Porosity and Permeability

Date: 30/10/2020 Porosity and Permeability Lab The terms porosity and permeability are related. • Porosity – the amount of empty ('pore') space in a soil, rock or other Earth material; this empty space is known as pore space. Porosity is how much water a substance can hold within itself. Porosity is usually stated as a percentage of the material’s total volume. • Permeability – is how well water flows through a soil, etc. Factors that affect permeability are how large the pores are and how well the particles fit together. Water flows between the spaces in the material. If the pores are close together, such as in clayrich soils, water will tend to cling ('sorb') to the material and not pass through it easily or quickly. In contrast, if the pores are large and the material does not 'fit' together closely, such as in the gravel, the water passes through quickly (c.f., the cartoon below). There are two other terms that are used with water: percolation and infiltration. • • Percolation – the downward flow of water from the surface into soil or porous rock. Infiltration – when the water enters the soil surface after falling from the atmosphere. In this lab, we will test the porosity (we tested permeability previously) of gravel, silt, and clay-rich soil. 1 Porosity and Permeability Lab Hypotheses: Which material do you think will have the highest permeability (fastest time)? Gravel, as the shape of the material is larger, and they have less ability to fit together, and therefore have more space in between for water to pass through at a faster rate. Which material do you think will have the lowest permeability (slowest time)? Clay, as there is less space in between the material due to the fact the grains are more tightly compacted together resulting in less and slower permeability. Which material do you think will have the highest porosity (largest spaces)? Gravel, due to the large spaces in between the material, as seen in the diagram above as well. The shape of the material does not allow it to fit together, therefore there is more space. Which material do you think will have the lowest porosity (smallest spaces)? Clay, as the grains fit together due to their shape and size, allowing there to be limited space in between. 2 Porosity and Permeability Lab Materials 2-to-6x 2L bottles ('mesocosms') Volumetric container (beaker or measuring cup) Marker Timer or watch Calculator 3x diverse soils or porous materials Procedure for measuring porosity 1. 2. 3. 4. 5. 6. 7. 8. 9. Measure out 600 mL of water in the measuring cup. Pour the 600 mL of water into one of the mesocosms and use the marker to mark the level. Pour the water back into the measuring cup. Fill the same cup with gravel up to the mark from Step #2. Pour the 600 mL of water slowly into the gravel. Stop when the water level just reaches the top of the gravel. Record the amount of water left in the measuring cup (c.f., table below). Calculate the pore space by subtracting the amount left in the measuring from the original 200mL. Repeat steps 4-7 with the silt and clay-rich soils. Calculate the %porosity and record in the table. Use this formula: % porosity = volume of pore space (amount of water) volume of material Volume left in Cylinder (mL) x 100 Pore Space Volume (total volume – volume left) % porosity Soil types Total Volume (mL) Example 600 mL 65 mL 100 - 65 = 35 mL 35/100 x 100 = 35% Gravel 600 mL 390 mL 210 mL 35% Silt 600 mL 300 mL 300 mL 50% Clay-rich 600 mL 140 mL 140 mL 23% 3 Porosity and Permeability Lab Answer the following questions in complete sentences. 1. Which earth material had the highest porosity? Explain why. Silt had the highest porosity. This is because of the distribution of grains, and the presence of roots and other irregular shapes which a farmer might have added. Grains were not as compacted as we had thought, due to their differences in size and shape, allowing water to seep through. 2. Which earth material had the lowest porosity? Explain why. Clay had the lowest porosity. This was due to the shape of the grains and their ability to fit together and be tightly compacted (like Lego). This made it more difficult for water to move down as there was less space to do so. 3. In what type of soil would you recommend locating a drinking water well? Explain why. Silt, because it was discovered that it has the highest porosity. This type of soil would enable any rainwater or water that touches it to seep down and fill up the well at a faster rate than some other soils, due to the spaces in between the grains, resulting in it being more porous. 4