Geology Chapter 17 Groundwater

In which ways do sinkholes form?

-Limestone is dissolved by rainwater, the bedrock surface lowers, and groundwater that is flowing in the passages below washes away the soil. -A roof of an already formed cavern collapses under its own weight.

What problem is caused by pumping groundwater for irrigation in the Southern High Plain?

A low precipitation rate and a high evaporation rate allow little water to recharge the aquifer.

Which of the following accounts for the largest usage of groundwater in the United States?

Agriculture and irrigation

What is the difference between an aquifer and an aquitard?

An aquifer is a rather permeable rock, whereas an aquitard is an impermeable rock.

During a summer with little rainfall, your house on a hill slope experiences an interval during which your well runs dry. You have to borrow water from your neighbor who lives downslope from you. Why does your neighbor have water when you don't?

As the water table drops due to the lack of precipitation, it goes below the base of your well.

Why might the ground subside after groundwater is pumped to the surface?

As water is withdrawn, the water pressure drops, and the sediment is left to support the weight on top of it. As a result, the sediment packs more closely together, causing the ground to subside.

Which acid, found in most groundwater, is responsible for most karst topography worldwide?

Carbonic

How does groundwater create caverns?

Caverns are created when acidic groundwater dissolves and carries away limestone over time.

Which of the following materials would make the best aquitard?

Clay

What feature develops if a well is withdrawing more groundwater than an aquifer can naturally replace, resulting in a dimple in the water table?

Cone of depression

________ is a term used to describe how the water table around a well becomes lower as water is taken from the subsurface.

Drawdown

What is drawdown, and how does it relate to the cone of depression?

Drawdown is when the water table around a well is lowered because of water discharge. This depression in the water table is conical and is called a cone of depression.

What is groundwater, and how does it relate to the water table?

Groundwater is water that occupies the zone of saturation within the ground. The water table is the upper limit of the groundwater.

What share of U.S. freshwater is provided by groundwater, and what is most groundwater used for?

Groundwater provides 23% of the U.S. freshwater. It is mostly used for irrigation and public use.

What geological roles does groundwater play?

Groundwater sustains during periods of no rainfall. Groundwater erodes bedrock through dissolution.

In order for an aquifer to be useful for reliable water extraction, what kind of porosity and permeability should it have?

High porosity, high permeability

What force pushes groundwater from pore space to pore space when below the water table?

Hydraulic gradient

What significant problem might arise when groundwater is heavily pumped at a coastal site?

If the groundwater withdrawal exceeds recharge, the saltwater will become high enough to be drawn into the wells.

California's Central Valley is an important agricultural area for U.S. fruit and vegetable production. Because of the semiarid climate there, farmers must irrigate their crops. Which environmental problem do you anticipate as a result?

Land subsidence

What geologic process is responsible for warming the water at nonvolcanic locations such as Hot Springs National Park, Arkansas, and Warm Springs, Georgia?

Normal geothermal gradient as groundwater circulates at depth

In what spaces or voids does most groundwater reside?

Pore space and fractures

Worldwide, the largest percentage of freshwater readily available to humans is stored in

Pore spaces between rocks and sediments

Please contrast porosity and permeability.

Porosity is the percentage of the total volume of rock or sediment that consists of pore spaces, whereas permeability is the ability of a rock or sediment to transmit fluid.

What is the term used to describe the hypothetical line the water in a confined aquifer would rise to if it weren't trapped?

Potentiometric surface

What is the source of much of water in geothermal features such as geysers and hot springs?

Precipitation

________ is when groundwater is being replenished. ________ is when groundwater is flowing back toward the surface.

Recharge; Discharge

Which environmental issue most commonly affects groundwater aquifers in coastal or island areas?

Saltwater contamination

What mineral substance makes up most geyser deposits?

Sinter

Which of the following materials would be the best natural filter, removing contaminants from groundwater over a long period of time?

Slightly clayey sand

What groundwater feature forms where the water table intersects with the Earth's surface, creating a natural outflow of groundwater?

Spring

What is the source of heat for most hot springs and geysers, and how is this reflected in the distribution of these features?

The heat sources for most hot springs and geysers are magma bodies and igneous rocks. Thus, hot springs and geysers are often located where igneous activity occurs.

What occurs to cause a geyser to erupt?

The heat underground within a geyser causes the water to expand, and some water is forced out of to the surface. Water deep within the geyser quickly turns to steam, causing the geyser to erupt.

How does the water table change around a pumping water well?

The water table elevation decreases.

The majority of groundwater is stored within which zone?

The zone of saturation.

Which of the zones is the location of the most biological activity?

The zone of soil moisture

What geologic process is responsible for the geothermal features in Yellowstone National Park?

Volcanic activity

How will the water table respond when there is a drought?

Water table will fall.

The ________ is the point in the subsurface where 100% of the pore spaces are filled with water.

Zone of Saturation

An artesian well must tap into ______.

a confined aquifer

Spring

a natural outflow of water where the water table intersects the surface.

Which of the following materials has the highest porosity?

clay

In terms of freshwater storage, groundwater is second only to which source?

glaciers

A ___ is an example of a groundwater recharge area.

grassy field

To be a gaining stream, the elevation of the water table must be ________ than that of the surface of the stream.

higher

Geysers are more common in the western United States because ______.

igneous activity has occurred more recently in the western United States

Aquitard

impermeable layer that prevents flow.

What uses the largest percentage of Earth's groundwater?

irrigation

Which aquifer listed below would be the most effective in purifying polluted groundwater, and why?

permeable sandstone; the pores between grains are large enough to let water through but small enough to block contaminants

Unconfined aquifer

rock or regolith that can store and transmit water and receives its recharge directly from the atmosphere.

Confined aquifer

rock or regolith that can store and transmit water but is contained between two impermeable layers.

Which of the following can result from excessive pumping of groundwater?

subsidence saltwater contamination reversing the direction of flow in the aquifer

What is the cone of depression?

the shape that the water table takes on near a pumping well

When rain falls on land, what factors influence the amount of water that soaks in?

the steepness of the slope the intensity of rainfall the nature of the surface material the amount of vegetation on the surface

In a losing stream, _____.

the water table is lower than the stream surface

Which household function has the highest daily water consumption?

toilets

Which of the following has the highest permeability?

unlithified sand

Hot spring

water in a basin that is 6-9∘C warmer than the mean annual temperature of the location.

Artesian well

water under pressure in a confined aquifer that will rise above the level of the aquifer.

When will a cone of depression stop enlarging?

when the amount of water flowing toward the well equals the amount of water being pumped out of the well

When might a well, Well A, go dry?

when the cone of depression of a second well intersects the deepest part of Well A

How do stalactites and stalagmites form?

~Stalagmites form when calcite-rich water falls to the floor from the ceiling, splattering minute amounts of calcite on the floor. After many such depositions, a stalagmite begins to take shape. ~Stalactites form on the ceiling when water seeps through the ceiling and deposits calcite. Eventually a soda straw develops, which develops into a stalactite after many more depositions of calcite.

Geyser

− intermittent hot springs that periodically eject water into the air with great force.