Groundwater Ground water does not occur as an underground river UNLESS it is found in a cave.

So where is groundwater stored and what does it look like? Most groundwater occurs in small pore spaces within rock and unconsolidated sediment Water flows through this sediment slowly…. A few centimeters to meters per day. Groundwater tends to collect above materials that are impermeable.

Porosity What kind of material has the highest porosity? Open space The amount of space in between sediment grains, cracks or fractures in rocks. These spaces may be filled with water, air or a mixture of both. What kind of material has the highest porosity? Sediment Porosity %   Gravel 25 to 40 Clean Sand 30 to 50 Silt 35 to 50 Clay 35 to 80 Glacial Till 10 to 20 More about Porosity

Permeability This is the ability to transmit water. Rocks may have high porosity, but if the spaces or pores are not interconnected, then the permeability will be very low. Materials that can transmit large quantities of water beneath the surface are called aquifers.

What kind of materials have a high permeability? Recall what types of material have the highest porosity. Sediment Porosity (%)   Gravel 25 to 40 Clean Sand 30 to 50 Silt 35 to 50 Clay 35 to 80 Glacial Till 10 to 20 What kind of materials have a high permeability?

Distribution of Water

Distribution of Groundwater Unsaturated Zone – Region that covers the ground surface down to the water table. This area may have pore spaces that are partially filled with water. The unfilled pore space contains air

Saturated Zone – Region of the subsurface where the pore spaces are completely filled with water. Water Table – Area where the unsaturated and saturated zones meet in unconfined aquifers. (Very top of the saturated zone) Capillary Fringe – Area above the water table where capillary forces pull water into pore spaces.

What is the difference between these to aquifers? Hint: Compare their layers Bedrock Bedrock Clay Bedrock

Aquifer is between 2 layers of impermeable mater. Confined Aquifer- Aquifer is between 2 layers of impermeable mater. Water is pressurized Wells are called artesian wells Unconfined Aquifer- Water is not confined on the top and bottom of saturated zone. Water can freely move to the surface if the water table rises. The level of the water in a well will be the same level as ground water level

Describe the difference of water flow with respect to the streams and the water table in the diagrams below Gaining streams occur when the surrounding groundwater is higher than the streams water level. Groundwater will flow into the stream and add water to the stream Losing streams occur when the groundwater is lower than the streams water level. Here we see the water flowing from the stream to the aquifer.

Confined groundwater may flow at the surface due to hydrostatic pressure.

Springs Water flows freely from the ground surface Springs Water flows freely from the ground surface. This happens when the water table intersects the ground surface.

Perched Aquifer

Observe a geyser animation along with an explanation of how a geyser occurs by clicking on Beehive geyser above.

Wells We use wells to remove groundwater. The water table goes down when water is pumped from a well. This is called a cone of depression.

Artesian – Groundwater that is under pressure rises above the level of the aquifer. One end of the CONFINED aquifer, must be able to receive water There must be aquitards above and below the aquifer. If the water does not rise to surface level we call this artesian well a non flowing artesian well. When a well is drilled into the aquifer and water rises to surface level or higher, then we have a flowing artesian well.

Over-pumping can be a real problem in areas of slow recharge. When the groundwater is not recharged at the same rate or faster than groundwater is removed, the water table is lowered, wells dry up, and there is an overall depletion of groundwater. Non-renewable resource in some areas – Can be referred to as groundwater mining. In the US there is a huge problem with over pumping in the Southwest and Southern California.

Subsidence When groundwater is over pumped, there is no longer the pressure of water between the pore spaces of unconsolidated material. The overlying material puts lots of weight on this material causing the grains to pack tightly together and the surface to collapse. Click on the picture to link to more information

Sink Holes http://ga.water.usgs.gov/edu/sinkholes.html

Florida Sinkhole

Cave

Cave

To see an animation of how a cave is formed, click on the image below

What are each of these features? B A C D

These are soda straws

What are each of these features? B A C D

B - Stalactites

What are each of these features? B A C D

C - Stalagmites

What are each of these features? B A C D

D - Column