Oceanography in the 21st Century - An Online Textbook
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Groundwater
The Sea Beneath Our Feet

With Contributions From Ethan Grossman and Jennifer McGuire

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:

  1. 50% of all drinking water,
  2. 40% of water used by industry,
  3. 20% of water used for irrigation.
    (Fry, 2005).

circular irrigation pattern
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.

Groundwater

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.

near-surface groundwater zones
Unsaturated and saturated zones below the earth's surface. The two zones are separated by the capillary fringe zone, which is a few centimeters thick.
From Office of Radiation, Chemical & Biological Safety Michigan State University FAQs on Wellhead Protection.

Groundwater Storage
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
Granite 0–5%
Shale < 10%
Sandstone 5–30%
Sand and Gravel (mixed) 22–35%
Sand and Gravel (well sorted) 25–50%
Clay 33–60%

pore space in sandstone
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 Gas Projects.

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:

  1. 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).

    hydraulic head in groundwater
    From University of Leeds Goodquarry web page on The Water Environment.

  2. 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.

  3. Hydraulic conductivity (k), which depends on:
    1. Permeability of the rock or sediment. It depends on:
      1. Size of the pores in the rock, and
      2. Connection between pores. Pores connected by very small, narrow channels have low permeability.
    2. Density of the fluid, and
    3. Viscosity of the fluid.
    4. The dimensions of k are meters per second.
    5. 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.
    6. 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].
From Karst Waters Institute.

karst regions in the US
From Karst Waters Institute, after Davies and others, 1984, U. S. Geological Survey, National Atlas, Engineering Aspects of Karst.

Aquifer Systems

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 Sparta aquifer
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 edwards aquifer texas
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. A larger image is available from the Edwards Aquifer Authority.
From Edwards Aquifer Authority.

Good Aquifers
Good aquifers provide adequate quantities of high quality water.

  1. The quality of the water depends on water chemistry discussed below.
  2. The quantity of water available from an aquifer depends on:
    1. Porosity - determines amount of groundwater storage.
    2. Size of aquifer - thickness and area.
    3. Permeability - affects recharge rate, ability to pump.

Artesian Wells
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 Aquifer

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, 2000):

  1. Efficient deep-well pumps,
  2. Low-cost energy to run gasoline or natural-gas engines,
  3. Inexpensive aluminum piping,
  4. 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.
  5. New management skills, and
  6. 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:

  1. 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 1999.
  2. Irrigated acreage is declining.
  3. Cost of pumping is increasing.

ogallala aquifer decline in water table
Change of depth of water table in the Ogallala Aquifer between 1980 and 1999. From McGuire (2001).

An Example of Land Subsidence: Brownwood Subdivision, Texas
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.

drop in potentiometric level houston land subsidence houston
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.

pool in brownwood subdivision houston house in brownwood subdivision Houston after subsidence
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 Archives.
Both from Houston Wet Scrapbook.

Water Chemistry

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.

Case 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:

  1. Carbon-based matter in soil and aquifer is oxidized, releasing acid.
    Carbon-based matter + oxygen   ->  carbonic acid

    CH2O + O2  ->   H2CO3

  2. The acid dissolves CaCO3 (calcite) in the aquifer.
    Calcite shell + acid   ->  calcium ion + bicarbonate

    CaCO3 + H2CO 3  ->   Ca2+ + 2HCO3 -

  3. 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 +

exchange of sodium and calcium cations in a clay matrix in an aquifer
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

Constituent Concentration (mg/liter)
  College Station Houston San Antonio
Sodium (Na+) 200 38 10
Calcium (Ca2+) 3 21 84
Magnesium (Mg2+) 0.7 2 16
Bicarbonate (HCO3-) 366 68 292
Chloride (Cl-) 57 38 16
Sulfate (SO42-) 7 38 36
Total Dissolved Solids 509 205 454

comparison of dissolved solids in water for three texas cities

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

  1. 16 inorganic, non-carbon compounds such as nitrates, arsenic, fluoride, selenium.
  2. 54 carbon-containing contaminants for which maximum contaminant levels (MCLs) have been established.
  3. The number and levels of contaminants is constantly changing as new problems are discovered. Currently, the EPA has developed MCLs for:
    1. 7 types of bacterial or viral microorganisms;
    2. 7 standards for disinfectants like chlorine or disinfectant products like bromate;
    3. 16 inorganic compounds;
    4. 54 carbon-based compounds, and
    5. 4 radionuclides.
  4. 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.
  5. 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 Perchlorate.

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 Water Quality.

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).

Major Concerns

  1. Depletion of aquifers almost everywhere in the world. Water tables are falling almost everywhere.
  2. Land subsidence.
  3. Contamination of aquifers.

References

General reference material:

  1. Groundwater and Wetlands, an on-line web book.
  2. Circular 1139: Ground Water and Surface Water A Single Resource published by the the US Geological Survey.
  3. Circular 1186: Sustainability of Ground-Water Resources published by the the US Geological Survey

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 Development: 16.

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.

Revised on: 4 August, 2009

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