Geology 100
Lecture 15 - Groundwater
I. Groundwater:
Groundwater is water contained in sediments, fractures, and caverns underground.
By far, the bulk of groundwater is contained in pores between sediment grains and should be thought of as behaving
like water in a sponge. Underground streams and pools in caverns ("underground lakes") are very rare indeed.
Groundwater flows very slowly between grains of sediment. Flow is slow enough that it is laminar;
turbulent flow occurs only in fractured bedrock or underground caverns.
Typical rates of groundwater flow are 4 cm/day (15 m/year).
The main controls on the occurrence and flow rates of groundwater are the porosity and permeability of the
groundwater reservoir.
Porosity is the percentage of void space in a reservoir that may be filled with water.
Permeability is a measure of the speed at which water can flow through pores in a rock and thus reflects the degree
to which pores are interconnected.
Different rock types have different porosity and permeability and therefore the ability of rocks and sediments to act
as good reservoirs of water is variable. Well sorted sand or sandstone is highly porous and permeable and usually
makes a good reservoir; on the contrary, mud or shale is highly porous, but very impermeable and thus does not form
a good reservoir but actually tends to form a barrier for underground water.
II. Aquifers, aquitards, and the water table:
An aquifer is a body of rock or sediment which contains economically significant quantities of groundwater
(e.g. the Missoula Aquifer). Aquifers may form in response to a variety of geologic situations and may be found in a
variety of different settings, but usually their depth is usually limited to several km. Below this depth, pressures are
high enough to seal pores, eliminating space for water.
Water in aquifers accumulates in and fills the pores above this impermeable barrier. The zone where pores are filled
with water is called the zone of saturation. The top of the zone of saturation is called the water table; above the
water table, pores of rocks and sediment are only lined with water and are mostly filled with air. This is called the
zone of aeration. Wells dug below the water table into the zone of saturation will fill with water up to the level of the
water table.
An aquifer may be confined if its upper surface is not the water table but rather an impermeable layer of rock, such
as shale. If the upper surface of an aquifer is the water table itself, then the aquifer is said to be unconfined. The
Missoula Aquifer is unconfined.
An aquitard is a layer of impermeable rock, such as shale, that blocks groundwater flow.
III. Movement of groundwater:
Groundwater is supplied from surface runoff called recharge. (The aquifer is "recharged" with more water.)
Groundwater moves from areas of high pressure towards areas of lower pressure. This may not always be
downhill. In general, the direction of groundwater movement is more or less parallel to the surface of the land in
unconfined aquifers. In confined aquifers, the movement is controlled by the pressure directions within the aquifer.
For example, if a confined aquifer is being recharged by rain runoff up slope, then the water in the aquifer at any
one point is under pressure from the weight of all the water within the aquifer upslope from that point.
The amount of this pressure is related to the vertical height of water above the point in the aquifer and is called
hydraulic pressure.
If a well is drilled into a confined aquifer under hydraulic pressure, water will naturally flow up the well. This is called
an artesian well and the water is artesian water. (Note that it is no different than any other water except that it
comes from a confined aquifer under pressure and naturally flows up the well. It is not more pure, better tasting, etc.)
Groundwater can flow into or out of surface streams. A stream which receives groundwater is an effluent (gaining)
stream; a stream which loses water to the groundwater system is called an influent (losing) stream. The Clark
Fork is both effluent and influent, depending on which stretch of the river is being considered and what time of the
year it is.
A spring is formed when groundwater flows out onto the surface. There numerous circumstances that can lead to the
formation of a spring. For example, if a layer of permeable, porous rock overlies impermeable rock and both layers
outcrop at the surface, then a spring will often form at the boundary between rock layers. This occurs because
groundwater can not flow through the impermeable layer and will only flow along the top of the impermeable layer
until it reaches the surface where it will flow out as a spring.
Sometimes, an impermeable layer of rock with porous, permeable rock above it will be situated above the regional
water table, but the porous rock will be filled with water (i.e. is an aquifer) since downward-flowing recharge water
can't flow past the shale aquitard. This is called a perched aquifer. Some geologists think the Missoula valley aquifer
is perched above an impermeable shale layer.
IV. Use of groundwater:
The success of a well is dependent upon the availability of a good aquifer system; the well
must penetrate into the aquifer (i.e. wells which are drilled into the zone of aeration only will be dry.)
In wells which do not draw much water or draw water very slowly (e.g. a hand-drawn village well), the surface of the
water table is not much affected. If water is drawn very rapidly from a well, however, the surface of the water table
may be drawn down to form a cone of depression, and ultimately, the level of the entire water table may be lowered,
leaving the well high and dry.
Overpumping of aquifer systems, even huge aquifer systems like the Ogallala Aquifer (see your book) may result in
eventual depletion of the aquifer that would take thousands of years to recharge. Thus, groundwater may be regarded
as a nonrenewable resource.
In areas next to the coast, fresh water aquifers overlie a "wedge" of salt water because fresh water is lighter than salt
water. If wells within the fresh water aquifer pump too much water too fast, a cone of depression will be formed and
salt water will eventually be drawn up into the well, rendering it useless. This is called saltwater intrusion.
V. Groundwater pollution:
Chemicals ad other pollutants that are spilled onto the ground act like rainwater and
eventually seep into the aquifer, polluting it.
Point source pollution occurs if the pollutants originate from a single site,
such as a chemical factory or gas station underground tank.
Point source pollution: a small collection of buried drums or a single underground tank
Non-point source pollution is caused by pollution over
a broad area, such as application of herbicides on agricultural fields and eventual seeping of the herbicide into the
groundwater. In some cases, if pollution is slow enough and enough time is available, micro-organisms in the soil will
destroy pollutants. Usually, however, the rate of pollution is so high that the ability of the natural system to destroy
pollutants is quickly overwhelmed, and the groundwater system becomes polluted. This is especially the case recently,
because many chemicals and other synthetic pollutants are not digestible (nonbiodegradable) by micro-organisms.
Non-point source pollution: pesticides, herbicides and fertilizers sprayed over crops
VI. Karst topography:
In areas underlain by limestone bedrock, large caverns may be dissolved by slightly acidic
groundwater. (Remember, limestone is mostly made up of calcite and calcite dissolves in acid.) These caverns may
eventually collapse, forming sinkholes and a distinctive form of topography called karst topography.
VII. Hot springs and geysers:
There are several ways that warm or hot water in hot springs may become hot:
1) In some unusual circumstances, groundwater can circulate to larger than normal depths and be heated by the
geothermal gradient, then return to the surface as warm or hot water.
2) In most hot springs as at Yellowstone Park, a groundwater system overlies a shallow magma chamber which heats
the water, giving rise to hot springs;
3) water can also be heated by certain chemical reactions which are exothermic (give off heat) as, for example the
reaction of pyrite with water.
Geysers form due to a constriction in an underground pipe connecting a series of water-filled chambers with the surface.
Between eruptions, the chambers are filled with water that is being heated by the surrounding hot rocks. Some steam is
being generated which pressurizes the chambers. The constriction in the vent of the geyser system does not allow the
pressure to be released for some time, however, so the water continues to heat above the boiling point for water but does
not turn to steam because of the pressure. Eventually, some of the pressure caused by the constriction is released and the
entire geyser chamber system is suddenly able to expand. Because it is already above the boiling temperature for water,
it all turns to steam at once and erupts suddenly out of the geyser vent.
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