Can Water Get Through This?
Objective: Compare the porosity and permeability of different substrates to determine their effect on movement of water through the soil and ground.
Background: Ground water is the water stored in the pores, cracks and openings of subsurface rock material and between the particles of sand and gravel below the surface. Ground water is one of Earth’s most valuable natural resources.
An aquifer is the layer of sand, gravel or rock that is capable of storing or transmitting water. The greater the permeability of an aquifer, the greater the volume of water that can be obtained from that aquifer over a given period of time.
Permeability is the ability of rock materials to transmit water. Different materials have different permeabilities. Porosity is the water-holding capacity to ground materials. Water must be able to flow through these pore spaces to be available for use.
Porosities of unconsolidated deposits depend on the range in grain sizes (sorting) and on the shape of the rock particles. Areas of well-sorted materials tend to have the highest porosities because they lack smaller particles to fill in voids and pres. Areas of poorly sorted materials have less available space between particles for storing water because they contain a wider range of particle sizes. So….porosity depends not on the absolute size of particles of the aquifer material but more the degree of sorting (range of particle sizes). Porosity can also be affected by the angularity of the material, especially in unconsolidated particles, and by the crystalline structure of the materials that compose a rock formation of consolidated materials. For example: limestone deposited.
Aquifers come in all shapes & sizes. Some aquifers cover hundreds of square miles and are hundreds of feet thick. Water quality and quantity vary from one aquifer to another and sometimes, within one system. Some aquifers yield millions of gallons of water per day and maintain adequate water levels, whereas others may be able to produce only small amounts of water each day. In some areas, wells have to be drilled thousands of feet to reach usable water, whereas, in other areas, water can be located only a few feet below the surface. One site may contain several aquifers located at different depths, whereas another site may yield little or no ground water.
The age of the ground water varies from aquifer to aquifer. For example, an unconfined surface aquifer might hold water that has been there only a few days, weeks or months. A deep aquifer that is covered by impervious layers may contain water that has been confined there for hundreds or thousands of years.
The rate of movement of ground water varies, according to the rock material in the formation through which the water is moving. The term water table refers to the top level of water collected below the ground. When water percolates down to the water table, it becomes ground water and starts to move slowly down the gradient. Water movement responds to differences in energy levels. The energies that cause ground water to flow are expressed as gravitational energy and pressure energy. Both are forms of mechanical energy. Gravitational energy comes from the difference in elevation between the recharge area (where water enters the ground water system) and the discharge area (where water leaves the system). Pressure energy (hydraulic head) comes from the weight of overlying water and earth materials. Gravity pulls ground water toward the area of least resistance. When ground water encounters a semi-impervious material such as clay, its movement is slowed significantly. When it is moved toward an open area such as a lake, water’s rate of movement will increase.
Hydrogeologists know that the above variable exist and that to really “get the ground water picture,” they must drill wells. Wells dug by hand or machine have been used throughout history to retrieve water from the ground. Wells provide the best method of learning the physical, hydrologic, and chemical characteristics of an aquifer. As a well is drilled deeper and deeper into the ground, the drill passes through different rock formations. The driller records the exact location of the well, records the depth of each formation, and collects samples of rock material that has been penetrated (sandstone, sand, clay, etc). These data become part of the well’s record, or well log. The well log becomes valuable information for determining ground water availability, movement, quantity, and quality. The well driller then caps and seals the well to protect it from contamination.
If hazardous wastes such as chemicals, heavy metals, or oil collect on the surface of the ground, then rain or runoff percolating into the soil can carry these substances into the ground water. When hydrogeologists or water-quality specialists analyze the quality of ground water, they consider land-use practices in the watershed and in the vicinity of the well. They also look at the characteristics such as permeability and porosity.
Part
1—Creating a Well Log:
1. Each
person needs a numbered paper strip and a copy of the “Well Log Data
Chart”. The paper strip represents
the depth of a well that has been dug.
Looking at the data about the location and type of rock materials
in your well, you will make a well log.
2. Divide
your strip into 12 1-inch sections.
3. Mark
the level of the water table by drawing a double line at the appropriate
point for your well number on the well log data chart. Draw a bold double line at the
appropriate point. This point
corresponds with the level found in the first column of the “Well Log Data
Chart”.
4. In the second column, find the level of fine sand. Measure this from the top of your well log column. Speckle this with dots to show fine sand.
5. Continue creating the well log, using the “Well Log Data Chart”. Use the same key to record the different rock types until you have completed all rocks on your well log.
6. Complete the drawing by coloring (light blue) the area between the water table and the top of the clay layer. Also color the gravel layer blue.
7. Fill
in the log using the number on the strip of paper and the information in
the “Well Log Data Chart”. If a
strip is labeled “6”, the student uses data from Well Number 6. Make sure to note the land use that
exists above the well site.
Questions—answered individually for your individual well log:
- The horizontal scale of the cross section is 1 inch = 1 mile. The vertical scale is 1 inch = 50 feet. How many miles are horizontally represented in the cross section? How many feet are vertically represented in the cross section?
- How many feet below the surface is the water table?
- Describe a drop of water’s movement down the column. Through which layers would it move the fastest? The slowest?
Part 2—Creating a Well Log Ground Water Chart (Cross Section)
- Assemble all well logs and tape them to the wall. Compare this cross section to the one attached to the back of this lab.
- Find the locations of each part of the ground water system in the well log cross section using the given well log definitions.
- What direction does the ground water move in the unconfined aquifer?
- What are water sources for the unconfined aquifer?
- How long would it take the water in the sandstone formation to move from Well Number 1 to Well Number 15? Assume the water moves at a constant rate and flows at 100 feet per day (1 mile = 5,280).
- At which layer might the drop’s movement be restricted? Remember—only a slight amount of water would pass through the clay. Speculate the source of water beneath the clay level (in the gravel layer).
- A cone of depression results from water being drawn up the well. Locate the cone of depression of the “Well Log Ground Water Chart (Cross Section).
- What are possible sources of water in the confined aquifer portion of your well?
- If you had to drill a well, which sites would you consider to be most favorable on the “Well Log Ground Water Chart (Cross Section)?