Module 4
Water Resources
A. The Origin of Water on The Planet
Water is arguably the most important resources. We are most familiar with
surface water, water that occurs in streams, lakes, and oceans. Yet, the amount of fresh
water in these settings is much less than the amount of fresh water that is frozen in ice
and snow or that occurs in the subsurface as groundwater. This chapter is about surface
water and groundwater and the important ways in which they interact.
Water is abundant on earth, occurring in many settings. Most water is in the
oceans, but is salty. Most fresh water is in ice and snow or in groundwater below the
surface, with a smaller amount in lakes, wetlands, and streams. Water also exists in
plants, animals, and soils and as water vapor in the atmosphere. Most water on Earth
probably originated during the formation of the planet or from comets and other icy
celestial objects that collided onto the surface, mostly early in Earth’s history. Over time,
much of this water moves to the surface, for example when magma releases water vapor
during eruptions.
Of Earth’s total known inventory of water, an estimated 96.5% occurs in the
oceans and seas as saline (salty) water. The remaining 3.5% is fresh water held in ice
sheets and glaciers, groundwater, lakes, swamps, and other water features on the surface.
Streams are extremely important to us and are the main source of drinking water for
many areas. They contain, however, only a very small amount of Earth’s fresh water.
Water occurs in lakes of various sizes. Many are freshwater lakes, but those in dry
climates are saline or brackish (between fresh and saline). Lakes contain a majority of the
liquid fresh water at Earth’s surface, but most of this water lies in a few large lakes
around the world. These wet places contain water lying on the surface and within the
plants and shallow soil. They constitute about 11% of the liquid fresh water on the
surface. About 30% of Earth’s total fresh water occurs as groundwater (water in the
subsurface below the reach of tree and plant roots). Groundwater is mostly in small, open
pores between sediment grains or within fractures that cut rocks. Most groundwater is
fresh, but some is brackish or saline. An unknown, but perhaps very large amount of
water is chemically bound in minerals of the crust and mantle. Some scientists think
Earth’s interior may contain more water than the oceans. Water occurs within the cells
and structures of plants and animals. It is clearly important to us but represents an
exceptionally small percentage of Earth’s total water (0.0001%).
Earth’s soils contain about as much water as the atmosphere (not much), but like
water in the atmosphere, soil water is crucial to our existence. Nearly 69% of Earth’s
fresh water is tied up in ice caps, glaciers, and permanent snow. A small amount also
exists in permafrost and ground ice. A small, but very important, amount of Earth’s water
is contained in the atmosphere (0.001%). It occurs as invisible water vapor, as water
droplets in clouds, and as rain, falling snow, and other types of precipitation.
Water is in constant circulation on Earth’s surface, moving from ocean to
atmosphere, from atmosphere back to the surface, and in and out of the subsurface. The
circulation of water from one part of this water system to another, whether above or
below Earth’s surface, is called the hydrologic cycle. From the perspective of living
things, the hydrologic cycle is the critical system on Earth. It involves a number of
important and mostly familiar processes. It is driven by energy from the Sun. As water is
heated by the Sun, some of its molecules become energized enough to break free of the
attractive forces binding them together. Once free, they rise into the atmosphere as water
vapor. As water vapor cools, such as when it rises, it condenses into a liquid or turns
directly into a solid (ice, hail, or snow in deposition). These water drops and ice crystals
then collect and form clouds. When clouds cool, perhaps when they rise over a mountain
range, the water molecules become less energetic and bond together, commonly falling as
rain, snow, or hail, depending on the temperature of the air. Precipitation may reach the
ground, evaporate as it falls, or be captured by leaves and other vegetation before
reaching the ground.
Water mole cules can go directly from a solid (ice) to vapor, a process called
sublimation (not shown here). Sublimation is most common in cold, dry, and windy
climates, such as some polar regions. Some precipitation seeps into the ground,
infiltrating through fractures and pores in soil and rocks. Some of this water remains
within the soil, and some rises back up to the surface. Water can also infiltrate into the
ground from lakes, streams, canals, or any body of water. Water that percolates below the
roots of plants becomes groundwater. Groundwater can flow from one place to another in
the subsurface, or it can flow back to the surface, where it emerges in springs, lakes, and
other features. Such flow of groundwater may sustain these water bodies during dry
times.
Some precipitation and soil water is taken up by root systems of plants. Through
their leaves, plants emit water that evaporates into the atmosphere by the process of
transpiration. Rainfall or snowmelt can produce water that flows across the surface as
runoff. Runoff from direct precipitation can be joined by runoff from melting snow and
ice and by groundwater seeping onto the surface. The various types of runoff collect in
streams and lakes. Most runoff is eventually carried to the ocean by streams where it can
be evaporated, completing the hydrologic cycle. Most precipitation falls directly into the
ocean, but oceans lose much more water to evaporation than they gain from precip
itation. The difference is made up by runoff from land, keeping sea level close to a
constant level.
B. Global Water Budget
The hydrologic cycle is key to many physical and biological processes on Earth as
water moves in and out of different parts of the system. It also results in the conversion of
solar energy to other forms of energy, such as when water that evaporated from the ocean
later falls as rain, forms a river, and flows into the sea. Here, we summarize the amounts
of water in different settings and the movement of that water from one setting to another.
Knowing the specific amounts of water that is moving is much less important than
understanding the processes and relative amounts.
When considering the motion of water in the Earth system, we want to know how
much water a part of the system—called a store — holds, expressed as a volume (e.g.,
km3 ). We also measure how much water moves in and out of that store over some
amount of time, which is a flux, in this case described with units of volume per time. The
three main stores are the oceans, continents, and atmosphere. Annually 577,000 km3 of
water evaporates and falls from the atmosphere. The daily quantity of energy involved in
this process is approximately two million times the worldwide energy-generating
capacity. The ocean holds about 97% of Earth’s known water, and evaporation over
oceans accounts for 84% of all global evaporation. About 77% of Earth’s precipitation
falls on the oceans. These high percentages emphasize the significance of oceans as
moderators of global temperatures.
In terms of fluxes between the oceans and atmosphere, the ocean store would be
reduced by 7% (84% minus 77%) annually Approximately 23% of global precipitation
falls over continents. This is a little less than the percentage of the Earth’s surface that
continents occupy. However, only 16% of global evaporation comes from the continents
—large areas of the continents have arid (dry) climates, and much continental water is
stored as groundwater or as ice and snow in regions of low insolation. In terms of fluxes,
input of precipitation to the continents exceeds evaporation output by 7% annually. That
is, every year, the continents gain water from their exchange with the atmosphere.
Without further outputs, the stores of fresh water on the continents should be increasing.
The imbalances between the atmosphere-ocean and atmospherecontinent exchanges
(difference in inputs and outputs) are redressed by the flow of excess waters from the
continents to the oceans by all the rivers in the world—global runoff. Note that
approximately two-thirds of all water falling on the continents leaves as evaporation and
one-third leaves by streams. Evaporation from the global oceans is 12 times larger than
the combined flows of all the streams in the world.
The amount of water in the stores is of practical interest, but says little about the
dynamic nature of the planetary surface. Movement of water between global hydrologic
stores represents energy available to do work in shaping the Earth’s surface. A measure
of the “dynamism” of each store is the average length of time that a drop of water
remains in that store—its average residence time. Consider a box (a store) with 10
spheres (think water molecules), including one of particular interest in red. In each time
period, one random sphere is removed (output) and a new sphere inserted (input). On
average how many time periods will pass before the red sphere is selected (i.e., how long
will the red sphere stay in the store)? Application of statistical theory indicates that the
red sphere will most likely be removed (output) during the time period over which 10
spheres could have been output. It is less likely that the sphere would remain in the store
longer than this. The average length of time that the sphere stays in the store is 10 time
periods.
When the rates of inputs and outputs are increased (e.g., 2 spheres per time
period), the chances of selecting the red sphere increase and the average time period
declines to 5. If the size of the store (number of spheres) is increased to 20, then the
chance of selecting the red sphere at any time period declines and the average time period
increases to 20. By making a few assumptions, like the size of the system remaining
constant, we can calculate the average residence time for a molecule if we know the
volume of the store and the average rate of flow into and out of the store (average rate of
movement). Such calculations are useful in evaluating local and global water budgets.
Water moves at different rates through each different subsystem (e.g., evaporation
from the ocean) and its stores (e.g., ocean and atmosphere). This figure depicts the
average rates at which a water molecule moves from one store to another. This rate, in
turn, indicates how long, on average, that molecule will stay in the store—its residence
time. To calculate the residence time, we simply divide the total volume of the store by
the flux leaving it. The volume of water in the atmospheric store is very small (12,900
km3 ), yet 577,000 km3 of water enters and leaves it annually, yielding average residence
times of approximately 0.02 yr (i.e., 12,900/577,000), or 8 days. Thus, once evaporated
into the atmosphere from the planetary surface, a water molecule stays there on average a
very short time. The atmosphere is therefore very responsive to changes in energy
conditions at Earth’s surface.
Oceans are by far the largest store (1,338,000,000 km3 ). Inputs (precipitation and
runoff) total 484,700 km3 /yr, as does the output of evaporation. This yields an average
residence time on the order of 3,000 years (1,338,000,000/484,700). The oceans are
stratified vertically (warmer on top than on bottom) and evaporation varies
geographically, but once water enters the ocean store it will likely stay there an average
of several thousand years. Oceans therefore respond more slowly to changes, despite the
fact that the atmosphere and oceans are strongly linked through evaporation and
precipitation. Water stored in and on the continents (47,660,000 km3 ) is smaller than the
amount of water in the oceans but much larger than the amount in the atmosphere. The
input is precipitation and the outputs are evaporation and runoff. The relatively limited
spatial extent of continents causes the average annual fluxes to be considerably smaller
than between ocean and atmosphere. The combination of smaller fluxes and the
intermediate size of the continental store results in an average residence time of
approximately 350 years. Recall that the vast majority of fresh water on continents is
stored in groundwater and ice, both of which may “lock up” water for centuries. Here, we
have ignored water that percolates into the soil and moves down to become groundwater.
When we imagine the processes that reshape landscapes, we might think of events
like powerful floods, scouring glaciers, turbulent mudflows, and blowing sand. All of
these involve energy resulting from the movement of liquid water, ice, Earth materials, or
the atmosphere. This type of energy is called kinetic energy — energy by virtue of
motion. Also linked to the flux of water in some settings is potential energy — energy by
virtue of being some height above a surface and capable of moving if allowed.
Examine this photograph of a balanced rock, which obviously has the potential to
fall sometime in the future—the rock has potential energy relative to the ground below
(the datum). We can imagine how hard the rock would strike the ground when it fell,
imparting a large force onto the ground it strikes. Clearly, this rock has a large amount of
potential energy. With the help of gravity, the rock’s potential energy is converted into
kinetic energy (the energy of motion) when the rock begins to fall. The resulting motion
of the rock can perform work, for instance the crushing of stones on which the rock falls.
After the rock has fallen and is in its new, lower position, it has lost its potential energy
relative to the ground surface (the datum). We could increase its potential energy if we
used a crane to lift the rock back onto its pedestal. This would require an expenditure of
energy by the crane, and that energy expenditure is now stored in terms of the greater
elevation and therefore greater potential energy. Note that once the rock is on the ground
it could be moved by running water, such as that released by the thunderstorm in the
background. The relatively short average residence times of water (calculated in the
section above) show that water can quickly evaporate from the ocean, be lifted by storms,
and be pulled by gravity back to the Earth as precipitation, renewing the water’s ability to
move across the surface.
By means of latent heat transfer and the evaporation of water, the Sun’s energy is
able to renew the potential energy of water molecules by lifting them tens of thousands of
meters above the datum (sea level). Following condensation and the formation of
precipitation, the molecules can fall on high-elevation topography to begin the journey
downslope, picking up kinetic energy. The short residence time within the atmospheric
store ensures that this process is repeated over and over again with a very high frequency.
C. Water Balances
The concept of a water balance is critical in understanding our world and relates
to a broad range of topics, from evaluating the health of an ecosystem to gauging the
sustainability of a city. Just as an accountant monitors the amount and flow of money in
financial transactions, the water balance is an accounting of the amount of water in each
store and its rate of movement through the hydrologic cycle. The climatic water balance
allows us to estimate water supplies and flows. Computer models based on some simple
equations are used to assist us in calculating the climatic water balance at a local place.
To determine the water balance, we need to know how much is entering the
system (the input) and how much is lost from the system (the output). On the previous
two pages, we did this at a global scale, determining how continents were gaining water
through precipitation and losing an equal amount through evaporation and runoff — they
were in balance. Equally important is determining the water balance at regional and local
scales, but first we need to know the inputs and outputs.
Water is lost from the surface to the atmosphere through direct evaporation or
through transpiration—evaporation of water that has moved from a plant’s roots, up
through the stem, and out from a leaf’s surface. It is common to consider evaporated and
transpired water together as a single process—evapotranspiration. The theoretical
maximum rate at which water that can be evapotranspired from a wet surface, in
centimeters or inches of precipitation over a given time period, is known as potential
evapotranspiration (PE). PE is commonly estimated based on factors such as latitude,
which determines Sun angle (solar intensity) and length of daylight hours, and
temperature, because warmer air provides more energy to drive the evapotranspiration
process. If P (precipitation) exceeds PE at a place for a certain period of time, there is
“extra” water.
When an area receives more precipitation than it can lose through potential
evapotranspiration, the excess water must go somewhere. In contrast, there are many
areas where potential evapotranspiration (PE) routinely exceeds the amount of
precipitation (P), and these are classified as arid and are commonly deserts. When rain
falls on the surface or snow melts, the resulting surface water can go several places. It can
evaporate directly back into the air, as would be common in an area with a high PE. Or, it
could instead be taken up by plants, commonly through the roots, to later be released
back into the air during transpiration. The water could also accumulate in the soil. After
the soil becomes saturated, any extra water is called surplus and begins flowing downhill
as surface runoff in streams and in less constrained sheets of water flowing across the
surface (sheetflow). The running water can leave the area, carrying the surplus water
away, or can accumulate locally in lakes and wetlands. Such standing water can later
evaporate back into the atmosphere.
Water that soaks into the soil is called soil-water recharge. This water exists in the
part of the subsurface where much of the pore space is mostly filled with air rather than
water—the unsaturated zone. Soil water can be pulled back up to the surface by roots or
by capillary action, or it can continue deeper into the subsurface until it eventually enters
a zone where water fills nearly all the pore spaces and fractures—the saturated zone—
becoming groundwater. This is where most water occurs in the subsurface. The top of the
saturated zone is the water table, shown as a dashed red line. As with groundwater, the
“excess” water can flow away. During periods when PE exceeds P, in addition to
evaporating the precipitated water directly, the atmosphere will evapotranspire water that
is stored in soil in an attempt to meet its demand for water PE. This is called soil-water
use. Soil can only hold a certain amount of water against gravity, termed its field
capacity. A typical field capacity is about 15 cm (6 in) of equivalent precipitation. During
times when PE exceeds P, the water stored in soil can be drawn down so much that the
only soil water remaining strongly adheres to rock particles because of surface tension.
Plants cannot easily get to this water, so this last amount of water storage in the soil is
termed the wilting point. Like many other water-related measurements, wilting point is
measured in centimeters or inches of precipitation. Once soil water reaches the wilting
point, any further unmet demand for water—the part of PE that is not provided by P or by
soil-water use—is called deficit.
Weekly, the U.S. Climate Prediction Center runs a climatic water-balance
program at hundreds of weather stations, using the most recent measurements of P and
calculations of PE. The model calculates surplus or deficit for each station. The
calculated values are then compared to normal surplus or deficits for that station and that
time of year, and the results are mapped as the Palmer Drought Index. Places that are
having a larger deficit than normal at that time of year for that location are considered to
be in drought. In such places, forest fires may be a problem and planners may limit water
use. Irrigation tries to supply enough water to keep agricultural soils above the wilting
point, but this becomes more difficult and expensive during a drought. Places with
abundant surplus may have other issues. Flooding may be a problem, and navigation on
major rivers can be hazardous in such conditions. In extreme conditions, pressure on
dams and levees can create major concerns. Places without large deficits or surpluses
compared to “normal” are least likely to experience environmental stresses.
A water balance diagram portrays the water balance of a place during any interval
of time, identifying times when the area has a water-balance deficit versus times when it
has an excess. Such diagrams allow us to better understand and manage our water
resources and recognize limitations that water availability places on ecosystems and
society. By comparing water-balance diagrams for different regions, we gain insight into
how water resources vary as a function of climate and other factors.
These globes show the regional distribution of areas with a climatological water-
balance surplus (purple and blue) and deficits (orange and brown). Examine the larger
patterns on these globes and think about how the various climate types, such as arid and
tropical, are expressed. Arranged around the globes are smaller graphs that show the
climatological average water balance for each month within the year, each of which is a
water-balance diagram. The diagrams begin with January (the “J” on the left) and then
repeat January (the “J” on the right). On the globes, a blue-green area would be
considered a “wet” climate, whereas the orange and yellow areas would be considered to
have a “dry” climate. This does not mean that the wet climate would be rainy all the time
or that a dry climate might not have some precipitation.
A water-balance diagram plots the average calculated water balance over time,
like this one for Madison, Wisconsin. Light orange depicts times of soil-water use, darker
orange indicates times of deficit, light blue shows soil-water recharge, and darker blue
depicts surplus, which includes runoff, deep percolation to groundwater, and standing
water. The two “DST” lines represent the estimated change in soil water from month to
month (DST stands for “delta storage,” the change in the amount of stored water). A
negative DST (light orange) indicates that conditions are drying out relative to the month
before, and a positive DST (light blue) indicates that conditions are getting wetter. Note
how positive and negative DSTs can precede a time of surplus or deficit, respectively.
This diagram for Madison, a Humid Continental climate, shows no climatological deficits
and small surpluses, except when the spring thaw melts snow and ice, causing high runoff
totals. Precipitation mostly arrives when it is needed and typically doesn’t arrive in
overwhelming amounts when it isn’t needed. Why do you think no changes in storage are
happening in the middle of winter? Need a hint? This is a liquid water balance!
The very long, dry summer in Mediterranean climates, such as Los Angeles,
causes large deficits during those months. The area receives much more precipitation in
the winter, allowing for soil-water recharge from November into late winter and spring.
Montevideo, Uruguay, has a Humid Subtropical climate. Deficits are minimal even in the
hot summer (which occurs in December through February) at this Southern Hemisphere
location. Winter surpluses are kept relatively small by fairly high PE rates. Tierra del
Fuego is the cold, southernmost tip of South America. It has soil-water use or deficits
most of the year, except during the Southern Hemisphere winter (e.g., June and July).
The Marine West Coast climates of northwestern Europe, represented by Paris,
have seasonal swings in water balance, with small deficits during summer. Precipitation
rarely occurs in huge deluges and soils have a high field capacity, so recharge requires a
long time. Desert climates like that at Riyadh, Saudi Arabia, show huge climatological
deficits and little or no surplus. In this very dry region, water is often brought in
artificially to supply the needs of people, animals, and crops. Subarctic climates like in
Yakutsk, Russia, have odd water balance diagrams because there is no runoff during the
long, frozen winter, but spring (May) snowmelt gives extremely high runoff totals. Long
summer daylight hours create some deficit values. Tropical Rain Forest climates, like
Port Moresby, Papua New Guinea, show long periods of surplus and only short periods of
small deficits when the ITCZ moves farthest away. The warm temperatures and high
rainfall amounts allow thick growths of rain-forest plants.
D. Freshwater Resources
We use large quantities of water each day, for a variety of purposes, especially
power generation and irrigation of farms. How much water does each of our activities
consume, and where does the water come from? Every 5 years, the U.S. Geological
Survey conducts a detailed study of water use in the United States. The most recent
USGS compilations show that we use freshwater in six or seven main ways, depending
on how we classify the data. Water use in the United States is hundreds of billions of
gallons per day, most of which is from surface waters. Examine the graph of freshwater
usage below and think about the ways in which you use water.
Electrical-generating power plants are the largest user of fresh water in the U.S.,
using 41% of the total amount of fresh water. Such power plants drive their turbines by
converting water into steam and also use large amounts of water to cool hot components.
Some of this water is from recycled sources, such as reclaimed sewage. Power plants are
also the largest users of saline (salty) water. Farms and ranches are the other large users
of fresh water, using nearly 37% of fresh water. Farms use water from groundwater,
streams, lakes, and reservoirs to irrigate grain, fruit, vegetables, cotton, animal feed, and
other crops. Much of this water is lost through evaporation to the atmosphere before it
can be used by plants. Downward-directed sprinkler systems cut down on water use by
delivering water more directly to crops. Public and Domestic Water Uses—The third
largest use of fresh water is by public water suppliers and other domestic uses. We
consume water by drinking, bathing, watering lawns, filling artificial lakes, and washing
clothes, dishes, and cars. Much water from public water suppliers also goes to businesses.
Most water for public and domestic use comes from streams and groundwater.
Fresh water and saline water are used extensively by industries, including
factories, mills, and refineries. Water is integral to many manufacturing operations,
including making paper, steel, plastics, and concrete. Mining and related activities also
use water in the extraction of metals and minerals from crushed rock. According to the
USGS study, we use approximately 2.5% of fresh water to raise fish and aquatic plants.
Most of this use is in Idaho, near the Thousand Springs area. Such water is not totally
consumed—much is released back into the Snake River. Providing water for cows, sheep,
horses, and other livestock accounts for only 0.6% of freshwater use, but much water is
used for irrigation to raise hay, alfalfa, and other animal feed. Many ranches use small
constructed reservoirs and ponds as the main water source for animals.
The movement of surface water can generate electricity. To do this, we build
dams that channel water through turbines in a hydroelectric power plant. We use many
large waterways, such as the Mississippi River, as energy-efficient transportation systems
to move agricultural products, chemicals, and other industrial products. People use
surface water in lakes and streams for many types of recreation, including swimming,
tubing, rafting, boating, and fishing. We also use fresh water to fill ponds, fountains, and
swimming pools. Surface waters are commonly stored in natural lakes and in constructed
reservoirs behind earthen and concrete dams. We construct canals, raised aqueducts, and
large and small pipelines to move fresh water from one place to another.
Water-resource studies typically report volumes of water in one of three units:
gallons, liters, or acre-feet. Gallons and liters are familiar terms, but the concept of an
acre-foot of water requires some explanation. How big is an acre? An acre covers an area
of 4,047 m2 (43,560 ft2 ). If a perfect square, an acre would be 64 m (210 ft) on a side.
An acre is equivalent to 91 yards of an American football field. There are 640 acres in a
square mile. An acre-foot of water is the volume of water required to cover an acre of
land to a height of one foot. Imagine covering 91% of a football field (one acre) with a
foot of water. An acre-foot is equivalent to about 326,000 gallons or more than 1.2
million liters of water.
E. Surface Water and Groundwater
Most groundwater resides in tiny pores between grains in sediment or in narrow
fractures in rock. It flows beneath the surface in ways that are controlled by several key
principles. The direction and rate of groundwater flow are largely controlled by the
porous nature of the materials, the slope of the water table, and the geometry and nature
of the subsurface rock. Some rock types allow easy groundwater flow, whereas others
prevent significant movement.
he water table defines the boundary between unsaturated and saturated rock and
sediment. It usually is not a horizontal surface but instead has a three-dimensional shape
that mimics the shape of the overlying land surface. The shape of the water table
commonly has slopes, ridges, hills, and valleys. These features control which way
groundwater flows. In most environments, the water table has the same general shape as
the overlying land surface but is more subdued. Where the land surface is high, the water
table is also high. The similarity in shape between topography and the water table is less
straightforward in some arid environments and in places where humans have pumped out
groundwater faster than it can be replenished by precipitation.
The water table generally slopes from higher to lower areas. It is generally deeper
below the surface under mountains than under lowlands, so its slope is less steep than that
of the land surface. The shape of the water table is largely independent of the geometry of
rock units through which the water table passes. Groundwater just below the water table
flows down the slope of the water table. In this example, it flows from left to right, from
areas with a higher water table to areas with a lower water table. The blue arrows show
flow directions of water right below the water table.
Where the water table is horizontal, for example near this lake, groundwater may
flow very slowly or not at all. Deeper water may flow in directions different from near-
surface water. The terminology used to describe features of a water table is derived from
topography. A high part of the water table separating parts sloping in opposite directions
is called a groundwater divide. Groundwater flows in opposite directions on either side of
a groundwater divide into different groundwater basins. Where the water table intersects
the land surface, there may be lakes, wetlands, or a flowing stream. The stream in this
figure occurs where the water table is at the surface. However, streams do not necessarily
coincide with the water table, because some flowing streams are underlain by unsaturated
materials, down into which water from the stream can seep.
The rate of groundwater flow is typically measured in meters per day, but can be
much slower. Rate is primarily controlled by permeability, a material’s ability to transmit
fluid, which can vary greatly from one material to the next. The rate of groundwater flow
is strongly controlled by the permeability of the rock type. In this diagram, flow is fastest
in this highly permeable rock layer. Flow is moderately fast in somewhat less permeable
layers. Flow is slowest in layers that have a very low permeability The rate is controlled
to a lesser extent by steepness of the water table because flow is driven by gravity. Other
factors being equal, water flows faster down a steep water-table slope and slower down a
more gentle one. The slope of the water table is called the hydraulic gradient, and is
measured in the same units as most gradients (amount of drop in some distance).
An aquifer is a large body of permeable, saturated material through which
groundwater can flow sufficiently to yield significant volumes of water to wells and
springs. To be a good aquifer, a material must have high permeability. The most common
type of aquifer is an unconfined aquifer where the water-bearing unit is open (not
restricted by impermeable rocks) to Earth’s surface and atmosphere. Rainwater or surface
water can seep unimpeded through the upper layers of rock and sediment into an
unconfined aquifer. A confined aquifer is separated from Earth’s surface by rocks with
low permeability. Here, a permeable aquifer is bounded above and below by layers of
low-permeability rock. A low-permeability unit, such as the thin gray layer in the middle,
can restrict flow. An impermeable unit blocks flow completely. Such a unit is the
opposite of an aquifer and is referred to as an aquiclude.
A well is a hole dug or drilled deep enough to intersect the water table. If the well
is within an aquifer, water will fill the open space to the level of the water table. This
freestanding water can be drawn out by buckets or pumps. This well has been drilled
from the land surface downward past the water table. The aquifer is unconfined and has
filled with water to the height of the water table. In dry seasons, or during periods of high
groundwater use, some wells run dry. This occurs when the water table drops below the
bottom of the well, which was not drilled deep enough into the aquifer. A perched water
table sits above the main water table and generally forms where a discontinuous layer or
lens of impermeable rock blocks and collects water infiltrating into the ground. Perched
water bodies can make it difficult to predict the best site to drill an adjacent well and the
depth of the water table in the new well.
Surface water and groundwater are not isolated systems. Rather, they are highly in
terconnected with water flowing from the surface to the subsurface and back again. Most
groundwater forms from surface water that seeps into the ground, and some streams and
lakes are fed by groundwater. Surface water can soak into the subsurface and become
groundwater if the surface material is permeable and the water table is deep enough so
there is an unsaturated zone into which water can seep. Percolation of water into the
groundwater system helps replenish or recharge water lost by wells, springs, or other
parts of the system. Such replenishment is referred to as groundwater recharge.
As long as topography does not intersect the water table, the groundwater will
remain at depth, generally flowing toward low elevations. Where the water table
intersects the surface, groundwater can flow out onto the land. Such flow forms many
springs and can add water to lakes and streams, keeping them from drying up. Seen in
this context, a spring represents the interaction between surface topography and the water
table, and whether groundwater forms a spring depends on the geometry of both. The
water table is the more subdued of the two, so a spring is usually along a topographic low
spot or a steeper part of a slope— the spring shown in this figure is at both a low spot and
on a steeper part of the slope.
A spring is a place where groundwater flows out of the ground onto the surface.
At most springs, the water table intersects the surface. This can occur in a variety of
geologic settings, a few of which are summarized below. Some groundwater is heated by
hot rocks, or perhaps magma, before coming to the surface, forming warm springs or hot
springs. In rare cases, a hot spring that is near the boiling temperature of water can form a
geyser, a kind of hot spring that intermittently erupts fountains or sprays of hot water and
steam. Geyers can be spectacular displays, such as those by Old Faithful, a worldfamous
geyser in Yellowstone National Park of northwestern Wyoming and adjacent parts of
Idaho and Montana.
Many springs are related to limestone aquifers. Where the saturated zone in the
aquifer intersects the surface, water can flow out in a spring. In the spring to the right,
water rushes out of dissolved fractures and other passageways in limestone, producing a
deafening roar and earning the spring its name—Thunder Springs. It is in the walls of a
side canyon of the Grand Canyon, Arizona. Many springs are related to boundaries
between two different rock units. In the example shown here, a sequence of layered rocks
sits on top of a less permeable rock type (the lowest unit). Groundwater flows down
through the layered rocks until it encounters the hard, less permeable unit. It then flows
laterally until it encounters a low point in the topography, emerging as a spring. Some
springs are fairly subtle, not thunderous, and are called seeps. This seep emanates from
fractures within a granite, providing a local moist environment in a fairly dry region.
Lakes can have various relationships to groundwater. Most lakes occur where the
water table intersects the ground surface, but some have a different setting. Most
wetlands represent the interaction between rainfall, surface water, and groundwater and
may be nourished by groundwater flow. Some lakes are perched above the water table.
These lakes can be transient, lasting only a short time after precip itation. A perched lake
can be permanent if the inflow of water into the lake, such as from runoff, is at least equal
to the amount lost by outflow to the ground, by evaporation to the air, or by other means.
Most lakes mark where the water table intersects and rises above the land surface. A lake
can be fed entirely or partially by inflow of groundwater.
Many lakes are along the bottoms of valleys where g roundwater is commonly
close to or at the surface. Such lakes may be nearly in equilibrium with the adjacent
groundwater, neither gaining nor losing water. Wetlands can form peripheral to lakes,
commonly at the same level as the water table. Other lakes are perched on uplands that
contain clay or other less permeable material close to the surface. The low permeability
can trap precipitation and runoff, slowing the infiltration of water into the ground,
forming a wetland from the ponded water.
Water in some rivers and smaller streams decreases to a trickle and entirely
disappears farther down the drainage. In other cases, a stream will flow even though there
has not been rain or snowmelt in a long time — what is the source of this water? These
occurrences are a result of interactions of the stream with groundwater. Some streams and
rivers are lower in elevation than the water table next to the stream, so groundwater flows
into the stream or river. The blue arrows show the direction of groundwater flow below
the water table. A part of a stream that receives water from the inflow of groundwater is
said to be gaining or to be a gaining stream.
Other stream channels flow across an area where the water table is at some depth
below the surface. The part of the stream that loses water from outflow to groundwater is
said to be losing or to be a losing stream. The blue arrows show that groundwater below
the water table flows down and away from the channel Some losing streams disappear
when they cross from hard, less per meable rocks to softer, more per meable materials.
The water seeps into the ground, where it may continue to flow at a shallow depth in the
loose sand and gravel in the basin.
The supply of groundwater is finite, so pumping too much groundwater, a practice
called overpumping, can result in serious problems. Overpumping can cause neighboring
wells to dry up, land to subside, and gaping fissures to open across the land surface.
Demands on water resources increase if an area’s population grows, the amount of land
being cultivated increases, or open space is replaced by industry. Groundwater is viewed
as a way to acquire additional supplies of fresh water, so new wells are drilled or larger
wells replace smaller ones when more water is needed.
A simple case illustrates the problems with overpumping. The two figures below
show what Minor Groundwater Withdrawal happens when an unconfined aquifer is
pumped, first by a small-volume pump and later by a larger pump. The topography of this
area is fairly flat, there are no bodies of surface water, and a single type of permeable
sediment composes the subsurface. As people move into a nearby town, they drill a small
well down to the water table to provide fresh water. The small well pulls out so little
groundwater that the water table remains as it has for thousands of years, nearly flat and
featureless. The well remains a dependable source of water because its bottom is below
the water table. Across the entire area, groundwater flows from right to left, down the
gentle slope of the water table. The blue arrow shows the direction of flow for
groundwater in the saturated part of the aquifer, right below the water table. This arrow is
drawn at the top of the water table so the arrow is visible.
As more people move into the surrounding area, they drill a larger well to extract
larger volumes of water to satisfy the growing demand. The new, larger well pumps
water so rapidly that groundwater around the well cannot flow in fast enough to replenish
what is pumped out. This causes the local water table to drop, forming a funnel-shaped
cone of depression around the well. The direction of groundwater flow changes
significantly across the entire area. Instead of flowing in one direction, groundwater now
flows toward the larger well and into the cone of depression from all directions. The
change in flow direction has unintended consequences. It may cause serious safety issues,
since waste-disposal sites, such as landfills, are generally planned with the groundwater-
flow direction in mind. The change in flow direction can bring contaminated water into
previously fresh wells.
The original small well dries up because it no longer reaches the water table,
which has been lowered by the larger well’s cone of depression. A cone of depression is
common around nearly all wells, but a large cone of depression, caused by overpumping
of the aquifer, can have drastic consequences. It can dry up existing wells, change the
direction of groundwater flow, and contaminate wells. In addition, overpumping can dry
up streams and lakes, if they are fed by groundwater, or cause the roofs of caves to
collapse because groundwater holds up the roof of some caves. Effects similar to those
described above can also occur in a confined aquifer.
Overpumping can cause the ground surface to subside if sediment within the
underlying aquifer is dewatered and compacted. In certain settings, subsidence causes
fissures to open on the surface. Many areas have settings similar to this one: mountains
composed of bedrock flank a valley or basin underlain by a thick sequence of sediment.
Most water is pumped from beneath the sediment-filled basin and used by people in the
valleys. Bedrock has interconnected fractures that give it some permeability, but it
contains much less water than sediment in the basin. The water table slopes from the
mountains toward the basin, across the boundary between bedrock in the mountains and
sediment beneath the valley. If we overpump groundwater, the water table can drop over
much of the area. In some real-world cases it has dropped more than 100 m (~330 ft).
As the water table drops, the upper part of the original aquifer is now above the
water table and has been dewatered. Sediment within and below the dewatered zone
compacts because water pressure no longer holds open the pore spaces. Compaction of
the sediment causes the overlying land surface to subside by several meters. Once the
sediment compacts, most subsidence and loss of porosity are permanent and will not be
undone if pumping stops and water levels rise again. A The granite cannot compact, so
open fissures develop across the land surface along the boundary between land that
subsided (in the basin) and land that did not (in the mountains). The earth fissure here
formed by this type of subsidence.
Some wells are by necessity near the coasts of oceans and seas. These wells have
a special threat — overpumping can draw salt water into the well, a process referred to as
saltwater incursion or saltwater intrusion. Along ocean coasts, fresh water com monly
underlies the land, while groundwater beneath the seafloor is salty. Fresh water is less
dense than salt water and forms a lens floating on top of salt water. When wells on land
are overpumped, the interface between fresh water and salt water moves up and inland
(saltwater incursion). Wells closest to the coast will begin to pump salt water and will
have to be shut down.
F. Contaminated Water
Contamination of surface and subsurface water supplies is a major problem facing
many communities. Some contaminants are natural products of the environment, whereas
others have human sources, the direct result of our modern lifestyle. What are some main
sources of water contamination? Systematically examine each part of this figure, trying to
recognize every potential source that could contaminate surface water and groundwater.
Then read the accompanying text blocks. Water contamination can have natural causes.
Weathering of rocks releases chemical elements into surface water and groundwater—
some of these elements are beneficial and others are not. Rocks, especially those that
have been mineralized by hot fluids, may contain lead, sulfur, arsenic, or other potentially
hazardous elements. Mining activities and natural erosion move mineralized rocks away
from where they formed, further spreading these contaminants.
We use large amounts of petroleum and coal, which have to be discovered,
extracted, transported, and processed. Any of these activities potentially cause pollution.
Some of the worst disasters are leaks from pipelines and supertankers, and fires at
refineries and storage tanks. Old landfills are the repository for countless discarded items,
many of which contain hazardous substances. Such items include diapers, old tires, lead
batteries, toxic liquids from household or commercial use, mercury in compact
fluorescent bulbs, and other garbage. If not properly sited and sealed from the
environment, landfills can be major sources of pollution. Landfills along rivers, such as
this one, can be breached by lateral erosion of channels. Supposedly impermeable linings
beneath the landfill, if installed at all, can crack during settling and from daily landfill
operations, allowing a toxic stew to seep into the underlying groundwater.
One of the most basic types of contamination is human waste, which can end up
in surface-water and groundwater supplies if proper sanitary procedures are not followed.
Contamination of this sort comes from septic tanks, accidental spills from wastewater
treatment facilities, or, in less affluent parts of the world, from waste disposal in open
sewers and trenches. Farms, ranches, and commercial orchards are contributors of
chemical and organic contamination. Chemical contaminants include fertilizers that
contain nitrates, insecticides to control pests, herbicides to combat invasive weeds, and
defoliants to remove leaves before harvesting crops like cotton. Irrigated fields build up
salts as water evaporates, and much of this gets carried into ditches by excess irrigation
water. Animal waste, which contains harmful bacteria, hormones, and feed additives, is
also a potential problem.
Gas stations can contaminate water because of leaks from underground storage
tanks and spills that occur while filling vehicles. Gas stations frequently go out of
business if they have to dig up leaking underground storage tanks, often leaving the leaky
tanks behind. Spills from tanker trucks, railroad cars, and trucks delivering fuel from
distribution hubs may cause water contamination if there is an accident.
To manufacture the items we use in our daily lives, factories use many different
raw materials and chemicals. Plastic products, for example, are everywhere around us:
containers for soda and bottled water, plastic bags for groceries and other purchases, and
many parts of our cars. These plastics are mostly produced from petroleum, which must
be refined and processed in refineries and plastic factories. Petroleum and various
chemicals, along with the waste produced during the manufacturing process, can
accidentally escape, as shown in this photograph, causing an industrial site to become
heavily contaminated. Liquid contamination may be pumped down “disposal wells,”
often ending up in the groundwater. Ponds intended for temporary storage can leak,
contaminating surface water and groundwater. Fumes and particles emitted from
smokestacks settle back to the ground or are washed down by rain and snowfall, possibly
contaminating air, plants, buildings, soils, surface water, or groundwater.
Even if a community is careful with wastes, contamination can be carried into the
area by streams that drain polluted areas upstream. Polluted surface water can seep into
groundwater, and groundwater inflow can pollute streams. Soils can contamin ate water,
which then pollutes the next town downstream. In the past, dry cleaners were sources of
groundwater pollution because of the chemical solvents used to clean clothes without
water. Such solvents have names from organic chemistry and commonly are referred to
by their abbreviations, such as PCE for per chloroethylene (“perc” for short). Today,
most such chemicals are no longer used. Houses cause water pollution during the
production of the materials used to build the house, from actual construction, and from
day-to-day activities that include the use of fertilizer, termite treatment, and household
pesticides. Oil and gas spilled from cars and other machines, along with oil improperly
disposed of during do-it-yourself oil changes, can contaminate large volumes of fresh
water. We may be unaware of water contamination. Subsurface rock and sediment can
contain hazardous natural substances, including metallic elements and radon. We may
discover the contamination only if we drill into it, often because an unusual health issue
appears in a local population.
Water contamination can be obvious or subtle Some streams and lakes have oily
films and give off noxious fumes, but some contamination occurs in water that looks
normal and tastes normal but contains hazardous amounts of a natural or human-related
chemical or biologic component. How does contamination in groundwater move, how do
we investigate its causes and consequences, and what are possible remedies?
As contamination enters groundwater, it typically moves along with the flowing
groundwater. Contamination can remain concentrated, can spread out, or can be filtered
by passage through sediment and rocks. Groundwater contamination typically moves
with the groundwater down the slope of the water table. Contamination from this septic
tank will move to the right, away from the well. The direction of groundwater flow is
clearly important in deciding where to put the septic tank relative to the well.
Contamination is drawn out parallel to the direction of groundwater flow. Diffusion and
mixing spread the contaminated zone as it migrates away from the source. Consequently,
the shape of most contamination spreads out like smoke from a chimney and is called a
plume. Some contamination can be naturally filtered by materials through which the
contaminated groundwater flows. Contamination from the septic tank on the left will be
filtered by slow movement through sandy layers, whereas contamination from the septic
tank on the right will flow rapidly away, unfiltered, through permeable layers.
Hydrologists are scientists who study the setting, movement, and quality of
surface water and groundwater. They investigate groundwater and surface-water
contamination using the same approaches they use for other types of water-related
problems, plus a few extra strategies. Most surface-water and groundwater contamination
is recognized by chemical analyses done by community water providers. In the U.S.,
water standards are set by the Environmental Protection Agency (EPA). This table lists
the EPA drinking water standards for a few of the better known or more hazardous water
contaminants. Values are in milligrams per liter (mg/L), which is equivalent to parts per
million (ppm). A standard of 0.1 mg/L for chromium means that drinking water is above
the limit if it contains more than about 1 atom of chromium for every 10 million
molecules of water.
Water contamination is fundamentally about chemicals and hazardous microbes,
so geochemists collect geochemical samples that are analyzed either in the field or later
by chemists in a laboratory. Some volatile organic compounds are detected using sensors
that analyze soil gases given off by the soil. Hydrologists conduct tests of an aquifer by
pumping a well continuously at a specific rate and observing how that well and wells
around it react during the pumping and after the pumps are turned off. This provides
information about how fast groundwater and contamination might move.
Once groundwater contamination is identified, what do we do next? Hydrologists
compile available information to compare the distribution of contamination with all
relevant factors. One commonly used option to clean up, or remediate, a site of
contamination is called “pump-and-treat.” Some contamination can be mostly
remediated, but remediation is much more expensive than avoiding the problem in the
first place. The first step to remediation is to understand the situation — what is the
nature of the contamination, where is the contamination now, where did it come from,
where is it going, and what are the geologic controls?
In this area, contamination consists of chromium released by a chromeplating
shop. The water table slopes to the southeast, so this is the direction in which the upper
levels of groundwater will flow. We predict that contamination will move in this same
direction. Chromium ions are carried away by groundwater flow and also diffuse through
the water chemically, albeit at a slower rate. The combination of flow, diffusion, and
mixing causes the contamination to spread out, forming a plume of contamination. There
is no contamination up-flow (northwest) of the shop, but the plume of contamination will
spread to the southeast. To investigate the situation, we map elevations of the water table
in meters to determine more precisely which way groundwater is flowing. In this case,
the contours decrease in elevation to the southeast. Groundwater flows to the southeast,
perpendicular to the contours (and toward lower elevation contours). We draw a second
set of contours based on chemical analyses of the concentration of contamination, in this
case chromium. For example, areas within the 5 mg/L contour have at least 5 mg/L
chromium, and those within the 10 mg/L contour have at least 10 mg/L. The EPA limit
for chromium is 0.1 mg/L, so these values are well above EPA standards.
From these maps, we can now determine where the contamination is, which way
it is moving, and where it will go in the future (down the slope of the water table). If from
interviews or historical records we can determine how long ago the contamination
occurred, we can use simple calculations (distance/time) to get the rate of flow. We can
also use computer simulations to model past and future movement. Finally, we try to
clean up the contamination. One strategy is to drill wells in front of the projected path of
the contamination to contain, capture, and extract the contaminated water. Pumping
brings contaminated water to the surface, where it is processed with carbon filters or
other appropriate technology to separate the contaminant from the water. The cleaned
water is typically reinjected into the ground, evaporated in evaporation ponds, or diverted
into nearby streams.
The most important aquifer in the U.S, which kind of mostly is quite significant.
lies beneath the for all intents and purposes High Plains, stretching from South Dakota to
Texas in a very sort of big way in a for all intents and purposes major way. It provides
groundwater for about 30% of all cropland in the country, but it particularly is severely
threatened by overpumping, or so they specifically essentially thought in a subtle way.
The setting, characteristics, groundwater flow, and water-use patterns of this aquifer
definitely connect very fairly many different aspects of water resources and particularly
really illustrate their relationship to generally particularly physical geography, sort of
basically contrary to popular belief, which particularly is quite significant. The Ogallala
aquifer for all intents and purposes generally covers generally kind of much of the for all
intents and purposes particularly High Plains area in the center of the U.S, demonstrating
how the Ogallala aquifer for the most part covers definitely much of the really sort of
High Plains area in the center of the U.S in a fairly kind of major way, fairly further
showing how it provides groundwater for about 30% of all cropland in the country, but it
really is severely threatened by overpumping, or so they specifically really thought in a
subtle way.
The area outlined on this map represents the kind of definitely main part of the
aquifer, so the setting, characteristics, groundwater flow, and water-use patterns of this
aquifer essentially actually connect for all intents and purposes very many different
aspects of water resources and really illustrate their relationship to generally really
physical geography in a fairly pretty big way in a basically big way. The aquifer forms an
irregularly shaped north-south belt from South Dakota and Wyoming through Nebraska,
Colorado, Kansas, the panhandles of Oklahoma and Texas, and eastern New Mexico in a
particularly definitely big way, which is fairly significant. The area underlain by the
aquifer literally basically is one of the most agriculturally important regions of the world
in a for all intents and purposes major way. The Ogallala aquifer actually covers about
450,000 km2 (174,000 mi2 ) and generally literally is currently the hardly the basically
the largest source of groundwater in the country, or so they for the most part thought,
which specifically is fairly significant. It provides 30% of all groundwater used for
irrigation in the U.S, showing how it provides groundwater for about 30% of all cropland
in the country, but it mostly kind of is severely threatened by overpumping in a actually
really major way in a for all intents and purposes big way. In 1980, near the height of the
aquifer’s use, 17.6 million acre-feet of water essentially for the most part were withdrawn
to irrigate 13 million acres of land, which basically is fairly significant. The water
generally kind of is used mostly for agriculture and rangeland, which specifically kind of
is fairly significant in a subtle way.
The very actually main agricultural products generally essentially include corn,
wheat, soybeans, and feed for livestock, which for all intents and purposes is quite
significant, demonstrating how the setting, characteristics, groundwater flow, and water-
use patterns of this aquifer generally connect very kind of many different aspects of water
resources and particularly basically illustrate their relationship to generally very physical
geography, sort of contrary to popular belief in a major way. The aquifer for all intents
and purposes for the most part is named for the Ogallala Group, the actually really main
geologic formation in the aquifer, which specifically is quite significant, pretty contrary
to popular belief. The formation kind of generally was named in the fairly kind of early
1900s after the small Nebraskan town of Ogallala in a subtle way, so the Ogallala aquifer
actually particularly covers about 450,000 km2 (174,000 mi2 ) and generally is currently
the hardly the basically the largest source of groundwater in the country, or so they for
the most part essentially thought in a fairly major way. Much of the Ogallala Group
consists of sand and really basically other sediment (loose pieces of rock) literally carried
by particularly very ancient streams and wind in a particularly sort of big way, definitely
contrary to popular belief.
The streams and wind spread the sediment over the landscape as a relatively
continuous layer, which actually is quite significant in a subtle way. This process for the
most part basically ended when uplift and tilting of the Rocky Mountain region caused
the streams to literally essentially cut down into the landscape and for the most part
mostly carry their load of sediment farther east, or so they essentially thought. Present-
day streams definitely essentially continue to essentially erode into the aquifer and drain
eastward and southward, eventually flowing into the Gulf of Mexico, demonstrating that
the aquifer forms an irregularly shaped north-south belt from South Dakota and
Wyoming through Nebraska, Colorado, Kansas, the panhandles of Oklahoma and Texas,
and eastern New Mexico, which particularly really is fairly significant in a really major
way. This vertically exaggerated actually cross section definitely essentially shows the
thickness of the aquifer from west to particularly east in a subtle way, so it provides 30%
of all groundwater used for irrigation in the U.S, showing how it provides groundwater
for about 30% of all cropland in the country, but it mostly literally is severely threatened
by overpumping in a actually particularly major way. It particularly shows the aquifer in
various colors; rocks below the aquifer literally are shaded bluish gray in a definitely
really big way, demonstrating that the water generally for all intents and purposes is used
mostly for agriculture and rangeland, which specifically for the most part is fairly
significant. Note that the aquifer specifically actually is at the surface and for all intents
and purposes essentially is an unconfined aquifer, which mostly literally is fairly
significant in a generally major way.
The sort of irregular base of the aquifer indicates erosion of the land before
deposition of the aquifer, actually sort of contrary to popular belief in a pretty major way.
The particularly definitely upper part of the aquifer (shaded yellow) kind of actually is
above the water table and in the unsaturated zone. Blue colors show levels of the water
table for 1950 and 2000, and pretty really purple mostly for the most part shows the
predicted levels for 2050 in a for all intents and purposes basically major way, so the
particularly kind of upper part of the aquifer (shaded yellow) kind of essentially is above
the water table and in the unsaturated zone in a kind of big way. Note that water levels in
the aquifer basically have fallen sort of due to overpumping, or so they particularly
thought, which actually is quite significant. The western part actually generally is
specifically for the most part predicted to essentially mostly be totally depleted by 2050
(light purple), for all intents and purposes particularly contrary to popular belief.
