Groundwater

Pamela J. W. Gore
Georgia Perimeter College

Objectives

Contrast porosity and permeability.
Understand the concept of the water table and the various factors which control its position.
Describe the character and behavior of groundwater in various settings.
Discuss environmental problems related to groundwater.
Understand what karst topography is and how it forms (sinkholes, disappearing streams, solution valleys).
Understand how caves and the various features in them form.

Porosity and Permeability

Porosity is the amount of pore space in a rock (the spaces between the grains)

Porosity is independent of grain size.

Porosity depends on:

Sorting of the grains (or uniformity of grain size)
Are the grains all the same size (well sorted), or are a variety of grain sizes present, with finer grains filling the spaces between the larger grains (poorly sorted)?
Shape of the grains
Packing and arrangement of grains.
Cubic packing. Porosity = 47.64%
Rhombohedral packing. Porosity = 25.95%

What are some examples of a rock with high porosity?

What are some examples of a rock with low porosity?

Permeability is the ease with which fluids flow through a rock or sediment.

A rock is permeable if fluids pass through it, and impermeable if fluid flow through the rock is negligible.

Permeability depends on:

Grain size
Coarser-grained sediments are more permeable than fine-grained sediments because the pores between the grains are larger.
Sorting
Grain shape
Packing (controls pore size)

What are some examples of a rock with high permeability?

What are some examples of a rock with low permeability?

The Water Table

When it rains, some of the water percolates or soaks into the ground. We call this infiltration. Some of the water is held in the soil because it clings to the soil particles because of molecular attraction. It may evaporate from the soil or be used by plants (zone of aeration, also called the vadose zone). Both air and water occupy the pores spaces.

Excess water penetrates downward until it reaches the water table. Below this point, all of the pore spaces are filled with water (zone of saturation, also called the phreatic zone).

The water table is the top of the zone of saturation.

The water table is not flat. It mimics the topography, but is more subdued. It stands somewhat higher under hills, and lower under valleys.

Where the water table intersects (or lies above) the ground surface, springs. lakes, swamps, or rivers are present. In humid areas, groundwater movement supplies a flow of water to a stream or river.

If a well is drilled, the waterlevel in the well is at the water table.

Think about when you went to the seashore and dug a hole in the sand near the sea. Remember how the water came up in the hole? The surface of the water in the hole was the water table.

The position of the water table may fluctuate with droughts.

If water is withdrawn from a well, the water table is lowered in the immediate vicinity of the well. The lowered surface of the water table around a well forms a conical depression in the water table. It is called the cone of depression.

If significant quantities of water are withdrawn from a well, the cone of depression may be so large that it affects the water level of other wells nearby.

In New Orleans, the dead are buried in above ground tombs because the water table is very near the ground surface.

Aquifers and aquicludes

An aquifer is a water-bearing rock.
Aquifers have high porosity and high permeability.

Examples of rock types that could be aquifers?

Aquicludes are water-excluding rocks.
They have little or no porosity or permeability.

Examples of rock types that could be aquicludes?

Note that some impermeable rock types may serve as aquifers if they are highly jointed. Water may be in the joints or cracks in the rock.

How much water can a well yield?

An average figure for Georgia is about 20 gallons per minute (gpm). With careful geologic study (such as lineament mapping), yield can be increased to 100 - 300 gpm. A few new wells in Cobb County (Piedmont region) yield 500-550 gpm. And in most cases, the groundwater is pure enough for drinking, but the surface waters must be treated (purified) before drinking. (Information from the State Geologist, William McLemore, May 30, 1996 at the Atlanta Geological Society meeting).

Artesian wells

Know (and be able to sketch and label) the basic geometry of the artesian system; aquifer sandwiched between two aquicludes (or aquitards); tilted up at one end. Look at diagrams in textbook. See location of recharge area (where water enters - or recharges -the aquifer). Note location of pressure surface (sometimes called potentiometric surface). Note height to which water rises above the top of the aquifer, or above the surface of the ground (flowing wells).

City water systems as artificial artesian systems

See diagram in textbook about how city water systems work.

Problems associated with groundwater withdrawal

Ground water depletion
Subsidence
The Atlanta Journal-Constitution (Sunday Feb. 22, 1998) reported the following in its column, "EarthWeek - A Diary of the Planet" by Steve Newman:

"Mexico's capital city is sinking at a rate of up to 18 inches per year because of the massive pumping of groundwater to supply the megalopolis of nearly 20 million people. The National Water Commission reported that the city has sunk by 33 feet because of the process this century. "If the current rate of drainage continues, not only will it affect the quality and quantity of underground water, it would also cause social and economic problems for the population", the report said."
Sinkhole formation
Lowering the water table through pumping may lead to an increase in sinkhole formation in areas underlain by limestone.
Examples of this can be seen in Shelby Co. Alabama (just south of Birmingham) and in northwestern Georgia near Fairmount and Kingston, GA where pumping in limestone quarries has led to the formation of sinkholes in the cone of depression.
Groundwater pollution or contamination
- Septic systems
  Pathogens may be introduced to the groundwater.
  Hepatitis is a particular problem.
- Pesticides, herbicides, fungicides, farm chemicals, fertilizers
  At present, there do not seem to be any groundwater problems in Georgia with nitrates or pesticides from Georgia, according to the State Geologist, William McLemore.
- Organic waste from farm animals (fecal colliform bacteria in water)
- Hazardous and industrial waste
- Urban runoff
  May include illegal dumping of chemicals, motor oil by homeowners
- LUST - leaking underground storage tanks (the #2 groundwater problem in GA)
- Leaking landfills
- Storage and disposal of radioactive waste
- Acid rain or runoff
- Naturally occurring elements in the water
  Magnesium and iron are the major problems in this area in Georgia.
  Other states have problems with lead and zinc.
Saltwater encroachment
The most significant groundwater problem in Georgia is saltwater encroachment, according to the State Geologist. Saltwater encroachment has become an environmental problem near Brunswick, GA, and near Hilton Head, South Carolina.