11 Water

Entrenched meander of the Colorado River, downstream of Page, Arizona. High cliffs, that lead down to a river with narrow shores.
Entrenched meander of the Colorado River, downstream of Page, Arizona.

11 Water

KEY CONCEPTS

Tall ancient pillars with arched entrances supporting a canal that piped water from the mountains to Roman cities.
Example of a Roman aqueduct in Segovia, Spain.
  • Components and definition of the hydrologic cycle
  • Water users and volume of water used
  • How water is shared among people
  • Distribution of water on the planet
  • Define aquifer and confining layer
  • Properties required for a good aquifer

All life requires water.  The hydrosphere (Earth’s water) is an important agent of geologic change. It shapes our planet through weathering and erosion, deposits minerals that aid in lithification, and alters rocks after they are lithified. Water carried by subducted oceanic plates causes melting in the upper mantle material. Communities rely on suitable water sources for consumption, power generation, crop production, and many other things.

Mayan stone figure with a long elephant-like nose representing a water deity.
Chac mask in Mexico.

In pre-industrial civilizations control of water resources was a symbol of power .  Two thousand year old Roman aqueducts still grace European, Middle Eastern, and North African skylines.  Ancient Mayan kings used water imagery such as frogs, water-lilies, water fowl to show their divine power over their societies water resources .  Mask facades of the hooked-nosed rain god, Chac, are prominent on Mayan buildings such as the Kodz-Poop (Temple of the Masks) at Kabah in the drier northern lowlands of the Yucatan peninsula but much rarer in the tropical, wet regions to the south.  Control over water continues to be an integral part of the governmental duties of most modern societies.  

11.1 Properties of Water

The hydrogen atoms are on one side, about 105° apart.
A model of a water molecule, showing the bonds between the hydrogen and oxygen.
The physical and chemical properties of water are what makes it essential to life and useful to civilization. Water is a molecule made of one negatively charged (-2) oxygen ion and two positively-charged (+1) hydrogen ions, giving it the chemical formula H2O, with strong covalent bonds between the oxygen and two hydrogen ions.  The shape of the water molecule allows for an uneven distribution of charge, where one side is slightly positive and one side is slightly negative. Because of this polarity, water molecules form hydrogen bonds with each other. Hydrogen bonds are electrostatic intermolecular bonds that are weaker than ionic and covalent bonds (see discussion in the Minerals chapter).  Water can self-ionize, breaking down into  an acidic hydrogen ion (H+) and a hydroxyl ion (OH), chemically a base. Because of its polarity and its ability to be amphoteric, water is a universal solventa chemical that can dissolve a wide range of other chemicals.

Surface tension keeps the paperclip on top of the water.
Surface tension keeps the paperclip on top of the water.
Capillary action by water in a narrow tube. Unlike water, mercury does not rise up in the tube.
Capillary action by water in a narrow tube. Unlike water, mercury does not rise up in the tube.
Other side-effects of water’s polarity are cohesion (water likes to stick to itself) and adhesion (water likes to stick to other things). Water has the highest cohesion of all nonmetallic liquids. Cohesion gives water surface tension, allowing water glider insects to float on the water surface. Surface tension is what gives rain drops a spherical shape.  Capillary action occurs when a combination of adhesive and cohesive forces causes water to move up narrow passages and tubes, rising higher than surrounding liquid. Capillary action happens when the adhesion of water to the tube is greater than the water’s internal cohesive forces. Paper towels have small pores that use capillary forces to clean up watery spills.  Plants use capillary forces to pump water into their tissues.

Water has a high specific-heat capacity.  Specific-heat is the amount of heat required to raise the temperature of a substance.  Water requires a relatively large amount of heat to raise its temperature. The high specific heat of water allows it to act as an energy buffer to extreme changes in air temperature.

Molecular arrangement of hydrogen dioxide (water) molecules in ice.
Molecular arrangement of water molecules in ice.
When water freezes, the molecules arrange themselves in a well-ordered crystal structure, creating a spacing between molecules that is greater than if water is in liquid form. The difference in molecular spacing causes ice to be less dense than water, making it more buoyant than liquid water, causing it to float on water.  Because of its high specific-heat capacity, ice floating on a lake’s surface insulates the liquid water and keeps it from freezing.

Because of its hydrogen bonds, water also has a high heat of vaporization.  A significant amount of energy is required to evaporate water.  As water evaporates, energy is absorbed by the breaking of hydrogen bonds and the air around the evaporating water is cooled.

11.2 Water Cycle

The water cycle.
The water cycle.

The water cycle describes how water changes between solid, liquid, and gas (water vapor) phases and changes location. Water can be evaporated, which is the process where a liquid is converted to a gas. Solar energy warms the water sufficiently to excite the water molecules to the point of vaporization. Evaporation occurs from surface water bodies such as oceans, lakes, and streams and the land surface. Plants contribute significant amounts of water vapor as a byproduct of photosynthesis in a process called transpiration. Geologists commonly combine these two sources of water entering the atmosphere in a term called evapotranspiration.

Water vapor in the atmosphere can migrate long distance from ocean to over land by way of prevailing winds. Over the ocean or land, the air can cool and cause the water to condense back into liquid water.  This usually happens in the form of very small water droplets that form around a microscopic piece of dust or salt called condensation nuclei.  These small water droplets are visible as a cloud. Clouds build and once the water droplets are big enough, they fall to earth as precipitation. Precipitation can take the form of rain, snow, hail, or sleet.

Once it has reached the surface it does two important things that are most relevant for geology.  At the surface, liquid water can flow as runoff into streams, lakes, and eventually back to the oceans (in most cases).  Water in streams and lakes is called surface water.  In addition, water can also infiltrate into the soil and finally the pore spaces in the rock or sediment deep underground to become groundwater, the name given to all subsurface water. Groundwater slowly moves through rock and unconsolidated materials and most of it eventually reaches the surface, where it discharges to the surface as springs or into streams, lakes, and the ocean. Also, surface water in streams and lakes can recharge water into groundwater.  Therefore, the surface water and groundwater systems are connected.

Your Score:  

Your Ranking:  

11.3 Water Basins and Budgets

11.3.1 Drainage basins

Map view of a drainage basin with main trunk streams and many tributaries with drainage divide in dashed red line.
Map view of a drainage basin with main trunk streams and many tributaries with drainage divide in dashed red line.

The basic unit of division of the landscape is the drainage basin.  A drainage basin, also known as a catchment or watershed, is the area of land that captures precipitation and contributes runoff to a stream or stream segment (Figure) . Drainage divides are local topographic high points that separate one drainage basin from another . If water falls on one side of the divide, that water goes to one stream, and if it falls on the other side of the divide, then the water goes to a different stream. Each stream has its own drainage basin. Further, a drainage basin for each tributary can also be designated. In areas with flat topography, drainage divides are not as easily identified .

Oblique view of the drainage basin and divide of the Latorita River, Romania.
Oblique view of the drainage basin and divide of the Latorita River, Romania.
Streams only flow downhill and smaller tributary streams combine downhill to make the larger trunk of the stream. Where a stream begins is called the headwaters and where it finally reaches its end is called the mouth. Most streams have the mouth of the stream at the ocean. However, a rare number of streams do not flow to the ocean, but rather end in a closed basin (or endorheic basins) where the water evaporates from a stream or lake before it can reach the ocean. Most streams in the Great Basin are in closed basins. For example, Little Cottonwood Creek and the Jordan River flow into the Great Salt Lake where the water evaporates.

Major drainage basins color coded to match the related ocean. Closed basins (or endorheic basins) are shown in gray.
Major drainage basins color coded to match the related ocean. Closed basins (or endorheic basins) are shown in gray.
In humid climates, many streams are perennial and in arid climates like Utah, many streams are ephemeral. Perennial streams flow all year round.  They occur in humid or temperate climates where there is sufficient rainfall and low evaporation rates.  Water levels rise and fall with the seasons, depending on the discharge. Ephemeral streams flow during rain events or the wet season. These are often dry washes or arroyos for much of the year. They are above the water table and occur in dry climates with low amounts of rainfall and high evaporation rates. They flow mostly during flash floods .

Special topic: Watershed Protection Areas

Along Utah’s Wasatch Front, there are several watersheds that are designated as “Watershed Protections Areas” that limit the type of use allowed in those drainages.  Dogs and swimming are limited in those watersheds because they could contribute harmful bacteria and substances to the drinking supply of Salt Lake City and surrounding municipalities.

Water in an area is very much like currency in a personal budget. There is income in the form of precipitation, stream inflow, and groundwater inflow, and there are expenses, in the form of groundwater withdrawal, evaporation, and stream and groundwater outflow. If the expenses outweigh the income, then the water budget is not balanced, and, if available, water will be removed from storage. Reservoirs, snow and ice, soil moisture, and aquifers can all act as storage in a water budget.

11.3.2 Water Budgets

Scientists can make groundwater budgets for any designated boundary, but they are generally made for watershed (basin) boundaries, because groundwater and surface water are easier to account for within these boundaries. However, water budgets can be created for state, county, or aquifer extent boundaries as well.  The groundwater budget is an essential component to the hydrologic model, where measured data are used with a conceptual workflow of the model to better understand the water system.

Your Score:  

Your Ranking:  

11.4 Water Use and Distribution

Agricultural water use in the United States by state.
Agricultural water use in the United States by state.

In the United States, 355 billion gallons of ground and surface water are withdrawn for use each day, of which 76 billion gallons is fresh groundwater.  The state of California accounts for 16% of national groundwater withdrawals .

Utah is the second driest state in the United States behind its neighbor Nevada, having a mean statewide precipitation of 12.2 inches per year.  Utah also has the second highest per capita rate of total domestic water use  of 167 gallons per day per person.  With the combination of relatively high demand and limited quantity, Utah is at risk for water budget deficits.

Trends in water use by source
Trends in water use by source

11.4.1 Surface Water Distribution

Surface water makes up only 1.2% of the fresh water available on the planet, and 69% of that surface water is trapped in ground ice and permafrost.  Water in rivers accounts for only 0.006% of all freshwater and lakes contain only 0.26% of the world’s fresh water .

Global circulation patterns are the most important factor in precipitation, and thus, the distribution of surface water. In general, due to the Coriolis effect and the uneven heating of the Earth, air rises near the equator and near 60° north and south latitude  and sinks at the poles and 30° north and south latitude. Land masses near rising air are more prone to humid and wet climates, while sinking air inhibits precipitation and creates dry conditions . Prevailing winds, ocean circulation patterns (e.g. the Gulf Stream’s effects on eastern North America, rain shadows (dry leeward sides of mountains), and even the proximity of bodies of water can affect local climate patterns. For example, when cold winds blow across the relatively warm Great Salt Lake, the air warms, which causes it to pick up moisture.  This local increase in moisture content of the air may eventually fall as snow or rain on nearby mountains, a phenomenon known as “lake-effect precipitation” .

Distribution of precipitation in the United States.
Distribution of precipitation in the United States. The 100th Meridian is approximately where the average precipitation transitions from relatively wet to dry. (Source: U.S. Geological Survey)

In the United States, the 100th Meridian roughly marks the boundary between the humid and arid parts of the country, where west of the 100th Meridian, irrigation is required to grow crops . In the West, surface water is stored in reservoirs and mountain snowpacks , then strategically released through a system of canals during times of high use.

Some of the driest parts of the western United States are in the Basin and Range. The Basin and Range has multiple mountain ranges that are oriented north to south.  Most of the basin valleys in the Basin and Range are dry, receiving less than 12 inches of precipitation per year. However, some of the the mountain ranges can receive more than 60 inches of water as snow (snow-water-equivalent). The snow-water equivalent is the amount of water that would result if the snow were melted, as the snowpack is generally much thicker than the equivalent amount of water that it would produce .

11.4.2 Groundwater Distribution

Water source Water volume

(cubic miles)

Fresh water (%) Total water (%)
Oceans, Seas, & Bays 321,000,000 96.5
Ice caps, Glaciers, & Permanent Snow 5,773,000 68.7 1.74
Groundwater 5,614,000 1.69
      — Fresh 2,526,000 30.1   0.76
      — Saline 3,088,000   0.93
Soil Moisture 3,959 0.05 0.001
Ground Ice & Permafrost 71,970 0.86 0.022
Lakes 42,320 0.013
      — Fresh 21,830 0.26 0.007
      — Saline 20,490 0.006
Atmosphere 3,095 0.04 0.001
Swamp Water 2,752 0.03 0.0008
Rivers 509 0.006 0.0002
Biological Water 269 0.003 0.0001
Source: Igor Shiklomanov’s chapter “World fresh water resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Fresh Water Resources (Oxford University Press, New York)

Groundwater makes up 30.1% of the fresh water on the planet, making it the most abundant source of fresh water accessible to most humans.  The majority of freshwater, 68.7%, is stored in glaciers and ice caps as ice .  

Your Score:  

Your Ranking:  

11.5 Water Law

Federal and state governments have put laws in place to ensure the fair and equitable use of water. Based on the distribution of precipitation in the United States, the states are in a position that requires them to create a fair and legal system for sharing water.

11.5.1 Water Rights

Because of the limited supply of water, especially in the western United States, some states have adopted a system of legally dispersing ownership of natural waters.  A claim to a portion or all of a water source, such as a spring, stream, well, or lake, is known as a water right.  Federal law mandates that states control water rights, with the special exception of federally reserved water rights, such as those associated with national parks and Native American tribes, and navigation servitude, which maintains navigable water bodies. Each state in the United States has a different way to disperse and manage water rights.

A person or entity (company, organization, etc.) must have a water right to legally extract or use surface or groundwater in their state.  Water rights in some western states are dictated by the concept of prior appropriation, or “first in time, first in right,” where the person with the oldest water right gets priority water use during times when there is not enough water to fulfill every water right.

The Law of the River and the Colorado River Compact

The Colorado River and its tributaries pass through seven states (Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming), Native American reservations, and Mexico. As the western United States became populated and while California was becoming a key agricultural producer, the states along the Colorado River realized that the river was important to sustaining population and agriculture in the West.  

The states enacted the Colorado River Compact in 1922 to ensure that each state got a fair share of the river water. The Compact granted each state a specific volume of water based on the total measured flow at the time.  However, in 1922, the flow of the river was higher than its long-term average flow, consequently, more water was allocated to each state than is typically available in the river .

Over the next several decades, many other agreements and modifications would follow the Colorado River Compact, including agreements that brought about the Hoover (formerly Boulder) and Glen Canyon Dams, and a treaty between the American and Mexican governments. Combined, the agreements became known as “The Law of the River.”  Despite adjustments to the Compact, many believe that over-allocation is still prevalent, as the Colorado River no longer reaches the Pacific Ocean, its original terminus (base level).  

The Law of the River highlights the complex and prolonged nature of interstate water rights agreements, as well as the importance of water.

Snake Valley

In 1989, the Southern Nevada Water Authority (SNWA) submitted applications for water rights to pipe up to 155,000 acre-feet of water per year from Spring, Snake, Delamar, Dry Lake, and Cave valleys to southern Nevada (mostly Las Vegas) . Unlike the other valleys, Snake Valley straddles the border of Utah and Nevada, where more of the irrigable land area is on the Utah side of the border.  Nevada and Utah attempted a comprehensive agreement, but negotiations have yet to be settled.

 

NPR story on Snake Valley

SNWA History

Dean Baker Story

11.5.2 Quality Protection

Two major federal laws that protect water quality in the United States are the Clean Water Act and the Safe Drinking Water Act.  The Clean Water Act, an amendment of the Federal Water Pollution Control Act, protects navigable waters from dumpage and point-source pollution. The Safe Drinking Water Act ensures that water that is provided by public water suppliers, like cities and towns, is safe to drink

The Superfund program ensures the cleanup of hazardous contamination, and can be applied to situations of surface water and groundwater contamination. It is part of the Comprehensive Environmental Response, Compensation, and Liability Act of 1980. It allows state governments and/or the U.S. Environmental Protection Agency power remediate polluted sites through either actions or funds provided by the pollutor that caused the contamination.

Your Score:  

Your Ranking:  

11.6 Surface Water

A stream or river is a body of flowing surface water confined to a channel. Terms such as creeks and brooks are social terms not used in geology. Streams are the most important agents of erosion and transportation of sediments on the earth’s surface. They create much of the surface topography and are an important water resource. Most of this section will focus on stream location, processes, landforms, and flood hazards . Water resources and groundwater processes will be discussed in later sections.

11.6.1 Discharge

Several factors cause streams to erode and transport sediment, but the two main factors are stream channel gradient and velocity. Stream gradient is the slope of the river channel. A steeper gradient causes the stream to erode downward. When tectonic forces lift up a mountain, the increased stream gradient causes the stream to erode downward and make a valley. Stream velocity is the speed of the flowing water in the channel. Velocity can increase by increasing the gradient, decreasing cross-sectional area (narrowing) of the channel, or by increasing the discharge.

Stream size is measured in terms of discharge – the volume of water flowing past a point in the stream over a defined time interval. Smaller streams have a smaller discharge, therefore generally stream discharge increase downstream. Volume is commonly measured in cubic feet (length x width x depth), shown as feet3 or ft3.  Therefore, the units of discharge are cubic feet per second (ft3/sec or cfs).  Smaller streams have less discharge than larger streams. For example, the Mississippi River is the largest river in North America, with an average flow of about 600,000 cfs .  For comparison, the average discharge for the Jordan River at Utah Lake is about 574 cfs  and the Amazon River annual discharge is about 6,200,000 cfs .   

Discharge can be expressed by the following equation:

Q = V A

  • Q = discharge (ft3/sec),
  • A = cross-sectional area of the stream channel [width times average depth] (ft2),
  • V = average velocity (ft/sec) .

When the channel narrows but discharge remains constant, the same volume of water flows through a narrower space causing the velocity to increase, similar to putting a thumb over the end of a backyard water hose. In addition, during rain storms or heavy snow melt, runoff will increase which increases stream discharge and thus velocity.

Velocity varies within the stream channel as well. Generally when the channel is straight and uniform in depth, the highest velocity is in the center of the channel along the top of the water where it is the farthest from frictional contact with the channel bottom and sides.When the channel curves, the highest velocity will be on the outside of the bend.

Thalweg of a river. In a river bend, the fastest moving particles are on the outside of the bend, near the cutbank.
Thalweg of a river. In a river bend, the fastest moving particles are on the outside of the bend, near the cutbank. Stream velocity is higher on the outside bend and the surface which is farthest from the friction of the stream bed. Longer arrows indicate faster velocity (Earle 2015).

 

 

11.6.2 Runoff vs. Infiltration

There are many factors dictating whether water will infiltrate into the ground or run off over the land after precipitation. These include but are not limited to the amount, type, and intensity of precipitation, the type and amount of vegetative cover, the slope of the land, the temperature and aspect of land, preexisting conditions, and the type of soil in the area of infiltration. High intensity precipitation as rain will cause more runoff than the same amount of rain spread out over a longer duration.  If the rain falls faster than the properties of the soil allow it to infiltrate, then the water that cannot infiltrate will become runoff.  Dense vegetation can increase infiltration, as the vegetative cover slows overland flow of water particles, giving them more time to infiltrate.  If a parcel of land has more direct solar radiation and/or higher seasonal temperatures, there will likely be less infiltration and runoff, as evapotranspiration rates will be higher.  As the slope of the land increases, so does runoff, as the water is more inclined to move downslope than infiltrate into the ground.  Extreme examples would be a basin and a cliff, where water infiltrates much quicker into a basin than a cliff having the same soil properties.  Because saturated soil does not have the capacity to take more water, runoff is generally greater over saturated soil. Clay rich soil cannot accept infiltration as quickly as gravel-rich soil.

11.6.3 Drainage Patterns

The pattern of tributaries within a drainage basin are called drainage patterns. They depend largely on the type of rock beneath, and on structures within that rock (such as folds and faults). The main types of drainage patterns are dendritic, trellis, rectangular, radial, and deranged. Dendritic patterns are the most common and develop in areas where the underlying rock or sediments are uniform in character, mostly flat lying, and can be eroded equally easily in all directions. Examples are alluvial sediments or flat lying sedimentary rocks. Trellis patterns typically develop where sedimentary rocks have been folded or tilted and then eroded to varying degrees depending on their strength. The Appalachian Mountains in eastern United States have many good examples of this. Rectangular patterns develop in areas that have very little topography and a system of bedding planes, joints, or faults that form a rectangular network. A radial pattern forms when streams flow away from a central high point such as a mountain top or volcano, with the individual streams typically having dendritic drainage patterns. In places with extensive limestone deposits, streams can disappear into the groundwater via caves and this creates a deranged pattern .

Various stream drainage patterns.
Various stream drainage patterns.

11.6.4 Fluvial Processes

Fluvial processes are the mechanisms that dictate how a stream functions. This section discusses the main factors controlling fluvial sediment production, transport, and deposition. These processes include velocity, slope and gradient, erosion, transportation, deposition, stream equilibrium, and base level.

Streams can be divided into three main sections: the tributaries in the source area, the main trunk stream in the floodplain and the distributaries at the mouth of the stream. These can be defined as zones of sediment production (erosion), transport, and deposition. The zone of sediment production is located near the headwaters of the stream. Downstream of the headwaters, the stream erodes less sediment but transports sediment easily in the zone of sediment transfer. Lastly, most streams eventually flow into the ocean which is the zone of sediment deposition located at the mouth of a stream . The longitudinal profile of a stream illustrates the location of three zones .

Zone of Sediment Production (Erosion)

The zone of sediment production is located near the headwaters of a stream where rills and gullies erode sediment and contribute to larger tributary streams. These tributaries carry sediment and water further downstream to the main trunk of the stream. Tributaries at the headwaters have the steepest gradient and most sediment production and erosion, especially downward erosion, occurs at the headwaters. Headwater streams tend to be narrow and straight with small or non-existent floodplains adjacent to the channel. Since the zone of sediment production is generally the steepest part of the stream, many headwaters are located near relatively high elevations. For example, the Rocky Mountains are generally the headwaters for the Colorado River which flows from Colorado through Utah, Arizona, Mexico.  

Zone of Sediment Transfer (Transportation)

A stream carries dissolved load, suspended load, and bedload.
A stream carries dissolved load, suspended load, and bedload.
Streams transport sediment great distances from the headwaters to the ocean, the ultimate depositional basins. Sediment transportation is directly related to stream gradient and velocity. Faster and steeper streams can transport larger sediment grains. When velocity slows down, larger sediments settle to the channel bottom. When the velocity increases, those larger sediments are entrained and move again.

Transported sediments are grouped into bedload, suspended load, and dissolved load. Sediments moved along the channel bed are the bedload and typically are the largest and densest. Bedload is moved by saltation (bouncing) and traction (being pushed along by the force of the flow). When stream velocity increases, smaller bedload sediments can be picked up by flowing water and held in suspension as suspended load. The faster streams can carry larger grains as suspended load. Dissolved load in a stream is the sum of the ions in solution from chemical weathering. The dissolved load includes ions such as calcium (Ca+2), chloride (Cl-1), potassium (K+1), and sodium (Na+1). The solubility of these ions is not affected by flow velocity.

Profile of stream channel at bankfull stage, flood stage, and deposition of natural levee (Earle 2015).
Profile of stream channel at bankfull stage, flood stage, and deposition of natural levee [zotpressInText item="{R6VDH77A}" format="%num%" brackets="yes" separator="comma"].
Stream flooding is a natural process that adds sediment to floodplains. A floodplain is the mostly flat area of land located adjacent to a stream channel that is inundated with flood water on a regular basis. A stream typically reaches its greatest velocity when it is close to flooding, known as the bankfull stage. As soon as the flooding stream overtops its banks and occupies the wide area of its floodplain, the velocity decreases. At this point, sediment that was being carried by the swiftly moving water is deposited near the edge of the channel, forming a low ridge or natural levée. In addition, sediments are added to the floodplain during this flooding process .

Zone of Deposition

The process of deposition is when bedload and suspended load come to rest on the bottom of the water column in a stream channel, lake, or ocean. The two major factors causing deposition are decreased in stream gradient and reduction in velocity associated with a decrease in discharge or increased in cross-sectional area. Deposition occurs temporarily in the zone of transportation such as along meandering stream point bars, floodplains, and alluvial fans (discussed later), however, ultimate deposition occurs at the mouth of the stream where it reaches a lake or ocean. These deposits at the mouth of a stream can form landforms such as deltas. Deposition at the mouth of a stream is generally the finest sediment such as fine sand, silt, and clay, because as the stream exits it channel, the energy of the water is dispersed, causing the deposition of the particles in the stream.

Equilibrium and Base Level

All three stream zones are illustrated in the longitudinal profile of the stream (see figure). The profile plots the stream gradient from headwater to mouth and represents the balance among erosion, transport, gradient, velocity, discharge, and channel characteristics at each point along the stream’s course. This balance is called equilibrium. When mountains are uplifted, streams become steeper which erodes downward cutting a valley. This uplift is balanced against downward erosion of the stream. Eventually, streams erode enough downward that the gradient is reduced, downward erosion slows, and the river starts to erode from side to side. This point is generally characterized by a stream with a floodplain .

Another factor influencing equilibrium is the location of base level. The elevation of the stream’s mouth generally represents base level, that is the lowest level to which a stream can erode and is ultimately sea-level. Base level for a stream can change if sea-level rises or falls or a natural or human-made dam is formed. When base level is lowered, a stream will downcut and deepen the canyon. When base level is raised, deposition increases along the head stream profile as the headwater area erodes back to adjust to the change and establish a new state of equilibrium (Figure).

11.6.5 Landforms

Stream landforms are the land features formed on the surface by either erosion or deposition. The primarily stream-related landforms described here are related to channel types.

Channel Types

Braided stream pattern on the Waimakariri River in New Zealand.
Braided stream pattern on the Waimakariri River in New Zealand.
Air photo of the meandering river, Río Cauto, Cuba.
Air photo of the meandering river, Río Cauto, Cuba.
Stream channels can be straight, braided, meandering, or entrenched. The gradient, sediment load, discharge, and location of base level all influence channel type. Straight channels are relatively straight, located near the headwaters, have steep gradients, low discharge, and narrow V-shaped valleys. Good examples of these are located in mountainous areas. Anastomosing are a variety of straight channels, formed in areas of high vegetation where the plant growth keeps the channel straight. Braided channels have multiple smaller channels splitting and recombining downstream creating numerous mid-channel bars. These are found in broad terrain with low gradients near sediment source areas such as mountains or in front of valley glaciers in Alaska. Meandering channels are generally a single channel that curves back and forth like a snake within its floodplain. Meandering channels tend to have a wide floodplain, high discharge, natural levees, and flood regularly. Meandering channels are usually located on low gradient slopes in the zone of transportation and close to the zone of deposition at the stream’s mouth. In areas of uplift, meanders can become entrenched or incised into the bedrock.

Panoramic view of incised meanders of the San Juan River at Gooseneck State Park, Utah.
Panoramic view of incised meanders of the San Juan River at Gooseneck State Park, Utah.

Alluvial Fans

 

Floodplains, Meandering Rivers, and Natural Levees

Many fluvial landforms occur in a floodplain near a meandering stream. A floodplain is the broad, mostly flat area next to a meandering river that is regularly flooded. (Make my own from a screenshot of Landsat imagery). A streams create its floodplain as the channel meanders back and forth over thousands, even millions of years. Regular flooding contributes to creating the floodplain by eroding uplands next to the floodplain Landsat imagery of Missouri River near KC showing uplands. The stream channels are confined by small natural levees that have been built up over many years of regular flooding. Natural levees can isolate flow from contributing channels from immediately reaching the main channel on the floodplain.  The smaller isolated streams, called yazoo streams, will flow parallel to the main trunk stream until there is an opening in the levee to allow for a belated confluence .

The location and width of floodplains naturally vary, however, humans build artificial levees on flood plains to limit flooding. Sediment that breaches the levees during flood stage are called crevasse splays delivering silt and clay into the floodplain. Floodplains are nutrient rich from the fine grained deposits and thus often make good farm land. Floodplains are also easy to build on due to their flat nature, however, when floodwaters crest over human-made levees, the levees quickly erode with potentially catastrophic impacts.

Sandy deposition at the inside of a bend (point bar) and erosion on the outside of the bend (cut bank) of a river in France.
Point bar and cut bank on the Cirque de la Madeleine in France.
Meandering rivers create additional landforms as the channel migrates within the floodplain. Meandering rivers erode side-to-side because  the highest velocity water having the most capacity to erode is located on the outside of the bend.  Erosion of the outside of the bend of a stream channel is  called a cut bank. The thalweg of the stream is the deepest part of the stream channel. When the channel is straight, the thalweg and highest velocity are in the center of the channel. But at the bend of a meandering stream, the thalweg moves to the cut bank. The inside bend of the channel has the lowest stream velocity and therefore becomes the area of deposition call a point bar.

The channels of meandering streams move over time. Sometimes on very broad floodplains with very low gradients, the meander bends can become so extreme that they cut back into themselves. The former channel becomes isolated from streamflow and forms an oxbow lake. Eventually the oxbow lake fills in with sediment and becomes a wetland and eventually a meander scar. Channels can migrate in a short amount of time. In the images, today’s Mississippi River channel is superimposed on channels from 1880 (green), 1820 (red), and 1765 (blue). Even earlier, prehistoric channels underlie the more recent patterns. An oxbow lake from 1785 is visible. A satellite image from 1999 shows the current course of the river and the old oxbow lake . Stream meanders migrate and form oxbow lakes. Where stream channels form geographic and political boundaries, this shifting of channels can cause conflicts. (http://www.nebraskahistory.org/publish/publicat/history/full-text/NH1973MissouriRiver.pdf).

 

Deltas

The area that contributes to the tributaries of the Mississippi River.
Location of the Mississippi River drainage basin and Mississippi River delta.

When a stream reaches a low energy body of water such as a lake or some parts of the ocean, the velocity slows and the bedload and suspended load sediment come to rest, forming a delta. If erosion is greater than deposition from the river, such as high energy waves at the mouth of the river, then deposition will not occur and a delta will not form. The largest and most famous delta in the United States is the Mississippi River delta formed by the Mississippi River where it flows into the Gulf of Mexico. The Mississippi River drainage basin is the largest in North America, draining 41% of the contiguous U.S. . Because of the large drainage area, the river carries a large amount of sediment that is supplied to the large delta. The Mississippi River is a major shipping route and human engineering has ensured that the channel no longer meanders significantly within the floodplain. In addition, the river has been artificially straightened so that it meanders less and is now 229 km shorter than it was before humans began engineering it . Because of these restraints, the delta is now solely focused at one area and thus has created a “bird’s foot” pattern. The two NASA images of the delta show how the shoreline has retreated and land was inundated with water while deposition of sediment was located at end of the delta. These images have changed over a 25 year period from 1976 to 2001. These are stark changes illustrating sea-level rise and land subsidence from the compaction of peat due to the lack of sediment resupply .

The formation of the Mississippi River delta started about 7500 years ago, when postglacial sea level stopped rising. In the past 7000 years, prior to anthropogenic modifications, the Mississippi River delta had several lobes that were sequentially created by the river, abandoned for a shorter route to the Gulf of Mexico, then reworked by the ocean waves of the Gulf of Mexico .  After the lobes are abandoned by the river, isostatic depression and compaction of the sediments caused basin subsidence (e.g. the mass and compaction of the new sediments caused the land to sink). 

Delta in Quake Lake Montana. Deposition of this delta began in 1959, when the Madison river was dammed by the landslide caused by the 7.5 magnitude earthquake.
Delta in Quake Lake Montana. Deposition of this delta began in 1959, when the Madison river was dammed by the landslide caused by the 7.5 magnitude earthquake.

A clear example of how deltas form came from an unlikely source, an earthquake. During the 1959 Madison Canyon 7.5 magnitude earthquake in Montana, a large landslide dammed the Madison River forming Quake Lake , which is still there today. A small tributary stream that once flowed into the Madison River, now flows into Quake Lake where a delta has been forming since.  This a modern example of a Gilbert-type delta, which is a delta composed primarily of coarse material actively eroded from the mountainous upthrown block to the north.

Deltas represent stream deposits protruding into a quiet water body and can be further categorized as wave-dominated or tide-dominated. A wave-dominated deltas occur where the tides are small and wave energy dominates. An example, is the Nile River delta in the Mediterranean Sea that has the classic shape like the Greek character (Δ) . A tide-dominated delta is when ocean tides are powerful and influence the shape of the delta. For example, Ganges-Brahmaputra Delta in the Bay of Bengal (near India and Bangladesh) is the world’s largest delta and mangrove swamp called the Sundarban. Tidal forces create linear segments in the delta shoreline by ocean intrusion into the delta deposits. This delta also holds the world’s largest mangrove swamp, and incidentally is the only place where the Bengal tiger still actively hunts humans as a prey .  

The Nile Delta is a triangular patch of green in an otherwise sandy brown area.
Nile Delta

Special topic: Ancient Deltas in Lake Bonneville

Contours of the Logan Delta, incised by the Logan River.
Map of the Logan Delta [zotpressInText item="{KV2FUXDI}" format="%num%" brackets="yes"].
Lake Bonneville was a large, pluvial lake that occupied the western half of Utah and parts of eastern Utah from about 30,000 to 12,000 years ago . The lake rose up to an elevation as great as approximately 5100 feet above mean sea level, covering the basins, leaving the mountains exposed. The presence of the lake allowed for deposition of fine grained lake mud and silt, as well as coarse gravels entrained by mountain streams that lost their sediment load and energy to the open area of the lake.  The lake’s average surface elevation varied over its existence. The variations in lake level were controlled by regional climate and a catastrophic failure of Lake Bonneville’s main outlet, Red Rock Pass .  Extended periods of time where the lake level remained stable allowed for the development of large swaths of deltaic deposits at the mouths of major canyons in Salt Lake, Cache, and other valleys.  As the lake regressed to its remnant, the Great Salt Lake, the progenies of the rivers that created the deltaic deposits incised stream valleys into the same deposits.

Deltaic deposits of Lake Bonneville near Logan, Utah.
Deltaic deposits of Lake Bonneville near Logan, Utah [zotpressInText item="{KV2FUXDI}" format="%num%" brackets="yes"].

Entrenched Meanders

Entrenched meander of the Colorado River, downstream of Page, Arizona. High cliffs, that lead down to a river with narrow shores.
Entrenched meander of the Colorado River, downstream of Page, Arizona.

In some rare cases, uplift will occur on a low-gradient landscape with a meandering river. This effectively increases the gradient of the stream causing it to erode downward instead of side-to-side. An example of this is where the Colorado River crosses the Colorado Plateau. As the Colorado Plateau has uplifted over the past several million years, the Colorado River has incised into the flat lying rocks of the plateau by hundreds of feet.  The entrenched meanders continue to erode side-to-side, and in some cases, create incised oxbows (Figure – Satellite image). An excellent example of entrenched meanders is Goosenecks State Park, Utah, where the San Juan River is deeply entrenched into the Colorado Plateau.  An entrenched channel is when a meandering channel rapidly down cuts due to a drop in rapid drop in base level. This causes the original meandering shape to be preserved within a deep entrenched channel. This channel type is rare worldwide, but is common in Colorado Plateau region which is a broad flat area near four-corners region where Utah, Colorado, Arizona, and New Mexico meet. For example, the Green, Colorado, and San Juan Rivers famously form entrenched channels.

A winding river cut deep through hundreds of feet of rock. The San Juan River incised the flat Colorado Plateau at Goosenecks State Park, southeastern Utah.
The San Juan River incised the flat Colorado Plateau at Goosenecks State Park, southeastern Utah.

Terraces

Stream terraces are remnants of older floodplains located above the existing floodplain and river. Like entrenched meanders, stream terraces form when uplift occurs and streams erode downward, leaving behind their old floodplains. In other cases, stream terraces can form from extreme flood events associated with retreating glaciers. A classic example of multiple stream terraces are along the Snake River in Grand Teton National Park in Wyoming .

Landscape with river cutting into plane. Three different levels of flat surfaces that represent different base levels.
Terraces along the Snake River, Wyoming.
River along several different levels of flat floodplains
Terraces in Glen Roy, Scotland

Your Score:  

Your Ranking:  

11.7 Groundwater

Groundwater is an important source of freshwater.  It can be found in all places under the ground, but is limited by extractable quantity and quality.

11.7.1 Properties

Permeability and Porosity

Most rocks are not entirely solid and contain a certain amount of open space, known as pores.  Porosity is a measure of the open space in rocks – the percentage of open space that makes up the total volume of the rock or sediment material. Porosity can occur as primary porosity, which represents the original pore spaces in the rock (e.g. space between sand grains, vesicles in volcanic rocks), or secondary porosity which occurs after the rock forms (e.g. fractures, dissolved portions of rock). Lithification of unconsolidated sediments will reduce porosity, because it compacts grains and adds cement (see chapter 5.3). Water trapped in the unconnected pores of the rock during the processes of deposition and lithification is called connate water.

Permeability is a measure of the interconnectedness of pores in a rock or sediment. The connections between pores allows for that material to transmit water. A combination of a place to put water (porosity) and a path to move water (permeability) makes a good aquifer—a rock unit or sediment containing extractable groundwater.  Well-sorted sediments have higher porosity, because there are not smaller sediment particles filling in the spaces between the larger particles. Clays can have very high porosity, but the pores are poorly connected, causing low permeability.  

While permeability is important as a measure of ability to transmit fluids, it is generally not the most commonly used descriptor among geologists for this property. Hydraulic conductivity is another common measure of connectedness of pore spaces, and is a function of both permeability and fluid properties. Because it considers fluid properties, hydraulic conductivity is used by both petroleum geologists and hydrogeologists to describe the production ability of oil reservoirs and aquifers. A high hydraulic conductivity indicates a rapid transmission of fluid through an aquifer. Unconsolidated gravels, highly fractured and dissolved rocks, and well sorted sandstones have high hydraulic conductivities.

11.7.2 Aquifers and Confining Layers

An aquifer is a geologic material capable of delivering water in usable quantities. Geologic material includes any rock or sediments. In order for a geologic material to be considered an aquifer, it must be at least partially saturated, where its open spaces are filled with water and be permeable, or able to transmit water. For drinking water aquifers, the water must also be potable. Aquifers can vary dramatically in scale, from spanning several formations, to being limited to a small area on the side of a hill. Aquifers adequate for water supply are both permeable and porous.

In a confined aquifer, the potentiometric surface can rise above the ground.
Potentiometric surface and water table in an aquifer system.

A good aquifer will provide sufficient quantity of water.  The quantity of water that an aquifer can hold and transmit is governed by its physical properties. Most simply, the aquifer’s porosity and permeability (defined above) are variables that govern its hydraulic conductivity and storativity.

A confining layer is a layer of low permeability geologic material that restricts the flow of water to or from the aquifer.  Confining layers include aquicludes (a.k.a. aquifuges), which are so impermeable that no water travels through them, and aquitards, which significantly decrease the speed at which water travels through them due to their low permeability.

11.7.3 Groundwater Flow

From the Surface to the Ground

The groundwater table is where the pores in an aquifer are completely saturated.
Zone of saturation.
When surface water infiltrates or seeps into the ground, it usually enters the unsaturated zone (a.k.a. vadose zone, a.k.a. zone of aeration).  The vadose zone is the area between the land surface and the zone of saturation, which consists of geologic materials in which the pore spaces are not completely filled with water .  Plants’ roots inhabit the upper vadose zone. In the vadose zone, fluid pressure in the pores is less than atmospheric pressure.  Below the vadose zone is the capillary fringe.  Capillary fringe is the usually thin zone below the vadose zone where the pores are completely filled with water (saturation), but the fluid pressure is less than atmospheric pressure.  The pores in the capillary fringe are filled because of capillary action, described in the Properties section above. Below the capillary fringe is the saturated zone (a.k.a. phreatic zone), where the pores are completely saturated and the fluid in the pores is at or above atmospheric pressure . The interface between the capillary fringe and the saturated zone marks the location of the water table.

Water Table

Wells are conduits that extend into the ground with openings to the aquifers, to extract from, measure, and sometimes add water to the aquifer. Wells are the generally the way that geologists and hydrologist measure the depth to groundwater from the land surface.

Water is found throughout porosity in sediments and bedrock. The water table is the area at which the pores are fully saturated with water.  The most simple case of a water table is when the aquifer is unconfined, meaning it does not have a confining layer. Confining layers can pressurize aquifers by trapping water recharged at a higher elevation underneath the confining layer, allowing for a potentiometric surface higher than the top of the aquifer, and sometimes higher than the land surface. The potentiometric surface represents the height that water would rise in a well penetrating the pressurized aquifer system. Breaches in the pressurized aquifer system, like faults or wells, can cause springs or flowing wells, also known as artesian wells.

The water table will generally mirror surface topography (i.e., a function of elevation) because hydrostatic pressure is equal to atmospheric pressure along the surface of the water table.  If the water table intersects the ground surface the result will be water at the surface in the form of a spring, lake, or wetland.  

Geologists measure the height of the water table and potentiometric surface using wells.  Graphs of the depth to groundwater over time are known as hydrographs.  Hydrographs can indicate changes in the water table over time. The water level in a well can change very frequently (every minute), seasonally, and over long periods of time, and is controlled by many forces.  

A hydrograph is a line graph that shows depth to water with time.
Example of a hydrograph.

Darcy’s Law

Darcy’s Law was an empirical relationship established by Henry Darcy in 1856 showing how discharge through a porous medium is controlled by permeability, pressure, and cross sectional area.  In his experiment,  Darcy used tubes of packed sediment with water running through them. The relationships described by Darcy’s Law have close similarities to Fourier’s law in the field of heat conduction, Ohm’s law in the field of electrical networks, or Fick’s law in diffusion theory. Darcy’s Law provides a quantitative measure of hydraulic conductivity.

Q=AKhL

Cylinder with a length (L), cross-sectional area (A), which is filled with a material of a specific hydraulic conductivity.
Pipe showing apparatus that would demonstrate Darcy’s Law. Δh would be measured across L from a to b.
  • Q = flow (volume/time)
  • K = hydraulic conductivity (length/time)
  • A = cross-sectional area of flow (area)
  • Δh = change in pressure head (pressure difference)
  • L = distance between pressure (h) measurements (length)

 

Cone of Depression

Pumping water from an aquifer lowers the water table or potentiometric surface around the well. In an unconfined aquifer, the water table is lowered as water is removed from the aquifer near the well. In a confined aquifer, the pressure around the well is reduced.  The amount of change from before pumping to pumping level is termed drawdown. Drawdown is greatest nearest the well, resulting in a concentric pattern of drawdown termed the cone of depression.

The shape of the potentiometric surface or water table around a pumping well is cone-shaped, where groundwater level has the greatest drawdown near the well.
Cones of depression (Heath 1983).

When one cone of depression intersects another cone of depression or a barrier feature like an impermeable mountain block, drawdown is intensified. When a cone of depression intersects a recharge zone, the cone of depression is lessened.

11.7.5 Recharge

The recharge area is where surface water enters an aquifer through the process of infiltration. Recharge areas are generally the topographically highest areas for an aquifer.  They are characterized by losing streams and sediment or rock that allows infiltration into the subsurface. Recharge areas mark the beginning of groundwater flow paths.

In the Basin and Range region, the recharge zones for the unconsolidated aquifers of the valley areas are along the valley margins, near the foothills of the mountains. In Salt Lake Valley, as mountain streams leave the mountainous areas, they lose water to the gravel-rich deltaic deposits of ancient Lake Bonneville.

Recharge can be induced through the aquifer management practice of aquifer storage and recovery.  Injection wells and infiltration galleries (basins) allow for humans to increase the rate of recharge into an aquifer system . Injection wells pump water into an aquifer.  Injection wells are regulated by state and federal governments to ensure that the injected water is not negatively impacting the quality or supply of the existing groundwater in the aquifer.  Some aquifers are capable of storing significant quantities of water, allowing water managers to use the aquifer system like a surface reservoir. Water is stored in the aquifer during periods of low water demand and high water supply and later extracted during times of high water demand and low water supply.

Different ways an aquifer can be recharged. Rivers, injection wells, and infiltration can all contribute to aquifer recharge.
Different ways an aquifer can be recharged.

11.7.6 Discharge

Discharge areas are where groundwater emerges at the land surface. Groundwater emerges at the land surface when the potentiometric surface or water table intersects the land surface. These areas are characterized by springs, flowing (artesian) wells, gaining streams, and playas. Discharge areas mark the end of groundwater flow paths. In the Basin and Range of the western United States, discharge zones are typically in the middle of the valley basins, where playa lakes, springs, and gaining streams signify groundwater emerging at the land surface.

11.7.7 Groundwater mining and subsidence

Groundwater as a Limited Resource

Like other natural resources on our planet, the quantity of fresh and potable water is finite.  Because of a slow rate of travel, limited recharge areas, and intensifying extraction and demand, in many places groundwater is being extracted faster than it is being replenished.  When groundwater is extracted faster than recharge can renew it, groundwater levels (potentiometric surfaces) decline, and areas of discharge can diminish or dry up completely. Regional pumping-induced groundwater decline is known as groundwater mining or groundwater overdraft. Groundwater mining can lead to dry wells, reduced spring and stream flow, and subsidence.   


Subsidence

The author stands in a large ditch-like
Example of an earth fissure in Cedar City.

Water actually helps hold up the skeleton of the aquifer, in many places, by the water pressure exerted on the grains in an aquifer.  If pore pressure decreases because of groundwater mining, the aquifer can compact, causing the surface of the ground to sink.  Areas especially susceptible to this effect are aquifers made of unconsolidated sediments.  Unconsolidated sediments with multiple layers of clay and other fine-grained material are at higher risk because clay can compact considerably when drained of water .

In many cases, the amount of compaction in one area will be greater than the amount of compaction in an adjacent area.  The different amounts of compaction in areas that are next to each other can cause the land to offset and develop cracks and fissures.

Subsidence from groundwater mining has been documented in southwestern Utah, notably Cedar Valley, Iron County, Utah.  Groundwater levels have declined more than 100 feet in certain parts of Cedar Valley, causing earth fissures and measurable amounts of land subsidence.

Your Score:  

Your Ranking:  

11.8 Water Contamination

Types of Contamination

Water can be contaminated by various human activities or by existing natural features, like mineral-rich geologic formations.  Agricultural activities, industrial operations, landfills, animal operations, and small and large scale sewage treatment processes, among many other things, all can potentially contribute to contamination. As water runs over the land or infiltrates into the ground, it dissolves material left behind by these potential contaminant sources. There are three major groups of contamination: inorganic chemicals, organic chemicals, and biological agents. Small sediments that cloud the water, causing turbidity, is also an issue with some wells, but it is not considered contamination.   The risks and type of remediation for a contaminant depends on what type of chemicals present.

Point source pollution can be attributed to a single, definable source, while nonpoint source pollution is from multiple dispersed sources. Point sources include waste disposal sites, storage tanks, sewage treatment plants, and chemical spills. Nonpoint sources are dispersed and indiscreet, where the whole of the contribution of pollutants is harmful, but the individual components do not have harmful concentrations of pollutants.  A good example of nonpoint pollution are residential areas, where lawn fertilizer on one person’s yard may not contribute much pollution to the system, but the combined effect of many residents using fertilizer can lead to significant nonpoint pollution.  Other nonpoint sources include nutrients (nitrate and phosphate), herbicides, pesticides contributed by farming, nitrate contributed by animal operations, and nitrate contributed by septic systems.  

Organic chemicals are common pollutants. They consist of strands and rings of carbon atoms, usually connected by covalent bonds. Other types of atoms, like chlorine, and molecules, like hydroxide (OH), are attached to the strands and rings. The number and arrangement of atoms will decide how the chemical behaves in the environment, its danger to humans or ecosystems, and where the chemical ends up in the environment. The different arrangements of carbon allow for tens of thousands of organic chemicals, many of which have never been studied for negative effects on human health or the environment.  Common organic pollutants are herbicides and pesticides, pharmaceuticals, fuel, and industrial solvents and cleansers.

Organic chemicals include surfactants (cleaning agents) and synthetic hormones associated with pharmaceuticals, which can act as endocrine disruptors. Endocrine disruptors mimic hormones, and can cause long-term effects in developing sexual reproduction systems in developing animals. Only very small quantities of endocrine disruptors are needed to cause significant changes in animal populations.

An example of organic chemical contamination is the Love Canal, in Niagara Falls, New York. From 1942 to 1952, the Hooker Chemical Company disposed of over 21,000 tons of chemical waste, including chlorinated hydrocarbons, into a canal and covered it with a thin layer of clay.  Chlorinated hydrocarbons are a large group of organic chemicals that have chlorine functional groups, most of which are toxic and carcinogenic to humans.    The company sold the land to the New York School Board, who developed it into a neighborhood. After residents began to suffer from serious health ailments and pools of oily fluid started rising into residents’ basements, the neighborhood had to be evacuated.  This site became a U.S. Environmental Protection Agency Superfund Site, a site with federal funding and oversight to ensure its cleanup.   

Inorganic chemicals are another set of chemical pollutants. They can contain carbon atoms, but not in long strands or links. Inorganic contaminants include chloride, arsenic, and nitrate (NO3).  Nutrients can be from geologic material, like phosphorous-rich rock, but are most often sourced from fertilizer and animal and human waste.  Untreated sewage and agricultural runoff concentrates nitrogen and phosphorus which are essential for the growth of microorganisms.  Nutrients like nitrate and phosphate in surface water can promote growth of microbes, like blue-green algae (cyanobacteria), which in turn use oxygen and create toxins (microcystins and anatoxins) in lakes . This process is known as eutrophication.

Metals are common inorganic contaminants. Lead, mercury, and arsenic are some of the more problematic inorganic groundwater contaminants.  Bangladesh has a well documented case of arsenic contamination from natural geologic material dissolving into the groundwater. Acid mine drainage can also cause significant inorganic contamination. See the Energy and Mineral Resources Chapter for a description of acid mine drainage.

Salt, typically sodium chloride, is a common inorganic contaminant.  It can be introduced into groundwater from natural sources, such as evaporite deposits like the Arapien Shale of Utah, or from anthropogenic sources like the salts applied to roads in the winter to keep ice from forming.  Salt contamination can also occur from saltwater intrusion, where fresh groundwater pumping near ocean coasts induces the encroachment of saltwater into the freshwater body.

Another common groundwater contaminant is biological, which includes harmful bacteria and viruses. A common bacteria contaminant is Escherichia coli (E. coli). Generally, harmful bacteria are not present in groundwater unless the source of groundwater is closely connected with a contaminated surface source, such as a septic system.  Karst is especially susceptible to this form of contamination, because water moves relatively quickly through the dissolved conduits of limestone. Bacteria can also be used for remediation (see below).

Table. Groundwater contaminants.

Remediation

Remediation is the act of cleaning contamination. Biological remediation usually consists of using specific strains of bacteria to break down a contaminant into safer chemicals.  This type of remediation is usually used on organic chemicals, but also works on reducing or oxidizing inorganic chemicals like nitrate. Phytoremediation is a type of bioremediation that uses plants to absorb the chemicals over time.

Chemical remediation uses the introduction of chemicals to remove the contaminant or make it less harmful.  One example is reactive barriers, a permeable wall in the ground or at a discharge point that chemically reacts with contaminants in the water. Reactive barriers made of limestone can increase the pH of acid mine drainage, making the water less acidic and more basic, which removes dissolved contaminants by precipitation into solid form.

Physical remediation consists of removing the contaminated water and either treating it (aka pump and treat) with filtration or disposing of it.  All of these options are technically complex, expensive, and difficult, with physical remediation typically being the most costly.

Your Score:  

Your Ranking:  

11.9 Karst

Karst refers to landscapes and hydrologic features created by the dissolution of limestone. Karst can be found anywhere where there is limestone.  Dissolution of limestone can create features like sinkholes, caves, disappearing streams, and towers.

Karst forms when natural water, in combination with carbon dioxide, creates carbonic acid, and dissolves calcite (calcium carbonate) in limestone.

H2O + CO2 = H2CO3

Water + Carbon Dioxide Gas equals Carbonic Acid in Water

CaCO3 + H2CO3 = Ca2++ 2HCO3-1

Solid Calcite + Carbonic Acid in Water Dissolved equals Calcium Ion + Dissolved Bicarbonate Ion

Calcium carbonate deposited at Mammoth hot springs encapsulates trees.
Mammoth hot springs, Yellowstone National Park.
After the slightly acidic water dissolves the calcite, changes in temperature or gas content in the water can cause the water to redeposit the calcite in a different place as tufa (travertine), often deposited by a spring or in a cave.  Speleotherms are secondary deposits, typically made of travertine, deposited in a cave.  Travertine speleotherms from by water dripping through cracks and dissolution openings in caves.  Speleotherms commonly occur in the form of stalactites, when extending from the ceiling, and stalagmites, when extending from the floor.

Cave deposits hanging and protruding from the base of a cave.
Varieties of speleotherms.
Meteoric (surface) water enters the karst system through sinkholes, losing streams, and disappearing streams.  Changes in base level can cause rivers running over limestone to dissolve the limestone and sink into the ground.  As the water continues to dissolve its way through the limestone, it can leave behind intricate networks of caves and narrow passages.  Often dissolution will follow and expand fractures in the limestone.  Water exits the karst system as springs and rises. In mountainous terrane, dissolution can extend all the way through the vertical profile of the mountain, with caverns dropping thousands of feet.

Your Score:  

Your Ranking:  

References