Ground water is about 60 times as plentiful as fresh water in lakes and rivers on the land surface . Groundwater is a tremendously important resource. How it gets underground, where it is stored, how it moves while underground, how we look for it, and, perhaps most important of all, why we need to protect it are the main topics of this chapter.

Also important is how groundwater is related to surface rivers and springs. Groundwater can form distinctive geologic features, such as caves, sinkholes, and petrified wood. It also can appear as hot springs and geysers. Hot groundwater can be used to generate power.

Key points

What is aquifer? What is aquitard?

What is ground water table?

What is capillary fringe?

What is saturated/unsturated zone?

What are the factors that control the hydraulic conductivity of bed rock?

What is permeability of soil and rock?

What is arrtesian well?

Is permeability proportional to porosity of soil?

Given different type of soil, i.e. Gravel, sand, silt, clay, rate the permeability of each soil?

What is unconfined aquifer and confined aquifer?

What are the effects of pumping wells?

What are the impacts if the ground water is prolonged withdraw from an aquifer?

What is cone of depression? What is drawdown?

What is the cone of depression?

What is the objective of pumping test?

Distinguish discharge and recharge?

Distinguish average velocity and seepage velocity?

Distingquish spring, gaining stream, losing stream?

Problems

Problem of ground water movement. Given are properties of aquifer, i.e. permeablity, thickness and other dimensions, water level or head. Students are required to determine velocity, duration of flow, flow rate, etc.

do the excercises of ground water movement

Technical terms

aquifer

A body of saturated rock or sediment through which water can move readily.

artesian well

A well in which water rises above the aquifer.

cave (cavern)

Naturally formed underground chamber.

concretion

Hard, rounded mass that develops when a considerable amount of cementing material precipitates locally in a rock, often around an organic nucleus.

cone of depression

A depression of the water table formed around a well when water is pumped out; it is shaped like an inverted cone.

confined (artesian) aquifer

An aquifer completely filled with pressurized water and separated from the land surface by a relatively impermeable confining bed, such as shale.

drawdown

The lowering of the water table near a pumped well.

gaining stream

A stream that receives water from the zone of saturation.

geode

Partly hollow, globelike body found in limestone or other cavernous rock.

geyser

A type of hot spring that periodically erupts hot water and steam.

ground water

The water that lies beneath the ground surface, filling the cracks, crevices, and pore space of rocks.

hot spring

An area of volcanic eruptions and high heat flow above a rising mantle plume.

karst topography

An area with many sinkholes and a cave system beneath the land surface and usually lacking a surface stream.

losing stream

Stream that loses water to the zone of saturation

perched water table

A water table separated from the main water table beneath it by a zone that is not saturated.

permeability

The capacity of a rock to transmit a fluid such as water or petroleum.

petrified wood

A material that forms as the organic matter of buried wood is either filled in or replaced by inorganic silica carried in by ground water.

porosity

The percentage of a rock’s volume that is taken up by openings.

recharge

The addition of new water to an aquifer or to the zone of saturation.

saturated zone

A subsurface zone in which all rock openings are filled with water.

sinkhole

A closed depression found on land surfaces underlain by limestone.

speleothem

Dripstone deposit of calcite that precipitate from dripping water in caves.

spring

A place where water flows naturally out of rock onto the land surface.

stalactite

Iciclelike pendant of dripstone formed on cave ceilings.

stalagmite

Cone-shaped mass of dripstone formed on cave floors, generally directly below a stalactite.

unconfined aquifer

A partially filled aquifer exposed to the land surface and marked by a rising and falling water table.

vadose zone

A subsurface zone in which rock openings are generally unsaturated and filled partly with air and partly with water; above the saturated zone.

Velocity of ground water

Velocity of ground water movement is the rate of ground water movement, expresses how fast the water move in soil and rock media. The velocity is determined based on Darcy law (V=ki) is average velocity through the cross section. The seepage velocity, Vs, is velocity of flow along the path of seepage. Vs=V/n, where n is the porosity (represented in a fraction). In other words, the average velocity is the velocity of groundwater though the cross section (the cross section includes solid particles and voids. The seepage velocity is the velocity of ground water through the voids. The average velocity is sometimes called apparent velocity and the seepage velocity is true velocity.

water table

The upper surface of the zone of saturation.

well

A hole, generally cylindrical and usually walled or lined with pipe, that is dug or drilled into the ground to penetrate an aquifer below the zone of saturation.

Extra Readings

1. Ground water represents percolation of about 15% of the precipitation falling to the earth, accounts for .61% of the world's water, and is second only to glaciers as a source of freshwater.

2. Porosity is the percentage of a rock's volume comprised of openings and it measures the rock's ability to hold water. Most rocks hold some water in either pores or joints, but porosity is highly variable. Permeability measures the capacity of a rock to transmit fluids. Many porous rocks are permeable, but shale has high porosity and low permeability.

3. Water percolates into the earth as far as porosity exists and saturates the lower portions of the porous intervals (saturated zone). The upper surface of the saturated zone is the water table. Above the water table, porosity is filled by air and water, forming an unsaturated zone (= vadose zone). Water is drawn by capillary action from the saturated zone into the vadose zone. Perched water tables result from local variations in permeability, such as shale lenses in sandstones.

4. Ground water movement reflects Darcy's Law . Water table slope influences ground water velocity: steeper slopes have faster movement.

5. Aquifers are porous and permeable rock bodies through which ground water moves easily. Aquitards have low permeability and/or porosity that prevent ground water movement. Unconfined aquifers have water tables and exhibit rapid ground water movement. Confined aquifers lack water tables and exhibit slow ground water movement.

6. Wells must penetrate a saturated zone to produce ground water. Water tables fluctuate with season. If the water table falls below the bottom of a well, it is dry. Drawdown from pumping wells produces local lowering of the water table. Recharge raises the water table. A cone of depression may form around a well because of drawdown. In artesian wells, water rises above its confined aquifer because the ground water is under pressure because of elevation differences.

7. Springs form where the water table intersects the surface, or structures bring water to the surface. The surface of gaining streams is the water table, while losing streams lie above the water table.

8. Rain can leach surface contaminants and move them into ground water. Human activity produces potential pollution from pesticides, herbicides, fertilizers, heavy metals and toxic compounds, bacteria, viruses and parasites from animal, plant and human waste, acid mine drainage, and radioactive waste (both low level and high level). Gasoline may float on the water table. Some pollutants are naturally occurring. Some filtration and purification can be expected through ground water flow, if it is slow. Heavily pumped wells near coasts can be contaminated by saltwater intrusion.

9. Dropping water tables create problems with supply and subsidence, through compaction. Artificial recharge may offset these problems.

10. Natural ground water is slightly acidic because of dissolved carbon dioxide from the atmosphere or soil gases. Its contact with calcite in limestone causes solution forming caves, sinkholes and karst topography. Calcium and bicarbonate in solution can be precipitated as calcite in the form of stalactites, stalagmites, columns and flowstone.

11. Ground water may also form petrified wood, concretions, geodes, cement sedimentary rocks, and develop alkali soils.

12. Hot springs have ground water warmer than the human body. Heating of the water is by either proximity to a magma chamber, or through the geothermal gradient. Geysers erupt periodically because of constrictions in conduits to the surface allow the temperature of the ground water to rise to vapor, which then condenses as the eruption proceeds. Hot ground water produces deposits of sinter (silica) or travertine (calcite). Geothermal energy is derived from hot ground water through the production of electricity from natural steam. It is also utilized for heating, paper manufacturing, ore processing, food preparation and other non-electric uses