Consumption
Recharge/Discharge --- lakes and rivers
Pollution/Contaminates
Slope Stability and Subsidence

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

Examples of a rock with high porosity: beach sand, aeolian sands, poorly consolidated material, poorly cemented coquina.

Examples of a rock with low porosity: well-cemented sandstone, shale, mudstone, very poorly sorted material, and micritic limestone.

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)

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 (water tension and the hydrogen bonds). 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 pore 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. At this interface, the capillary fringe is located. Water tension allows some water to creep up into the vadose zone.

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 water level 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.

Aquifers and aquicludes

An aquifer is a water-bearing rock.

Aquifers have high porosity and high permeability.

Aquicludes (aquitardes) are water-excluding rocks.

They have little or no permeability. An aquiclude could have high porosity, but the permeability is so low that water does not flow.

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

How much water can a well yield?

An average figure is about 20 gallons per minute (gpm). With careful geologic study (such as lineament mapping), yield can be increased to 100 - 300 gpm.

Artesian wells are wells in which water pressure in the well is great enough that it rises above the water table at the well. This may allow the water to flow freely at the surface.

All of this is related to Darcy's Law. Darcy's Law relates the velocity (V) of the water to its head (h), flow length (l), and the permeability (K) of the material:

Problems associated with groundwater withdrawal
Ground water depletion
Subsidence
Sinkhole formation

Lowering the water table through pumping may lead to an increase in sinkhole formation in areas underlain by limestone.

Named for the topography and features found in the Kars region of Slovenia - it commonly forms in areas underlain by limestone.
Karst Landforms

Dolines

sinkholes

swallow holes

blue holes

cenotes

cockpits

tower karst

• compound sinkholes
• blind valleys
• disappearing streams

• large uvalas

• from the ceiling

• from the floor

Groundwater pollution or contamination

• Septic systems
• Pathogens may be introduced to the groundwater. Hepatitis is a particular problem.
• Pesticides, herbicides, fungicides, farm chemicals, fertilizers
• Organic waste from farm animals (fecal coliform bacteria in water)
• Hazardous and industrial waste
• Urban runoff
• May include illegal dumping of chemicals, motor oil by homeowners
• UST - underground storage tanks
• Leaking landfills
• Storage and disposal of radioactive waste
• Acid rain or runoff
• Saltwater encroachment

Over the past half century, water-quality improvements in the basin were numerous because of advances in municipal and industrial waste treatment. The effects of industrialization and urbanization on the quality of rivers and ground-water resources, however, remain a primary concern to water-resource managers; planners; Federal, State and local governments; and citizen groups. Many of these issues relate to point and nonpoint pollution sources and are the subject of ongoing research and management programs. Examples of these programs include State and Federally funded projects like the Tunnel And Reservoir Project in the Chicago metropolitan area and program-related initiatives, such as Total Maximum Daily Loads, Best Management Practices, and Wetland Restoration; and State and local programs, such as Ambient Water-Quality Monitoring, Illinois Wellhead Protection, Side Stream Elevated Pool Aeration, Pesticide Management and Monitoring, Wisconsin Watershed Protection Plan, and Conservation 2000. The following is a list of some major water-quality issues that currently face water-resource managers in the upper Illinois River Basin:

• Atmospheric deposition of pesticides and trace metals and its effect on aquatic biota;
• Endocrine disrupting compounds in surface and ground water;
• Effectiveness of Federal pollution-control projects on water quality;
• Restoration of the Grand Kankakee Marsh and its effect on hydrology and water quality;
• Nutrient enrichment of surface and ground waters;
• Recovery of water levels in bedrock aquifers and its potential effect on water quality;
• Surface-water sampling strategies for computing total maximum daily loads;
• Occurrence, transport, and fate of pesticides, trace elements, and volatile organic compounds in surface and ground water;
• Effects of urbanization on biodiversity, habitat, and water quality.

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URL http://www.ux1.eiu.edu/~cfjps/1300/gwlecture.html
Revised 10/21/99
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