Materials Included In Kit

Additional Materials Required

Prelab Preparation

Activity B. Groundwater Simulation Model

1. Cut the clear plastic tube into six 6-inch pieces.
2. Cut the cheesecloth into six 1-inch pieces. Excess cheesecloth has been given for future use.
3. Obtain the three 6" pieces of clear tubing. Place a 1" x 1" piece of cheesecloth over the end of each tube and rubber band to secure them (see Figure 12).
   {12844_Preparation_Figure_12}
4. Tape the tubes to the plastic container as shown in Figure 13. Tubes 1 and 3 should be 1" from the bottom of the container and taped to the front corners of the container. Tube 2 should be ¼" from the bottom of the container and taped to the front face of the container.
   {12844_Preparation_Figure_13}
5. Cut the sponges into 1" x 1" pieces.

Activity C. Permeable Reactive Barriers

1. Prepare 20 ppm methylene blue solution: Add 1 mL of 1% methylene blue solution to 500 mL of distilled or deionized water in an Erlenmeyer flask or beaker. Stir the solution with a stirring rod to obtain a uniform concentration.
2. Prepare 20 ppm indigo carmine solution: In a 250-mL beaker, dissolve 0.25 g of indigo carmine in 100 mL of distilled or deionized water. Dilute 4 mL of this 0.25% solution to 500 mL with water in a 500-mL Erlenmeyer flask to obtain a 20 ppm solution. For best results, prepare this solution fresh the day of use.

Safety Precautions

Iron powder is a possible fire and explosion risk. Keep away from flames, sparks and other sources of ignition. Avoid breathing fine metal dust. The dye solutions will stain skin and clothes. Wear safety glasses or chemical splash goggles whenever working with chemicals, heat or glassware in the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. For Activity C, filter the heterogeneous reaction mixtures through a funnel to separate the iron powder. The iron powder and all other solids in this kit may be disposed of in the solid trash according to Flinn Suggested Disposal Method #26a. All solutions may be disposed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• In Activity A, sand and gravel samples may be dried and saved for additional tests.
• Have students perform the Acitivity A experiments again using their own soil samples or soil that has been obtained from the school grounds.
• In Activity B, the tubes and syringes should be reused each time this activity is performed.
• Perform Activity B near a sink or use a waste container disposal for all of the extracted water solutions.
• Students should wear gloves when working with the dye solutions in Activity B.
• In Activity B, students may indeed collect dye pollution solutions from tubes 2 and 3. This indicates a leaking barrier. This would be a good point to discuss the importance of restrictive barriers.
• In Activity C, filter the mixtures as described in the Disposal section and rinse with distilled water in between each activity or have students place all mixtures into one container and filter after all activities have been performed.
• Permeable reactive barriers are a passive technology, relying on the natural flow of water underground to clean groundwater. To illustrate the passive nature of PRBs, set up a series of three capped test tubes containing methylene blue solution. Add iron to the test tubes in sequence over a 3-day period.
• Methylene blue and indigo carmine are used in classic demonstrations, the blue-bottle reaction and the “stop-and-go” light, respectively, to illustrate reversible oxidation–reduction reactions. Other redox indicators that give interesting color changes include resazurin and dichloroindophenol.

Further Extensions

Activity A. Permeability and Porosity

• Additional topics, such as soil profiles, soil analysis, soil conservation and erosion, may be discussed with the students at this time. Flinn Scientific has many other kits that cover a wide range of soil topics.

Activity C. Permeable Reactive Barriers

• See a “Citizen’s Guide to Permeable Reactive Barriers” published by the U.S. Environmental Protection Agency for more information about this innovative technology. The publication may be downloaded from the EPA website at http://www.epa.gov/tio/pubitech.htm (accessed July 2009).

Alignment with AP^® Environmental Science Topics and Scoring Components

Topic: Earth Systems and Resources. Global Water Resources and Use (Freshwater/saltwater; ocean circulation; agricultural, industrial, and domestic use; surface and groundwater issues; global problems; conservation).
Scoring Component: 2-Earth Resources, Use.

Sample Data

Activity A. Permeability and Porosity

Activity B. Groundwater Simulation Model

When the water was poured onto the surface of the final layer of sand, the dye solutions/pollution began to seep toward the bottom of the container. However, the dye solutions did not make it past the clay layer. The water extracted from syringe 1 was slightly red. The water extracted from tubes 2 and 3 were colorless. The water in the cup/pond became lower each time water was drawn from the container.

Activity C. Permeable Reactive Barriers

The methylene blue solution went from its original blue color to colorless after the bottle was shaken. The indigo carmine solution started out blue, changed to green, and then finally to yellow. The original blue color of both solutions returned after the bottles were allowed to sit for 15 minutes.

Answers to Questions

Activity A. Permeability and Porosity

1. Define porosity and permeability. How do they compare?

   Porosity is the total volume of air and water soil can hold. Permeability is the ease in which water and air can move through soil. In general, the higher the porosity the faster the permeability will be.

2. Use Equation 1 from the previous page to calculate the porosity of each soil sample.

   Percent porosity = pore space volume/total volume of soil x 100
   i.e., for sand 29.8 mL/100 mL x 100 = 29.8%
   Sand ___29.8%___
   Gravel ___38.6%___

3. What is the relationship between the porosity and the grain (particle) size of each soil sample? Porosity increases when the size of the particles increases.
4. Which type of soil retained more water? Why?

   The sand retained the most amount of water. The smaller the particle size of soil particles, the higher the water retention.

5. Calculate the permeability of each soil type using the following equation:

   Permeability = 1/Initial time for water to reach bottom of tube
   Sand ___0.04___
   Gravel ___0.25___

6. What is the relationship between the permeability and the grain (particle) size of each soil sample?

   Permeability increases when the size of the particles increases.

7. Which soil type tested in this activity would cause the most water runoff? The least?

   The sand would promote the highest amount of runoff because of the small particle size. The gravel would cause the least amount of runoff.

Activity B. Groundwater Simulation Model

1. Where is the aerated zone located in the assembled groundwater model? The top layer of sand in the model best defines the aerated zone.
2. Where is the saturated zone located in the assembled groundwater model? The saturated zone was located in the lower gravel surface of the container.
3. What portion of the model best defines a confined aquifer?

   The portion of the model below the clay layer best defines a confined aquifer.

4. What type of groundwater withdrawal was performed when the syringe was used to remove the water from the model?

   Active withdrawal. The water in the container was physically withdrawn from a “well.”

5. Describe what happened to the water in the pond during this activity.

   The water level in the medicine cup became lower each time water was withdrawn from the container. Eventually, the pollution from both the leaking barrel (red dye solution) and the fertilizer from the farm (blue dye solution) would reach the pond.

6. Would the leaking barrel (sponge) in this model be considered a point or non-point pollution source? What about the fertilizer runoff from the farm?

   The leaking barrel (sponge) is a point pollution source. The widespread fertilizer from the farm is a nonpoint pollution source.

7. Compare and contrast the water withdrawn from tubes (wells) 1, 2 and 3.

   Detail any contamination that occurred near the three tubes. Contamination was seen from well 1. Tubes 2 and 3 were not affected by the pollution due to the protection offered by the clay layer.

Activity C. Permeable Reactive Barriers

1. Describe the color and appearance of the methylene blue and indigo carmine solutions before and after mixing with iron powder. What are the colors of the oxidized form and the reduced form of each dye? Why does the original color of the dye solution gradually reappear upon standing?

   The methylene blue solution changed from blue (oxidized form) to colorless (reduced form) after mixing with the iron powder for about 5 minutes. Indigo carmine went through a series of color changes, starting out blue (oxidized form), then green and finally yellow (reduced form). The original colors of the dye solutions gradually reappeared upon standing due to re-oxidation of the reduced forms by reaction with oxygen in air.

2. What characteristics of an organic redox indicator allow it to function as a model substrate for the reduction of pollutants using metallic iron?

   Redox indicators exist in at least two different oxidation states or forms that are different colors. The color of the indicator therefore provides a visible “clue” that the substrate has been reduced by reaction with metallic iron. This is a good model for the environmental fate of pollutants.

3. Predict how (a) the “mesh” or grain size of the iron particles and (b) the concentration of a pollutant in contaminated groundwater would affect the performance of a permeable reactive barrier.
   1. Reducing the size of the iron particles in a PRB will increase the surface area of the reactive metal, which should increase the rate of the heterogeneous reaction with any possible contaminants in the groundwater.
   2. As the concentration of a pollutant increases, the rate of the reaction with iron should also increase until all of the surface sites on the iron are occupied.

References

Activity C, Permeable Reactive Barriers, was adapted from Chemistry in the Environment, Flinn ChemTopic™ Labs, Volume 22; Cesa, I., Editor; Flinn Scientific: Batavia, IL (2006).

Introduction

Explore permeability, porosity, permeable reactive barriers, water pollution and other groundwater-related concepts in this series of activity-station based activities.

Concepts

• Permeability
• Porosity
• Groundwater
• Water purification
• Permeable reactive barriers
• Point vs. nonpoint pollution

Background

Activity A. Permeability and Porosity

Soil is an important natural resource. By providing both structure and nutrients for plant growth, healthy soil ensures a bountiful and healthy food supply for life on Earth. Soil is also a vital component of the hydrologic (water) cycle. Soil acts as a natural filter, adsorbing chemicals that may be applied to the soil or incorporated into the soil from other sources. “Chemical waste” that may be processed by the soil includes fertilizers, herbicides and pesticides, biological and agricultural waste products, and industrial waste chemicals. The ability of soil to protect against runoff and groundwater contamination depends on the mixture of particles in the soil, its pH and oxygen content, the amount of organic matter, and on the presence of microorganisms.

Often thought of as “just dirt,” soil is actually a complex mixture of inorganic minerals and organic matter, as well as air, water, and even biological organisms. As a rough estimate, only about 50% of soil consists of “solids” (inorganic and organic substances)—the rest is air and water.

The size of the particles in the soil is a major factor that influences both the amount of air in the soil (aeration) and the capacity of soil to retain water. The volume of air and water that soil can hold is known as the soil pore size or porosity. The larger the soil particles, the larger the soil pore size will be (see Figure 1). The reverse is also true—the smaller the soil particle size, the smaller the pore size.

The percent porosity of soil is measured using the following equation. Water tends to drain more rapidly through larger soil pore size than small pores. As water runs through any type of soil, it pulls small amounts of air along with it. When water enters soil that has a small pore size, the air fills the pores or voids in the soil. As the small pore spaces are filled, the soil holds or retains a greater amount of water. This is why it is important to have a good mixture of different types of soil for plant growth. A combination of large and small pores provides both better aeration and water retention in soil.

Permeability is another key characteristic of soil. Permeability is the relative ease in which water and air can move through soil. Water flows through soils with high permeability very easily. Soils with low permeability allow much less water flow or drainage. Soils that have high permeability can be pictured as being loose and soils with low permeability can be thought of as being tight or compacted.

Permeability decreases when soil becomes saturated with water. Saturation of soil and high levels of introduced water (rainfall for example) lead to runoff of water. Runoff is water that is not absorbed by the soil and flows to lower ground, eventually draining into streams, lakes, rivers, and other bodies of water. Excessive amounts of water runoff can cause severe flooding, which can lead to extensive property damage.

Activity B. Groundwater Simulation Model

At the molecular level, water is engaged in an endless cycle; from streams, to lakes, to rivers, to oceans, then to be evaporated, carried aloft and returned to the surface as precipitation. The water cycle is one of the major biogeochemical cycles upon which all living things on our planet depend.

When rain falls on the ground where does it go? To put it simply, some of it evaporates, some runs off to streams, and some soaks into the soil. Some of what soaks into the soil is taken up by plants, transpired, and returned to the atmosphere. The remainder continues to percolate down through the soil and becomes groundwater. As this water percolates downward it passes first through what is called the aerated zone. The aerated zone is characterized by having mostly open pore spaces with some residual water held by surface tension. Water continues down through the aerated zone to the saturated zone—where all of the pore spaces are completely full of water. The upper boundary of the saturated zone is known as the water table (see Figure 2). The lower boundary of the saturated zone is usually an impermeable layer of rock or clay preventing further downward percolation of the water. The saturated zone can be likened to a flowing underground reservoir and is commonly called an aquifer. Where the land surface falls below the level of the water table, the aquifer will be visible as a lake, pond or stream. An aquifer in which water is confined within upper and lower earth layers or manmade barriers is called a confined aquifer. The flow of water in these underground systems is driven by gravity and will be in the same general direction as that of the surface waters. Rates of flow may range from millimeters to meters per day.

Groundwater provides one-fifth of all the freshwater used in the United States and one-half of the drinking water. In some regions of the country it provides more than one-half of the freshwater used for crop irrigation, industrial processes and livestock maintenance. The importance of groundwater as a major source of fresh and potable water cannot be overstated.

Withdrawal of groundwater can occur by either active or passive means. Active withdrawal requires drilling wells to the depth of the water table or saturated zone and pumping the water to the surface. Passive withdrawal simply requires tapping a free-flowing spring—where the pressure of water trapped between impermeable layers of rock or clay is sufficient to force it to the surface. As most groundwater systems are in a state of dynamic equilibrium, with water flow into the system ultimately equivalent to water flow out of the system, any significant withdrawal is going to alter that equilibrium. If withdrawal remains steady at a low enough rate the result will be a new equilibrium at a lower water level. If withdrawal exceeds a certain rate, and continues to increase, the result is a condition known as overdraft. Under overdraft conditions wells will run dry, land may subside as it settles to fill the void left by the water, and in coastal areas saltwater may intrude into the aquifer. Any of these conditions can have severe consequences.

Groundwater naturally contains microorganisms (decomposers naturally present in the soil), gases produced by metabolic processes and decomposition and dissolved organic and inorganic compounds. Groundwater is by no means pure and all of the basic properties used to describe water are due to naturally present constituents. Hardness is a measure of the levels of calcium and magnesium ions; salinity is defined by the quantity of dissolved salts; and color, taste and odor are caused by a wide variety of compounds.

Water quality is an abstract concept that relates the suitability of water to a particular use. Water being considered for a particular use is subjected to a battery of tests to measure concentrations and levels of a number of constituents and properties. To “pass” these tests the results must fall within defined parameters—or else the water may be judged unsuitable. As an example, water that is too salty is unfit to drink and would be considered contaminated. Drinking water with excessive levels of lead, mercury or pesticide residues would also be considered contaminated. These examples illustrate that contaminants can be either natural or caused by man.

Major sources of non-natural groundwater contaminants vary regionally. A few of the most recently cited include: waste from over-applied agricultural and domestic fertilizers, pesticides, sewers, landfills, septic systems, industrial wastewater lagoons, leakage from petroleum transport and storage systems, chemical spills, illegal dumping and highway de-icing salts.

A contaminant follows the same route to the aquifer as the water itself. Percolating down through the soil, the contaminant reaches the saturated zone and enters the normal flow-pattern. The contaminant will move downstream, in most cases tending to fan out and form a plume. As such, contaminant concentration is greatest nearest its source, decreasing as it radiates downstream and away. The contaminant may arise either from a discreet, localized, “point” source or from a dispersed nonpoint source. An example of a point source would be a leaking pipe or compound dumped down a well. An example of a nonpoint source would be an agricultural chemical spread over many acres.

Once entering the soil numerous possible fates await contaminants. The contaminant may form insoluble precipitates with soil constituents and be rendered harmless. The contaminant may be adsorbed onto various substrates present in the soil and spread no further. Or the contaminant may be biologically or chemically degraded, converted or decomposed. The contaminant may also be either diluted to harmless levels or mechanically filtered out as it passes through the soil. Any one, or none, of these processes may take place to offset the potential harm caused by the contaminant.

Detection and treatment of contaminants can be understandably very difficult. Aquifers may run tens, hundreds or thousands of feet below the surface making extensive testing and monitoring of the water challenging and extremely costly. There may be no sign or indication of potential sources when and where a contaminant is detected, necessitating painstaking procedures to trace it. If the interval of distance or time between contaminant detection and its source is too great, tracing it may be impossible. Testing must also be conducted carefully to determine the boundaries of the contaminant plume and the affected area. Also, random testing may not analyze for a particular contaminant that might be present.

Activity C. Permeable Reactive Barriers

Since 1980, when Congress passed the first “Superfund” legislation to identify and clean up hazardous waste sites across the country, scientists and engineers have developed many innovative methods to remove contaminants from soil, surface water and groundwater. Permeable reactive barriers (PRBs) are a good example of new technology that was created to solve environmental problems. A PRB is a wall built below ground to remove pollutants from contaminated groundwater. The walls are permeable, so water will flow through, but are made of reactive materials that will trap or detoxify pollutants. PRBs made of metallic iron are used to remove chlorinated organic solvents and heavy metals from groundwater. The chemical principle is simple—iron is a good reducing agent. It reduces toxic organic compounds and converts them to less harmful substances. The reaction of iron powder with organic redox indicators (dyes) demonstrates the “potential” of this method to reduce toxic organic compounds.

The first full-scale permeable reactive barrier (PRB) was built in 1994. Since then, PRBs have been installed at more than 50 hazardous waste sites in the United States and Canada. PRBs are installed underground, beneath the water table, to clean up contaminated groundwater (see Figure 3). A barrier is built by digging a long, narrow trench and installing the reactive material in the natural flow path of the polluted groundwater. The advantages of PRBs for cleaning up groundwater are that they do not require pumps or expensive machinery, there are no energy costs to operate the barriers, and the process does not generate additional waste that would need to be disposed of in a landfill or by incineration. There are three major classes of PRBs. Barriers are designed to (1) trap pollutant chemicals by adsorption, using charcoal; (2) precipitate dissolved pollutants or ions, using limestone; or (3) react with toxic chemicals and convert them into less harmful substances. Metallic (zerovalent) iron is the most important reactive chemical used in the third class of PRBs. Iron is inexpensive, readily available and a good reducing agent, capable of reducing a wide range of organic and inorganic compounds in high oxidation states. So-called “iron walls” are commonly used to remediate groundwater contaminated with chlorinated organic solvents, such as trichloroethylene and perchloroethylene (dry-cleaning solvents) and are also effective for removing pesticides, nitrates, and chromates from water. The detoxification of trichloroethylene, a known carcinogen, occurs via a sequence of two-electron reductions and loss of chloride ions. The ultimate product is ethylene, which is easily biodegraded (Equation 2). In this activity, redox dyes are used as model substrates to illustrate the ability of metallic iron to reduce organic compounds. The organic dyes are redox indicators that exist in two oxidation states, having different colors. The structures of the oxidized and reduced forms of methylene blue and indigo carmine, along with their colors, are shown in Figure 4.

Experiment Overview

The purpose of this “activity-stations” lab is to investigate and explore concepts related to groundwater. There are three activity stations set up around the lab. Each activity focuses on a different aspect related to groundwater and is a self-contained unit, complete with background information and discussion questions. The activities may be completed in any order.

Activity A. Permeability and Porosity
Activity B. Groundwater Simulation Model
Activity C. Permeable Reactive Barriers

Materials

Safety Precautions

Iron powder is a possible fire and explosion risk. Keep away from flames, sparks and other sources of ignition. Avoid breathing fine metal dust. The dye solutions will stain skin and clothes. Wear safety glasses or chemical splash goggles whenever working with chemicals, heat or glassware in the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Activity A. Permeability and Porosity

1. Obtain the tube with one end closed.
2. Using a graduated cylinder, measure out 100 mL of sand and place it in the tube.
3. Measure 100 mL of water into a graduated cylinder. This will be the initial amount of water.
4. Start a timer (use a stopwatch or watch with a second hand) and slowly pour water from the graduated cylinder into the tube until the sand is saturated (water reaches the bottom of the sand) (see Figure 5).
   {12844_Procedure_Figure_5}
5. Record the amount of time it takes the water to reach the bottom of the tube in the data table under Initial Time.
6. Measure and record the amount of water remaining in the graduated cylinder in the data table.
7. Subtract the amount of water remaining in the graduated cylinder from the initial volume of water (100 mL). The difference is equal to the volume of the pore spaces in the sand. Record the pore space volume in the data table.
8. Empty the graduated cylinder.
9. Pinch the tube and pour the water retained in the sand from the tube into the empty graduated cylinder. Be sure not to pour any of the sand into the graduated cylinder. Record this amount of water as Water Drained from the Tube in the data table.
10. Subtract the Water Drained from the Tube from the Pore Space Volume of the sand. This value will be the Volume of Water Retained. Record this value in the data table.
11. Repeat steps 1–10 using the gravel sample. Record all data in the data table.

Activity B. Groundwater Simulation Model

1. Place ½" to ¾″ of gravel on the bottom of the plastic container. This layer represents a layer of bedrock (see Figure 6).
   {12844_Procedure_Figure_6}
2. Place sand on top of the gravel layer as shown in Figure 7.
   {12844_Procedure_Figure_7}
3. Pour water into the model until the gravel layer is completely surrounded by water and the sand layer is also saturated. Make sure that there is no standing water on top of the sand layer.
4. Roll out a piece of clay large enough to cover the surface of the sand layer. Place this layer of clay on top of the sand layer (see Figure 8). Tightly pack the clay around tubes 2 and 3 and around the edges of the plastic container. The bottom of tube 1 should be above the clay layer.
   {12844_Procedure_Figure_8}
5. Pour a small amount of water on top of the clay and record all observations in the data table.
6. Obtain a plastic medicine cup and a pushpin. Poke 10 holes around the sides and/or bottom of the paper cup.
7. Place the medicine cup into the plastic container as shown in Figure 9. Add additional sand to the container and pack sand around the medicine cup to keep it upright. This medicine cup represents a pond.
   {12844_Procedure_Figure_9}
8. Obtain a 1" x 1" piece of sponge. Place 10 drops of red dye solution onto the sponge. Bury this sponge in sand near the clay layer touching the front face of the plastic container (see Figure 10). This sponge represents a leaking barrel.
   {12844_Procedure_Figure_10}
9. Randomly place 10 drops of blue dye solution on the surface of sand on the right-hand side of the plastic container. This food coloring represents fertilizer runoff from a large local farm (see Figure 11).
   {12844_Procedure_Figure_11}
10. Pour water into the cup and the plastic container until the sand is saturated but no standing water is left on top of the sand. Allow the model to sit undisturbed for five minutes and record all observation during this time.
11. Obtain a syringe and attach it to the free end of plastic tube (well) 1. Using the syringe, remove 5 mL of the water from well 1. Record all observations in the data table.
12. Repeat step 11 for wells 2 and 3. Rinse the syringe in between each water withdrawal. Record all observations in the data table.
13. After use, carefully remove the contents (sand, clay, gravel) of the model and add to a large bucket for disposal.

Activity C. Permeable Reactive Barriers

1. Weigh approximately 2 g of iron powder into a small weighing dish. Using a funnel or weighing paper, transfer the iron into a square-cut plastic bottle.
2. Observe and record the initial color of the methylene blue solution in the data table. Pour the 20 ppm methylene blue solution into the bottle containing iron powder until the liquid is just about overflowing. (Remove as much air space or air bubbles in the liquid as possible.)
3. Cap the bottle and shake vigorously for 3–5 minutes. Record all observations in the data table.
4. Allow the bottle to sit for 15 minutes. Continue to step 5 while waiting.
5. Using a second clean plastic bottle, repeat steps 1–4 to treat the 20 ppm indigo carmine solution with 5 g of iron powder. Record all observations in the data table.

Student Worksheet PDF

12844_Student1.pdf