It condenses into clouds and falls as precipitation to earth, where any one of
four things can happen. It can be taken up by plant roots and return to the atmosphere in the
process of transpiration. It can soak into the ground to become part of the groundwater system. It
can runoff over the surface of the earth into rivers and then into the ocean. These last two
represent significant sources of freshwater for human consumption. Wherever there is standing
water, the water table is at the surface. All surface water is intimately connected to groundwater.
Springs, marshes, rivers, lakes are notable examples and are thus surface extensions of
groundwater. Water spends an average of 2 – 3 years in rivers and lakes before it evaporates or
flows into the sea. Once underground, water can remain for as much as a thousand years on
average. In deep rock layers, this time can be much longer. Water will spend an average of 3800
years in the ocean.
Underground water is found in permeable rocks (those allowing water to flow through rock) which
are called aquifers. Permeability is affected by grain size, shape and sorting of rocks. Pores must
be large enough to minimize friction and they must be connected. Gravels that are well sorted and
rounded make the best aquifers. Some aquifers are artesian, that is water is under pressure and
rises to the surface without pumping. This situation occurs when a permeable rock is sandwiched
between two impermeable rocks such as shale. The pore spaces in shale are so tiny that friction
prevents water from moving through the rock, so it is impermeable and is called an aquaclude.
e) dependence on freshwater resources and the effects of human usage on water quality
Although Earth has a huge amount of water, 97% of it is salty, and another 2.4% is frozen in
permanent ice caps. These, plus deep groundwater (>0.5 km) are economically useless for human
consumption. This basically means that we are limited to rivers and relatively shallow aquifers for
our needs. Rivers are renewed frequently, but worldwide, aquifers are being over exploited, largely
for irrigation.
When a well is pumped, the water table in the vicinity of the well is lowered in a cone of
depression. Think of drinking a slushy through a straw. The ice around the straw is emptied first.
If numerous wells are drilled in the same area, these cones of depression can overlap, resulting in
the depletion of the resource. Contamination of groundwater can result from anything placed in or
on the soil layer (landfills, improperly maintained septic systems, dumping oil on the ground,
leaking gas tanks, fertilizer, etc.). Once contaminated, it is a very expensive and time consuming
proposition to clean up groundwater. Pumping can draw pollutants into the family well. Since
groundwater and surface water are connected, contamination of one can lead to contamination of
the other. Historically, rivers have been convenient waste disposal systems. As development
increases, rivers become contaminated throughout their length with pollution increasing
downstream. Permanent diversion of water for irrigation or other uses diminishes a rivers ability to
dilute waste products. Please see ES.7 for discussion of water as a non-renewable resource.
f) identification of major watershed systems in Virginia, including the Chesapeake Bay and its
tributaries
A watershed (a.k.a. drainage basin) is all the land that sheds or drains water into a river and its
tributaries. A watershed is surrounded by a divide. If you begin at the mouth of a river and walk to
the highest ground and continue walking, always taking the highest route, never crossing water
and keeping the river and its tributaries on the same side, you will outline the watershed and
eventually return to your starting point, but across the river. Small watersheds make up larger
watersheds.
The Chesapeake Bay is the mouth of the Susquehanna River that has been flooded by rising sea
level. The Chesapeake Bay watershed is made up of the watersheds of many Virginia Rivers,
including the James, York, Rappahannock, and Potomac Rivers.
After reviewing material in your textbook and exploring the websites below, go to the file labeled
ES.9 Review Responses and open it in Word. Type your answers below each question and make
them a distinctive readable color or font. E-mail this file as an attachment word document to the
address provided in the introduction and please keep your answers succinct.
Relevant Internet Resources
http://soil.gsfc.nasa.gov/
Groundwater
http://ga.water.usgs.gov/edu/mearthgw.html
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/grnd.rxml
http://www.epa.gov/safewater/
Chesapeake Bay
http://www.chesapeakebay.net/
http://www.cbf.org/site/PageServer?pagename=homev3
http://chesapeake.usgs.gov/index.html
http://www.epa.gov/region3/chesapeake/