Introduction to Groundwater

Linkage to surface water

Origin of Groundwater

Occurrence and movement of groundwater

Porosity and Permeability

Lab: Porosity and Permeability

Water wells

Groundwater Contamination and Pollution

Geological Role of Groundwater


Introduction to Groundwater

Review Water Cycle

Evaporation to precipitation (hard rain video - Pooh & Blustery Day?)

Weathering & erosion

Produces fractured and broken rock, and a soil horizon

In a natural setting soil soaks up precipitation

Results in a groundwater system

Which in places leaks onto the surface

General description of groundwater

Floodplain water vs. bedrock aquifers

Groundwater has always held a special and mystical place in human development

Groundwater and wells have been the basis of life forever

Wells have always been important to the development of an area

Source of unpolluted water

Walled cities needed a good INTERNAL source of water to withstand sieges

Water availability represents the ultimate demographic control

Rome - did lead poisoning from water pipes cause the Empire to fall?

Wells - some were quite deep

Orvieto, Italy - 200' deep and had 2 spiral ramps used by donkeys to bring water to the surface

Some Chinese wells were >4900' deep! - How did they dig these?

Technology has allowed humans to seriously impact the water balance

Increased agriculture and urbanization in fundamentally arid lands

The ability to economically retrieve deep groundwater

As opposed to near-surface "floodplain" water

The ability to move it long distances

This is a poor practice in the long term

Will probably result in severe social, economical and political unrest in the N-T-D-Future


Linkage to surface water

Groundwater and surface water are part of the same system

Lots of factors can force the water to leak onto the surface

Climates where there is too much precipitation for the ground to hold

Tropics vs. arid lands

Climates where the weathering & erosion processes are incomplete

Lack of fractures and/or soil

Places where there is some sort of blockage which forces the water out

Springs - places where water flows or seeps onto the surface

Occur where the water table intersects the surface

Can be caused by many different sub-surface conditions

Effluent stream - Gets its water from the water table

Common to temperate climates

Actually, effluent streams are just springs with a lot of water!

Associated with relatively stable water tables (in a natural setting)

Directly reflects the water table

Deer Creek bridge example

Influent stream - Adds water to the groundwater supply

Common in arid regions

The water is usually from more humid areas upstream which are destined to flow down into a desert

EXAMPLE: the Colorado River


Origin of Groundwater

Nearly all comes from surface precipitation which soaks into the ground

Estimates of the water budget in the U.S. indicates that:

Precipitation averages 30" per year

Evapotranspiration returns 21" to the atmosphere

Runoff returns 9" to the sea

So, where does groundwater come from?

Obviously, major groundwater systems develop very slowly

And, they are in a very delicate balance relative to use vs. recharge

DEFINE: Recharge area - where water is added to the aquifer

In many areas, groundwater can be considered a non-renewable resource

This is especially true of deep aquifers and arid environments


Occurrence and movement of groundwater

Water Table - the upper surface of the groundwater

All openings in the rock below are saturated with water

Not necessarily completely level - Roughly parallels the ground surface

Rises with the hills & sinks with the valleys

Intersects the surface in springs, streams and lakes

Vadose Zone - above the water table

Completely dry to partially wet (but not saturated!)

Water moves downward through this zone to the water table

The water table doesn't extend down into the ground forever

Increased heat and pressure creates a lower limit

Sub-surface storage reservoirs:

Aquiclude - can't hold or transmit water

Aquifer: a sub-surface layer of rock which can hold and transmit water

Two basic types of aquifers

Unconfined aquifer

Water level stands at the water table

The water level will drop as a result of pumping

A "Cone of Depression" will form around the well

Over-pumping can cause adjacent cones to intersect

Can result in a regional lowering of the water table

Confined aquifer

A permeable horizon between two impermeable rock layers

Can result in artesian wells


Porosity and Permeability


The percentage of the total volume of rock occupied by void space

Factors which determine porosity

Igneous and metamorphic rocks

Generally very dense crystalline rocks

Porosity usually controlled by fractures and faults

Sedimentary rocks

Degree of sorting - probably most important

Amount of cement already filling up the void spaces

Grain size: the effect is confusing

Some shales can actually have up to 90% open space!

However, these extremely fine-grain rocks usually make poor aquifers due to surface tension which holds the water in the rock, leading us to...


The measure of a rocks capability to transmit liquid through it's pore spaces

Generally a very slow process (one reason why the ground leaks!)

Size of the pore spaces can be more important than the amount of space!

As I said above, extremely fine-grain rocks usually make poor aquifers

Porosity alone is useless as far as aquifers is concerned

Without interconnected pore spaces, the water won't flow


Lab: Porosity & Permeability (1 day)

Fill beakers with different types of sediments and weigh

Pour in water to fill - measure amount of water in milliliters (1 ml = 1 cm^3)

Weigh again (1 gram = 1 cm3 = 1 ml)

Cover with cheesecloth and pour out water

Record time to completely drain

Weigh again

Note difference - represents the water held due to surface tension


Water wells

Drilled or dug to tap water from aquifers

Over-pumping can be a real problem in areas of slow recharge

Almost like mining - non-renewable resource in most areas

Southwest - relate the problems in Tucson (Dave Christopherson)

Southern California - relate some of the water haggles

Mono Lake a good example

Now they want to divert water from up here

Open for class discussion?

I say give them the water, even if it means some additional dams

That's better than them all moving up here

Remember-water is the ultimate demographic control, and population centers will adjust to reflect availability

Problems related to water wells

Coastal areas - encroachment of sea water

EXAMPLE: Fountain Valley, California

Land subsidence due to over-pumping

Due to compaction of the sediments after the water is removed

Also results from over-production of oil and/or gas

EXAMPLE: Gulf Coast island


Groundwater Contamination and Pollution

Aquifers can become contaminated easily and in many ways

Axiom: the closer to the surface, the easier to contaminate

Many in developed areas have already been polluted beyond rational use

There are lots of ways an aquifer can become contaminated

Unfortunately, many aquifers were contaminated in the past without an understanding of, or regard for, the long-term consequences

Direct pumping of pollutants underground

"Out of sight, out of mind"

Sanitary landfills in recharge areas

Percolation of water through the dump and into an aquifer

Not-so-sanitary landfills

Hazardous waste dumps

Poor underground mining practices

Relate the problems in the Tri-State lead/zinc district

Nuclear waste disposal

Possibly the most significant long-term problem

Relate the current findings at Hanford

The radioactive waste is reaching the Columbia much sooner than expected

The Columbia River basalt is more permeable than they thought!

Search on for a "permanent" storage facility

Yucca Mountain, Nevada

In areas of poor recharge, restricted permeability, etc., contaminants can persist far longer than we have!


Geological Role of Groundwater

Important to the formation of many sedimentary rocks

As a cementing agent

Transports natural cementing agents into unconsolidated sediments

Geysers and hot springs

Percolating groundwater is heated by a near-surface magma

Mineral deposits commonly surround them

These are leached from subsurface rocks which the super-heated water passes through

2 kinds of hot-water deposits occur

Siliceous sinter - composed of silica

Travertine - composed of calcium carbonate

Geothermal energy

Often these areas can be tapped as a source of geothermal energy

Limestone and Karst topography

Most rain is slightly acidic

Carbonic acid (H2O + CO2 = H2CO3)

Dissolves limestone

Results in underground caverns

Stalactites, Stalagmites, Columns

Karst topography - surface features common to areas underlain by limestone

Distinctive features include:

A lack of surface drainages

Water sinks into underground caverns and waterways

Re-appears farther away, often as a river emerging from a giant spring

Large surface depressions

Can be kilometers across

Probably caused by subsurface solution cavities


Smaller depressions

Definitely the result of solution of the underlying limestone

Some extend down into caverns

Can act as natural wells if they intersect the water table