Name __________________
Environmental Conservation
Permeability and Porosity Lab
Background: Hidden beneath the earth’s surface, filling the crevices and natural pore
spaces between particles of rock and soil lies a natural reservoir of water which is called
“groundwater.” As water from various sources and forms of precipitation seep into the
ground, it passes through various layers of soil and rock. Soil layers that allow the water to
move quickly are described as “permeable.” Certain soil types, such as clay, have small pores
between their particles and do not allow water to pass through as easily. These soils are
termed “impermeable.”
Porosity is defined as the percentage of the total volume of the soil that consists of pore
spaces. Mathematically, it is found through the equation:
Porosity (%) = Volume of pore space
Total holding volume
x 100
Permeability is a measure of how quickly water pass through, or permeates, the soil.
Permeability is dependent upon the size and shape of the soil particles, the amount of pore
space between the particles and whether or not the pore spaces interconnect. Sand and
gravel particles tend to have larger interconnected pore spaces and are both more
permeable and more porous than silt or clay.
Procedure:
1. Obtain 3 clear tubes and end caps. Place a cap at the bottom of each tube
2. Using a graduated cylinder, measure 125 mL each of fine sand, coarse sand, and
gravel into each tube.
3. Fill a graduated cylinder with water to the 100mL mark.
4. While one person slowly pours water into the tube, a second lab partner will time
how long it takes the water to reach the bottom of the tube.
5. The person pouring the water will stop pouring when the water level has just
reached the top of the gravel material in the tube. RECORD THE RESULTS!
6. Repeat steps 3.-5. with each gravel sample. RECORD THE RESULTS.
7. Lightly pinch shut the top of tube with the fine sand and tilt the tube to drain
out what water you can into a plastic cup. Be careful not to spill the sand into
the cup! Measure how much water was retained by the sand by subtracting the
water drained out from the initial volume added. RECORD THE RESULTS
8. Using the porosity formula to determine the porosity. The “volume of pore
space” was found by the amount of water the sample could hold when you filled
it to the top. The “total holding volume” is the volume of the sample you added,
which in this lab was 125mL.
9. Repeat steps 7 and 8 for the other samples. RECORD YOUR RESULTS
10. Permeability is expressed as the reciprocal of the wetting time (the number of
seconds it took from first pouring the water until the water reached the
bottom). For example, if the wetting time was 25 seconds, the permeability
measurement is 1/25, or 0.04. RECORD YOUR RESULTS.
Data Table:
Fine sand
Coarse Sand
Gravel
Total volume of soil in
tube (mL)
Water volume needed to
fill pores (mL)
Wetting time (sec)
Permeability
Water volume recovered
(mL)
Water volume retained
in tube (mL)
Bar graphs:
Permeability Bar Graph
Water Retention Bar Graph (mL)
0.20
50
0.18
45
0.16
40
0.14
35
0.12
30
0.10
25
0.08
20
0.06
15
0.04
10
0.02
5
0
0
Fine Sand
Coarse Sand
Gravel
Fine
Sand
Coarse
Sand
Gravel
Questions:
1.
Explain the relationship between soil particle size and the percent of pore space
(porosity).
2. What is the relationship between soil particle size and the volume of water
retained.?
3. What is the relationship between soil particle size and permeability?
4. How do porosity and permeability differ?
5. Explain how soil particle size can affect infiltration, runoff, and flooding during a
rainstorm.
6. What would be most effective in purifying polluted water, an aquifer composed of
fine sand, coarse sand, or gravel?