unit 4 lab
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Broward College *
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1000L
Subject
Geography
Date
Feb 20, 2024
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7
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Groundwater Lab –
Porosity and Permeability The exercises that you will complete today focus on the occurrence and movement of groundwater. The highest percentage of freshwater that we use, comes from beneath Earth’s surface. This water resource is extremely valuable –
in the United States alone it provides over 40% of the country’s freshwater supply, especially in very arid western states. You might imagine an underground river of groundwater, but in actuality, groundwater moves through openings in earth materials. The only exceptions to this are in caves, where openings are so large that water can move freely, and in lava, where lava tubes (tunnels evacuated by lava that flows beneath the surface) and extensive cooling joints allow for water to move rapidly. The terms porosity and permeability are related, but do not mean the same thing. Each of these terms also relate to the movement of ground water, how Earth materials store water and how ground water is mapped across areas. Porosity Openings in Earth material give it a porosity, which is defined as the volume of void space within a given material. Porosity is typically measured as a fraction or percentage, using the following formula: Porosity (%) = ?????? ?? ???? ??𝑎𝑐? (𝑐?
3
)
?????? ?? ?𝑎???? (𝑐?
3
)
(for a percent value multiply by 100) The shapes and sizes of pore spaces can vary widely. For example, gravel has pore spaces that typically range between 2mm and 75mm, while clay particles typically have a pore space of less than .002 mm. Activity 1: Calculate the porosity of the given samples in the table below.
Type of Material Volume of Sample (
𝒄𝒎
𝟑
)
Volume of Pore Space (
𝒄𝒎
𝟑
)
Porosity (%) Sandy Soil 210 89 42% Unsorted Subsurface Soil 500 100 20% Limestone (Solid Rock) 950 123 12% Shale (Solid Rock) 435 57 13% Procedure for Measuring Porosity Now, you will calculate the porosity of three samples in the laboratory setting. Materials (Porosity and Permeability) •
Three large beakers •
Marker •
Three graduated cylinders •
Calculator •
Three funnels •
Ring stand •
Timer •
Water •
Food Coloring •
Gravel •
Sand •
Clay Hypothesis Considering what you know about the three sample materials in this part of the experiment –
sand, clay and gravel, predict which of your samples will have the greatest porosity. Explain your reasoning in your answer. Procedure 1.
Measure out 100mL of water in a graduated cylinder. 2.
Add two drops of food coloring to the sample and rotate the container to mix it. 3.
Fill your first large beaker to the 150mL with sand. 4.
Very slowly!! Pour the water into the sand. Stop when the water level just reaches the top of the sand. 5.
Record the amount of water left in the graduated cylinder. Subtract this number from 100 and it will tell you the volume of pore spaces in the sample. 6.
Repeat steps 1-5 again with your gravel and clay samples. 7.
Make sure all data is recorded in the table below!
Sample Material Sample Volume (
cm
3
)
Final Water Volume (mL
)
Pore Volume Porosity Percent (P. Volume / 150mL x 100)
Sand 150 32 68 45% Gravel 150 40 60 40% Clay 150 15 85 57% Activity 1 –
Analysis Questions 1.
Describe the porosity of sand and gravel using your collected data. Water travels better through sand and gravel compared to clay where the water cannot travel down as easily as it did with the sand and gravel. There was more space between the water and the gravel and sand compared to the clay. 2.
Read ahead. In your own words, describe the difference between porosity and permeability. Then, describe why porosity and permeability both play an important role in the movement of groundwater. Porosity is the percentage of void space in a rock while permeability is the determination is the ability for fluids to flow through rocks. Porosity is more about the storage of water while permeability is movement and flow.
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Activity 2: Calculate the permeability of the given samples. Permeability Permeability is defined as how easily water flows through a given material. Factors that impact permeability are the size of pores and how well the pores are connected to one another, as well as grain size of sediments. Just because a material is porous, does not mean that it is also highly permeable. Two terms often associated with permeability are percolation, which refers to the downward movement of water from the land surface through the soil or porous rock, and infiltration, which refers to when water enters the soil surface after falling from the atmosphere. For this section of the lab, we will use mostly the same materials, but a timer is required in addition to what we used previously, because we are going to measure permeability as a function of time. Hypothesis Based on what you learned in the previous section, predict the permeability of the given materials in order from most permeable to least permeable. Procedure 1.
Insert 150 mL of sand into a funnel with a narrow opening. 2.
Insert 100mL of water into a graduated cylinder (with two droplets of food coloring). 3.
Insert a flask under the base of the funnel to catch water. 4.
Using a timer, pour the 100mL water sample from the graduated cylinder, directly into your 150mL sand sample and stop the timer when the first droplet of water reaches the flask. 5.
Enter the recorded time in your data table. 6.
Repeat steps 1-5 with gravel and clay samples. Data Sample Material Time (s) Sand 25.91 Gravel 7.71
Clay impermeable Analysis Questions 1.
Explain why the porosity of clay would be greater than sand, while being significantly less permeable. Clay has a greater surface area than sand and more water can remain in it while the water in sand travels through much easier. clay is impermeable because unlike sand, it prevents the flow of water. 2.
Aquifers are areas of underground layers of water-bearing permeable rock or unconsolidated materials. Aquitards are impermeable layers that restrict the flow of groundwater from one location to another. Based on your results, describe the materials that would likely make up aquifers and aquitards. Aquifers are made up of gravel sand or other permeable rocks while aquitards are made up of soil.
Activity 3: Florida State Comparison Below, we have provided you with a geological survey of Florida State. Carefully observe the map and what each area on the map represents. Activity Instructions: Using methodology learned from previous sections, you will conduct an experiment that shows the permeability of each of the areas represented in the map. Area Samples 1-4 should be represented by the following: 1.
Limestone 2.
Area 2 Sand Sample 3.
50% Sand/Clay Mixture 4.
Limestone, Dolostone, and Sand Mixture
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Conduct your experiment, collect data in an appropriately designed data table and then write a summary of the areas based on the permeability of your area data. Samples The time Area 1 1.85 Area 2 22.13 Area 3 Imperable 86.39 Area 4 8.36 \ Knowing how these areas' permeability affects water flow during hurricanes and rainfall and how flooding may result from it is crucial. This experiment indicates that compared to areas 1 and 4, areas 2 and 3 have a higher likelihood of flooding during rainy or hurricane seasons. I am from Miramar, which is area 1, and I have only ever experienced severe flooding once. It might be more difficult for water to flow out of or into the sewers due to the levels of permeability in these areas. Where the beaches are seems to be a more likely place for flooding. During severe weather, flooding is also a possibility in a portion of the Everglades.