The Sea Beneath Our Feet
With Contributions From Ethan
Grossman and Jennifer
We learned in the chapter on the Water
Cycle that almost all fresh water available for human use is ground
water. In this chapter we will learn more about the distribution of
groundwater, its chemical properties, and its flow through aquifers.
Groundwater is water that that fills pores in soil and
rock. It is formed when precipitation percolates into the ground. If
groundwater is available in useful quantities, the layer of groundwater
is called an aquifer. Aquifers are the
most important water source for irrigation and and domestic use, especially
in the western US. Overall, they supply about half the household water
used by people in the the United States. Worldwide, groundwater supplies:
- 50% of all drinking water,
- 40% of water used by industry,
- 20% of water used for irrigation.
Finney County in southwestern Kansas is
now irrigated cropland where once there was shortgrass prairie. Common
crops are corn, wheat and sorghum. Green areas in the image are healthy
vegetation. Light colored cultivated fields are fallow or recently
harvested. The image shows center-pivot irrigation systems that are
800 and 1,600 meters in diameter (0.5 and 1 mile). This area utilizes
irrigation water from the Ogallala aquifer, that underlies an area
from Wyoming to Texas. The image was acquired on June 24, 2001, is
centered near 37.4 degrees north latitude, 100.9 degrees west longitude,
and covers an area of 37.2 x 38.8 km.
From U.S./Japan Advanced
Spaceborne Thermal Emission and Reflection Radiometer Science Team.
Click on image for zoom.
Water beneath the land surface occurs in two principal
zones, the unsaturated zone and the saturated zone. In the unsaturated
zone, the voids--that is, the spaces between grains of gravel, sand,
silt, clay, and cracks within rocks--contain both air and water. Although
a considerable amount of water can be present in the unsaturated zone,
this water cannot be pumped by wells because it is held too tightly
by capillary forces.
In contrast to the unsaturated
zone, the voids in the saturated zone are completely filled with
water. Water in the saturated zone is referred to as ground water.
The upper surface of the saturated zone is referred to as the water
table. Below the water table, the
water pressure is great enough to allow water to enter wells, thus
permitting ground water to be withdrawn for use. A well is constructed
by inserting a pipe into a drilled hole; a screen is attached, generally
at its base, to prevent earth materials from entering the pipe along
with the water pumped through the screen.
From US Geological Survey Concepts
of Ground Water, Water Table, and Flow Systems
The unsaturated zone is also called the vadose
zone. It includes the capillary fringe,
the lower part of the unsaturated zone just above the saturated zone,
where capillary action pulls water upward from the saturated zone.
The capillary fringe can be up to 60 cm thick.
The water table is not fixed. The height of the water table
is controlled by water entering the saturated zone by precipitation,
storage in the zone, and the rate that groundwater is pumped from the
ground by wells. The water table often slopes toward streams and lakes,
and it tends to follow the shape of the land surface.
Groundwater often flows into streams and rivers helping
maintain flow in summer and dry periods. It sometimes flows onto the
land at springs. About 30% of river flow comes from springs and groundwater.
Unsaturated and saturated zones below the earth's surface. The two
zones are separated by the capillary fringe zone, which is a few
From Office of Radiation, Chemical & Biological Safety Michigan
State University FAQs
on Wellhead Protection.
Pore Spaces are the voids in rocks, which holds air or water.
The volume of pore space, and the size of the pores depends on rock type.
Igneous and metamorphic rocks tend to have little or no pore space, although
they can have fractures (cracks) that allow water to move through the
rock. Sedimentary rocks have pore space that range in size from a fraction
of a micrometer in clays to several millimeters in coarse sandstone.
Porosity is the term used
to describe the volume of pore space. It is defined to be (volume of
pores) divided by (total volume, including pore volume and solid volume).
Porosity has a value between 0 and 1 or between 0 and 100%. It controls
the volume of water that can be stored in the saturated zone. For more
than you ever want to know about porosity read Predicting
Sandstone Reservoir System Quality and Example of Petrophysical Evaluation by
Dan J. Hartmann (Search
and Discovery Article #4005).
Porosity of Rock Types
|Sand and Gravel (mixed)
|Sand and Gravel (well sorted)
Image of a cross-section through a rock showing pore space and small
connections between larger pores. The small connections limit the
permeability of the rock. Click on image for a zoom.
From National Energy Technology Laboratory web page on Oil & Natural
Groundwater Flow Rates
Groundwater in the saturated zone flows very slowly downhill from where
the water table is high to where it is low. The flow rate depends on:
- Hydraulic Head.
This is the height of the water in a test well relative to a datum
such as sea level. For example, it the water level in a test well is
500 feet above sea level, the hydraulic
head is 500 feet. If the land surface is 600 feet above sea
level, water in the well is 100 feet below the surface. Hydraulic
head can be reported in units of height (meters or feet) or pressure
(Pa or pounds per square inch).
From University of Leeds Goodquarry web page on The
- Hydraulic Gradient or Pressure Gradient.
This is the hydraulic head (or pressure) at one location minus the
hydraulic head (or pressure) at another location divided by the distance
between the locations. It is proportional to the slope of the water
table. Typically the two points used to calculate the flow are at the
recharge zone where water enters the aquifer, and the point where groundwater
leaves the aquifer, such as at a spring or well.
- Hydraulic conductivity (k), which depends
- Permeability of the rock or sediment.
It depends on:
- Size of the pores in the rock, and
- Connection between pores. Pores connected by very small,
narrow channels have low permeability.
- Density of the fluid, and
- Viscosity of the fluid.
- The dimensions of k are meters per second.
- k has a very wide range of values, ranging from 10-1 m/s
for gravels to 10-8 m/s for silt and clay.
- Because density and viscosity do not vary much, the variability
of hydraulic conductivity is due almost entirely to variability
of the permeability of the different rock types.
Sandstone has high porosity and high permeability. Clay has high porosity
and very low permeability. Fractures greatly increase permeability Good
aquifers such as sand, gravel, sandstone, porous limestone, and highly
fractured bedrock have high porosity and permeability. Poor aquifers,
such as shale, mudstone, clay, or unfractured igneous and metamorphic
rock, have low porosity or permeability.
Flow Rate (V) is calculated
from Darcy's Equation (Sometimes called Darcy's Law).
V = (k) (head) / (distance) = k (hydraulic gradient)
where k = hydraulic conductivity.
Flow rates in aquifers tend to be slow. 5–10 feet/year
for clays, 3 feet/day for sorted sand, and up to 1000 feet/day or more
for karsts. The flow rate in our local sand aquifer is roughly 1 m/yr.
Karst is a special type of landscape that
is formed by the dissolution of soluble rocks, including limestone
and dolomite. Karst regions contain aquifers that are capable of
providing large supplies of water. More than 25 percent of the world's
population either lives on or obtains its water from karst aquifers.
In the United States, 20 percent of the land surface is karst and
40 percent of the groundwater used for drinking comes from karst
aquifers. Natural features of the landscape such as caves and springs
are typical of karst regions. Karst landscapes are often spectacularly
scenic areas. Examples include the sinkhole plains and caves of central
Kentucky, the large crystal-clear springs of Florida, and the complex,
beautifully decorated caves of New Mexico [and the Edwards
Aquifer System in Texas].
Waters Institute, after Davies and others, 1984, U. S. Geological
Survey, National Atlas, Engineering Aspects of Karst.
The infiltration of groundwater through the unsaturated
zone (vadose zone) into the saturated zone is called recharge.
The area where water enters the aquifer is called the recharge
zone. As water moves through the aquifer, it is confined by layers
of less permeable rock, such as clays or silts, called aquitards,
which can split the aquifer into separate units. An aquifer
system consists of aquifers and aquitards. An aquifer that is
confined above and below by an aquitard is called a confined
aquifer (see the figure above and
below for examples).
Cross section of the Sparta Aquifer in southern Arkansas, showing Cook
Mountain and Cane River aquitards, and the cone of depression at
the El Dorado municipal well. The recharge zone is on the far left
where the aquifer (yellow) outcrops. The aquifer that supplies College
Station, Texas (see case
study below) is similar to this, with the aquifer being the Simsboro
Sand in the Carrizo-Wilcox formation.
From US Geological Survey web page on Reports
and Fact Sheets on the Sparta.
Cross section of the Edwards Aquifer, Texas. It is the primary source
of water for approximately 1.7 million people. Rain falling in the
drainage area soaks into the limestone of the plateau (a karst region)
forming spring-fed streams. The recharge zone is located in an area
geologically known as the Balcones Fault Zone. In the recharge zone
porous and permeable Edwards Limestone is exposed at the surface
and provides a path for water to reach the artesian zone. Recharge
is water that enters the aquifer through features such as fractures,
sinkholes and caves. The artesian zone is a complex network of interconnecting
spaces varying from microscopic pores to open caverns in the karst.
image is available from the Edwards Aquifer Authority.
Good aquifers provide adequate quantities of high quality water.
- The quality of the water depends on water
chemistry discussed below.
- The quantity of water available from an aquifer depends on:
- Porosity - determines amount of groundwater storage.
- Size of aquifer - thickness and area.
- Permeability - affects recharge rate, ability to pump.
If the hydraulic head (potentiometric surface) is higher than the land
surface, water pressure in a well is so high that water rises to the
surface without the need to pump the water. This is an artesian well.
See the figure for hydraulic head.
Overpumping From Aquifers and Groundwater Mining
Aquifers are such a convenient source of water that it
is easy for cities, irrigation districts, and farmers to pump more water
from the aquifer than recharge can supply when averaged over a period
of many years. This is called groundwater
mining. Almost everywhere in the world pumping exceeds
recharge. Eventually the wells will run dry. But before this happens,
over pumping leads to to reduction of water level in the wells and sometimes
to land subsidence.
Cone of Depression
When water is pumped from an aquifer faster than the water can be reach
the well, the hydraulic head is depressed around the well, recovering
to original values at some distance from the well. This is the depressed
potentiometric surface (surface of constant hydraulic head) drawn with
a blue line in the sparta-aquifer figure above.
If the aquifer is unconstrained, the water table coincides with the
potentiometric surface, and it is pulled down in a cone
of depression centered on the well.
If the cone of depression reaches another well,
it may run dry. If it reaches nearby streams, water may no longer enter
the stream and the stream could dry up.
An Example of Groundwater Mining: The Ogallala
The Ogallala Aquifer (High-Plains Aquifer) is a good example
of groundwater mining. The aquifer is the largest in the US with an area
of 450,000 km2. It occurs in Tertiary and Quaternary sediments,
and the aquifer thickness averages 60 m, and is as great as 180 m. Pumping
from the aquifer increased rapidly after WWII due to (Guru and Horne,
- Efficient deep-well pumps,
- Low-cost energy to run gasoline or natural-gas engines,
- Inexpensive aluminum piping,
- Center-pivot sprinklers and other watering technologies. They produced
the circles in the image of Finney County.
A well and center-pivot sprinkler is in the center of each circle.
- New management skills, and
- An increased scale of operation.
Now, nearly nearly 170,000 wells pump around 2 x 1010 cubic
meters per year (19 million acre feet) to irrigate > 65,000 km2 of
land. As a result:
- Water table has declined. 3 to 15 m decline is common, and in some
regions the decline is greater than 60 m. Parts of the aquifer are
already depleted in Kansas. The average decline was 3.7 m (11.9 feet)
from predevelopment times to 2000 and 1.0 m (3.2 feet) from 1980 to
- Irrigated acreage is declining.
- Cost of pumping is increasing.
Change of depth of water table in the Ogallala Aquifer between 1980
and 1999. From McGuire (2001).
An Example of Land Subsidence: Brownwood Subdivision,
Water pressure in the pores of an aquifer helps support the weight of
the overlying rocks or sediments. As water is withdrawn and the hydraulic
head drops, the rock and sediment is compacted and the land surface subsides.
Excessive pumping of water from beneath Houston, Texas has led to widespread
subsidence and the destruction of the once prosperous Brownwood subdivision.
Left: Drop in the potentiometric surface
of permeable zone B below Houston, Texas due to pumping of groundwater. Right:
Drop in land-surface elevation of Houston, Texas from 1906 to 1980
due to groundwater pumping. Click on images for a zoom.
From US Geological Survey Ground
Water Atlas of the United States Oklahoma, Texas HA 730-E.
About 500 upper-income homes were built in Brownwood, many were for
executives of Humble Oil Company. Pumping of groundwater at a rate exceeding
540 million gallons per day in the Houston Area led to subsidence of
a large area between Baytown and Houston. Brownwood, which was originally
10 feet above sea level, sank as much as 9 feet between 1915 and 1978.
Brownwood subdivision was once the most elite
residential section of Baytown. Perched on a bay fronting the Houston
Ship Channel, it featured the expensive homes of executives and engineers
for the nation's largest petrochemical complex, which had sprouted
nearby. Excessive use of groundwater -- both by industry and the
municipalities that grew around it -- caused the subdivision in the
1960s to begin to sink into the bay, slowly at first. Eventually,
Brownwood was destroyed almost completely by Hurricane Alicia in
1983. Today, the abandoned peninsula has returned to Nature, partly
on its own and partly through the efforts of a marshland restoration
project funded by chemical companies under court order.
From Houston Wet.
Left: Suzanne Brown and children at her
in-law's house, Brownwood subdivision, 1968. Courtesy J.T. and Suzanne
Brown. Right: House in Brownwood subdivision,
outside perimeter road, 1976. Courtesy Brownwood Civic Association
Both from Houston
The quality of well water depends on the total amount and
type of dissolved solids, gases, and contaminants. Dissolved solids depend
on the type of rock in the aquifer, and how long the water was in the
aquifer. Water that contains high amounts of Ca2+ and/or Mg2+ is
called hard water. Soft water may contain Na+ or other minerals.
Water may also have a bad smell due to hydrogen sulfide, it may contain
reactive minerals such as pyrite (FeS2), or it may turn sinks
red due to dissolved iron (Fe2+).
Water flowing through limestone tends to be high in dissolved
calcium bicarbonate (CaHCO3). It is hard, with moderate amounts
of anhydrite (CaSO4). If the limestone has shale aquitards,
the calcium bicarbonate is changed to sodium bicarbonate (NaHCO3),
and the water is soft, with moderate to high total dissolved solids.
Water flowing through cracks in igneous rocks tends to
have low concentrations of total dissolved solids, and it is sometimes
called mountain spring water.
Study: College Station, Texas Drinking Water
Water in College Station, Texas comes from the Carrizo-Wilcox Aquifer,
Simsboro Sand at a depth of 2,500 feet to 3,000 feet. Rainwater enters
the aquifer in northern Robinson County where the sands outcrop. As the
water moves through the aquifer, the groundwater chemistry changes due
to a three-step process:
- Carbon-based matter in soil and aquifer is oxidized, releasing acid.
Carbon-based matter + oxygen -> carbonic acid
CH2O + O2 -> H2CO3
- The acid dissolves CaCO3 (calcite) in the aquifer.
Calcite shell + acid -> calcium ion + bicarbonate
CaCO3 + H2CO 3 ->
Ca2+ + 2HCO3 -
- Calcium cations are exchanged with sodium cations in clays in the
aquifer. The aquifer acts as a natural water softener.
Ca2+ + 2Na+ (on clay) ->
Ca2+ (on clay) + 2Na +
As water moves through an aquifer containing clay particles dissolved
calcium cations are exchanged for sodium cations in the clay, changing
water from hard to soft.
Natural Groundwater Quality
Example: Why Our Water Tastes As It Does
|Total Dissolved Solids
Drinking Water Quality
The Safe Drinking Water Act
(SDWA) [enacted in 1974 and amended in 1986, 1991, and 1996] authorizes
the Environmental Protection Agency to set national health-based
standards for drinking water to protect against both naturally-occurring
and man-made contaminants that may be found in drinking water. US
EPA, states, and water systems then work together to make sure that
these standards are met. ... Originally, SDWA focused primarily on
treatment as the means of providing safe drinking water at the tap.
The 1996 amendments greatly enhanced the existing law by recognizing
source water protection, operator training, funding for water system
improvements, and public information as important components of safe
drinking water. This approach ensures the quality of drinking water
by protecting it from source to tap.
Environmental Protection Agency Basic
Information on Safe Drinking Water Act.
Under changes made in the 1986
Safe Drinking Water Act amendments, the EPA required all public water
systems to monitor for
- 16 inorganic, non-carbon compounds such as
nitrates, arsenic, fluoride, selenium.
- 54 carbon-containing contaminants for which
maximum contaminant levels (MCLs) have been established.
- The number and levels of contaminants is constantly
changing as new problems are discovered. Currently, the EPA has developed
- 7 types of bacterial or viral microorganisms;
- 7 standards for disinfectants like
chlorine or disinfectant products like bromate;
- 16 inorganic compounds;
- 54 carbon-based compounds, and
- 4 radionuclides.
- In addition, the EPA has National Secondary
Drinking Water Regulations in place for 15 compounds or contaminants,
including chloride, sulfate and pH, which can cause cosmetic -- skin
discoloration -- or aesthetic effects, such as taste, odor or color.
- The 1996 Amendments to the Safe Drinking Water
Act require monitoring of other "unregulated" carbon-based
chemicals for which levels have not yet established. Currently, some
35 contaminants are on the list, including Acetochlor, Diazinon and
In Texas, the EPA has delegated authority for regulating
drinking water to the state government. Currently, under Chapter 341
of the Texas Health and Safety Code, the state requires water systems
to test for 126 chemicals, of which 81 have maximum contaminant levels.
In addition, the state requires public water suppliers to test for
16 secondary contaminants which can lead to odor or taste problems.
From Texas Environmental Profiles Drinking
All municipal water agencies must test regularly for all
the contaminants listed above, and to provide report the results to everyone
who uses their water. The goal is to remove contaminants, but not naturally
occurring minerals, many of which are essential for good health, including
many trace elements such as selenium that are toxic in high concentrations.
According to the U.S. National Academy of Sciences
(1977) there have been more than 50 studies, in nine countries, that
have indicated an inverse relationship between water hardness and mortality
from cardiovascular disease. That is, people who drink water that is
deficient in magnesium and calcium generally appear more susceptible
to this disease.
The U.S. National Academy of Sciences has
estimated that a nation-wide initiative to add calcium and magnesium
to soft water might reduce the annual cardiovascular death rate by
150,000 in the United States.
From Foster (1994).
- Depletion of aquifers almost everywhere in the world. Water tables
are falling almost everywhere.
- Land subsidence.
- Contamination of aquifers.
General reference material:
and Wetlands, an on-line web book.
1139: Ground Water and Surface Water A Single Resource published
by the the US Geological Survey.
- Circular 1186: Sustainability
of Ground-Water Resources published by the the US Geological
Foster, Harold D. (1994) Groundwater and Human Health. Groundwater
Resources of British Columbia, Ministry of Environment, Lands, and Parks
and Environment Canada, pp 6.1-6.3.
Fry, A. (2005). Water Facts and Trends, World Business Council for Sustainable
Guru, M. V. and J. E. Horne (2000). The Ogallala Aquifer. Poteau, OK,
The Kerr Center for Sustainable Agriculture: 35.
McGuire, V. L. (2001). Water-Level Change in the High Plains Aquifer,
1980 to 1999, US Geological Survey: 2.
4 August, 2009