Volcanoes

The major source of all fresh water drinking supplies in the United States is groundwater. Groundwater is stored underground in aquifers, and is highly vulnerable to pollution. Understanding groundwater processes and aquifers is crucial to the management and protection of this precious resource.

The vadose zone includes all the material between the Earth’s surface and the zone of saturation. The upper boundary of the zone of saturation is called the water table. The capillary fringe is a layer of variable thickness that directly overlies the water table. Water is drawn up into this layer by capillary action

The vadose zone has an important environmental role in groundwater systems. As with water, surface pollutants must filter through the vadose zone before entering the zone of saturation. Subsurface monitoring of the vadose zone can locate plumes of contaminated water, tracking the direction and rate of plume movement.

Large amounts of water are stored beneath the earth’s surface in aquifers. To be an aquifer, the stored water must be accessible at a usable rate. Aquifers consist of porous material such as sand, gravel, and fractured rock.

Aquifers can be confined or unconfined. Confined aquifers have non-porous layers above and below the aquifer zone. The non-porous layers hold water and restrict water movement. Such layers are referred to as aquitards or aquicludes. Clay soils, shales, and non-fractured, weakly porous igneous and metamorphic rocks are examples of aquitards. Sometimes a lens of non-porous material will be found in material that is more permeable. Water percolating through the unsaturated zone will be intercepted by this layer and will accumulate on top of the lens. This water is a perched aquifer. An unconfined aquifer does not have confining layers that retard water movement.

Some aquifers are confined under pressure. These aquifers are called artesian systems. Sufficient pressure results in free-flowing water, either from a spring or from a well. Water flow in an artesian well rises above the height of the recharge zone .

Water is continually recycled through aquifer systems. Groundwater recharge is any water added to the aquifer zone. Processes that contribute to groundwater recharge include precipitation, streamflow, leakage (reservoirs, lakes, aqueducts), and artificial means (injection wells). Groundwater discharge is any process that removes water from an aquifer system. Natural springs and artificial wells are examples of discharge processes.

Pumping water from a well causes a cone of depression to form in the water table at the well site. Overpumping can have two effects. It can cause a change in the groundwater flow direction. It also lowers the water table, making it necessary to dig a deeper well.

TASK 1

Use this internet site to compare two aquifers: http://wwwnj.er.usgs.gov/

1. Click on "groundwater"

2. Click on "list of water-level monitoring sites"

3. Click on the word map

4. Click on GLOUCESTER (county)

5. Scroll down and click on these two well sites: 150323, 151054

Record the following:

aquifer name
well depth
datum
period of record
extremes

6. Using the hydrographs for the 2 well sites compare the aquifer data for the same years of record,make a chart with these headings: year, maximum water level, minimum water level,determine the elevation of the maximum and minimum for each year you are comparing and record the values in your chart

7. For each well- is the average water level increasing, decreasing, or maintaining a consistent level?

8.What does this tell you about the water use in each aquifer?

TASK 2

Answer the following by doing research in your home community!

1. Where does the water that you use come from?

2. Which industry uses the most potable water?

3. Does it receive treatment to improve its quality before it reaches you?

4. Is it treated after you use it and return it to the system?

5. How much does your community spend on water treatment in a year?

6. How much water do you use in a day? A week? A month?

NAWQA info on the Apalachicola-Chattahoochee-Flint (ACF) River basin, which includes groundwater hydrology.

Movement of groundwater depends on rock and sediment properties and the groundwater’s flow potential. Porosity, permeability, specific yield and specific retention are important properties of groundwater flow.

Porosity is the volume of pore space relative to the total volume (rock and/or sediment + pore space). Primary porosity (% pore space) is the initial void space present (intergranular) when the rock formed. Secondary porosity (% added openings) develops later. It is the result of fracturing, faulting, or dissolution. Grain shape and cementation also affect porosity.

PERMEABILITY is the capability of a rock to allow the passage of fluids. Permeability is dependent on the size of pore spaces and to what degree the pore spaces are connected. Grain shape, grain packing, and cementation affect permeability.

SPECIFIC YIELD (S_y) is the ratio of the volume of water that drained from a rock (due to gravity) to the total rock volume. Grain size has a definite effect on specific yield. Smaller grains have larger surface areas. Larger surface areas mean more surface tension. Fine-grained sediment will have a lower specific yield than more coarsely-grained sediment. Sorting of material affects groundwater movement.Poorly sorted material is less porous than well-sorted material.

SPECIFIC RETENTION (S_r) is the ratio of the volume of water a rock can retain (in spite of gravity) to the total volume of rock.

Specific yield plus specific retention equals porosity (often designated with the Greek letter phi):

Porosity, permeability, specific yield, and specific retention are all components of hydraulic conductivity.

The definition of hydraulic conductivity (denoted "K" or "P" in hydrology formulas) is rate at which water moves through material. Internal friction and the various paths water takes are factors affecting hydraulic conductivity. Hydraulic conductivity is generally expressed in meters per day.

Note: Hydraulic Head = dh
(i.e., the difference in elevation of water table between two points.)

Hydraulic head (denoted "h" in hydrology formulas)is the name given to the driving force that moves groundwater. The hydraulic head combines fluid pressure and gradient, and can be though of as the standing elevation that water will rise to in a well allowed to come to equilibrium with the subsurface. Groundwater always moves from an area of higher hydraulic head to an area of lower hydraulic head. Therefore, groundwater not only flows downward, it can also flow laterally or upward.

The hydraulic gradient (I) is approximately the slope of the water table—in a simple unconfined water system (remember however, that confined aquifer systems can be more complex, in such systems, fluid pressure must also be considered).

REMEMBER: JUST BECAUSE POROSITY(n) IS HIGH DOESN'T MEAN HYDRAULIC CONDUCTIVITY (K) WILL BE HIGH! For example, clay has a high n, but a low K (because it has very small pores).

Water table contour lines (or flow lines) are similar to topographic lines on a map. They essentially represent "elevations" in the subsurface. These elevations are the hydraulic head mentioned above.

Water table contour lines can be used to tell which way groundwater will flow in a given region. Lots of wells are drilled and hydraulic head is measured in each one. Water table contours are drawn that join areas of equal head (like "connect-the-dots"!). These water table contours lines are also called equipotential lines. The map of contour lines is called a flow net. Remember: groundwater always moves from an area of higher hydraulic head to an area of lower hydraulic head, and perpendicular to equipotential lines.

TAKE A LOOK
at some flow net sketches that will help clarify the relationships between aquifer matrix, and groundwater movement.

In 1856, Henry Darcy studied the movement of water through porous material. He determined an equation that described groundwater flow. The following description tell how Darcy determined his equation:

A horizontal pipe filled with sand is used to demonstrate Darcy’s experiment. Water is applied under pressure through end A, flows through the pipe, and discharges at end B. Water pressure is measured using piezometer tubes (thin vertical pipes installed at each end of the horizontal pipe). The difference in hydraulic head (between points A and B) is dh (change in height). Divide this by the flow length (i.e. the distance between the two tubes), dl, and you get the hydraulic gradient ( I ).

The velocity of groundwater is based on hydraulic conductivity (K), as well as the hydraulic head (I). Therefore, the equation determined by Darcy to describe the basic relationship between subsurface materials and the movement of water through them is Q = KIA where Q is the volumetric flow rate (or discharge) and A is the area that the groundwater is flowing through. This relationship is known as Darcy’s law.

Now, rearrange the equation to Q/A = KI, which is known as the flux (v), which is an apparent velocity. Actual groundwater velocity is lower than that determined by Darcy, and is called Darcy Flux (vx)

Darcy’s law is used extensively in groundwater studies. It can help answer important questions such as what direction an aquifer pollution plume is moving in, and how fast it is traveling. (SEE Module 8 - Water Pollution)

TASK 3

To help you understand Darcy’s law and Darcian velocity, try the following:

This requires an outside faucet, 5-gallon bucket, 100’ garden hose (a shorter hose will probably work), and stopwatch (or watch or clock).

Using just the faucet, turn the water on full force and fill the bucket.
Time how long it takes to fill the bucket.
Follow this procedure at least 3 times, then determine the average time it took to fill the bucket.
Using the garden hose (attached to the faucet), turn the water on full force and time filling the bucket.

Was there a difference in the rate of flow? What should happen: using a hose to fill the bucket takes longer! The water is following a restricted path, therefore takes longer to get from the source to the reservoir. In an aquifer, water follows tortuous paths around grains as it moves through the system. Hence, the Darcian velocity is less than the true velocity.

Do you have indoor plumbing and a water heater (Think about the implications of this question)? If so, note where the water heater is located. Consider it the source of your water. Is your bathtub or shower located near the water heater or at the opposite end of the house? The water pressure will be greater near the source. You will also get hot flowing water more quickly. The water’s pathway has a major effect on how fast it moves and how soon it is usable. I hope that these examples help you understand Darcy’s law and the Darcian velocity.

TASK 4

Apply what you know and learn how to use Darcy's law by completing the following calculations:

Turn to page 270 of your text book, (Environmental Geology, E.A. Keller). Work through the problem, then change some of the values and work the problem again.

1. Change porosity from 30% to 15%. How does the change affect the result?

2. What if the difference in elevation between well 1 and well 2 was 10 meters (Water surface elevation of Well 1 is 107m)?

Surface streams have an effect on the groundwater table. Influent streams recharge groundwater supplies. Influent streams, located above the groundwater table, flow in direct response to precipitation. Water percolates down through the vadose zone to the water table, forming a recharge mound.

Effluent streams are discharge zones for groundwater. Effluent streams are generally perennial (flow year round). Groundwater seeps into stream channels, maintaining water flow during dry seasons.

The Big Lost River in Idaho is a good example of an intermittent, ephemeral influent stream. Natural flow of the Big Lost River terminates in the Big Lost River Sinks, located on the INEEL. But, local irrigation now diverts the Big Lost River from its natural terminus.

See Module Preview for initial information and a Powerpoint Presentation.

Terms for Understanding

artesian aquifer
aquifer
cone of depression
confined aquifer
Darcy's Law
discharge
effluent stream
groundwater
hydraulic conductivity
hydraulic gradient
hydraulic head
infiltration
influent stream
overland flow

perched aquifer
permeability
pores
porosity
recharge
residence time
soil water
specific retention
specific yield
unconfined aquifer
vadose zone
water table
water table contour lines

Required Tasks (tasks are found throughout module)

TASK 1: use this internet site to compare two aquifers: http://wwwnj.er.usgs.gov/
1. Click on "groundwater"
2. Click on "list of water-level monitoring sites"
3. Click on the word map
4. Click on GLOUCESTER (county)
5. Scroll down and click on these two well sites: 150323, 151054
Record the following: aquifer name; well depth; datum; period of record; extremes
6. Using the hydrographs for the 2 well sites compare the aquifer data for the same years of record,make a chart with these headings: year, maximum water level, minimum water level,determine the elevation of the maximum and minimum for each year you are comparing and record the values in your chart
7. For each well- is the average water level increasing, decreasing, or maintaining a consistent level?
8.What does this tell you about the water use in each aquifer?
TASK 2: Answer the following by doing research in your home community!
1. Where does the water that you use come from?
2. Which industry uses the most potable water?
3. Does it receive treatment to improve its quality before it reaches you?
4. Is it treated after you use it and return it to the system?
5. How much does your community spend on water treatment in a year?
6. How much water do you use in a day? A week? A month?
TASK 3:To help you understand Darcy’s law and Darcian velocity, try the following: This requires an outside faucet, 5-gallon bucket, 100’ garden hose (a shorter hose will probably work), and stopwatch (or watch or clock). Using just the faucet, turn the water on full force and fill the bucket. Time how long it takes to fill the bucket. Follow this procedure at least 3 times, then determine the average time it took to fill the bucket.Using the garden hose (attached to the faucet), turn the water on full force and time filling the bucket.
TASK 4: Apply what you know and learn how to use Darcy's law by completing the following calculations: Turn to page 270 of your text book, (Environmental Geology, E. A. Keller). Work through the problem, then change some of the values and work the problem again. 1. Change porosity from 30% to 15%. How does the change affect the result? 2. What if the difference in elevation between well 1 and well 2 was 10 meters (Water surface elevation of Well 1 is 107 m)?

E-mail your tasks to: hughscot@isu.edu

Referenced Web Sites

http://www.groundwater.org/GWBasics/gwbasics.htm
http://www.groundwater.org/
http://wwwnj.er.usgs.gov/
http://www.eardc.swt.edu/
http://www.edwardsaquifer.net/
http://www.tnrcc.state.tx.us/EAPP/
http://www.meto.umd.edu/~owen/CHPI/IMAGES/EA-location.html
http://gisdasc.kgs.ukans.edu/dasc/snapshots/glacial.html
http://wwwga.usgs.gov/nawqa/basinall.html

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