Does Media Matter? Infiltration Rates and Storage Capacities

The goal of low-impact development and green infrastructure design is to manage stormwater at its sources using natural means, and to establish conditions so that the hydrology and, more specifically, water quality, of developed sites approaches that of pre-development conditions. (Davis, 2006) An understanding of porosity, volume storage, void space and infiltration rates is necessary for the correct design of these systems. Civil and geotechnical engineers must have a comprehensive understanding of in-situ soil mechanics and groundwater flow in order to design effective systems to manage stormwater and design safe foundations for buildings, roads and bridges. Environmental engineers must understand the movement of water as it percolates through different soil layers in order to design technologies that address water quality and treatment of stormwater, groundwater and remediation projects.

After this activity, students should be able to:

• Describe the key soil mechanics and properties of media that determine infiltration rate and storage volume.
• Calculate the storage capacity and infiltration rate of different types of media and media combinations.
• Create a media layer that promotes infiltration, maximizes below-ground water storage, and provides an environment for healthy plants and microbial communities

Remember from our Green Infrastructure and Low-Impact Development Technologies lesson that soil is a combination of sand, silt, clay, minerals and other organic and inorganic compounds. The organic matter gives soil a black or dark brown color and provides nutrients for plants and bacteria. Typically, the darker the soil, the healthier both plant and microbial communities will be. So that we do not confuse soil with other earth materials that do not contain organic matter, such as rocks, gravel, limestone and granite, we refer to the combination of earth materials as media. The texture of the media is based on the proportion of mineral particles, sand, silt and clay within the media.

Have you ever thought about how water moves once it goes into the ground? Where does it go? What controls how fast it moves into the ground?

From the Natural and Urban "Stormwater" Water Cycles lesson, remember that infiltration is the movement of water into the media layer and percolation is the movement of water within the media layer. The infiltration rate is dependent on intensity and duration of precipitation, vegetative cover and the percolation rate. The percolation rate is affected by gravity, grain size, geology, depth to groundwater table and ability of the surrounding soil to hold water.

Once in the ground, water can be accessed by the plant roots and transpire through the stomata in the plant's leaves. Stomata are pores found in plants' leaves that control gas exchange (that is, carbon dioxide, oxygen and water). An entire world of bacteria and microbial activity also takes place below the ground and within the soil. Water is a critical element to all life and most soil bacteria would not survive if water did not percolate into the ground. The water may then take weeks, months or even years to reach the groundwater table, depending on the permeability of the media that it is passing through. Permeability is the ability of water to pass through media, typically dependent on the porosity and connectivity of open spaces within the media. This water plays a key role in recharging the groundwater table and providing sources for fresh drinking water that makes its way into our everyday lives.

Soil properties are used to classify the infiltration rate and storage capacity, which are both important design considerations for green infrastructure and low-impact development projects. Soil properties are also a key factor in stormwater infrastructure design (for example, dry detention/retention ponds and swales). Storage capacity is defined as the volume of water that can be absorbed within a given media layer. Some of the properties that relate to infiltration rate and storage capacity are texture, porosity, permeability, surface area, grain size distribution, mineralogy and friction.Figure 1. How does the overall surface area differ between different size particles?CopyrightCopyright 2013 Ryan Locicero, WARE raingardens.us (author)

Let's talk about particle size. Smaller particles have more surface area for the same mass or volume of larger particles. This increase in surface area causes an increase in soil friction due to water contact area. (Illustrate this by drawing a picture of open spaces and surface area such as shown in Figure 1, or by describing how the powdered activated carbon works in a Brita water filter.) Looking at this drawing (Figure 1), we can see that 40 of the smaller orange particles fit into the same area as one of the blue particles. The smaller particles have a diameter that is 1/8 of the larger particle. However, when we calculate the total surface area, we see that the smaller particle has five times the surface area of the larger particle for the same overall area.

Now let's review a few examples to help us understand some of the lesser-known soil properties terms. Imagine we have a bucket of sand. The space between the sand particles is known as the porosity, or the amount of open space between sand particles. Now imagine that we add water to the bucket filling in all of the spaces between the sand particles until the water level reaches the top of the sand. If we kept track of the volume of water added to the bucket, we would know the storage capacity of the sane. If we then drained the water from the bucket and subtracted the drained volume from the storage capacity we would be left with the field capacity. The field capacity is the amount of water remaining in the soil after all water has been drained. The water that is held within the field capacity can be removed through evaporation, plant uptake and transpiration, or by bacteria living within the media. The water remaining in the sand is held in place by capillary action as a result of adhesion, cohesion and surface tension between water and the open spaces within the media. Smaller-sized particles hold more water within their available open spaces due to their increased surface areas. This results in a higher permeability in course sand particles vs. fine clay pa rticles, which is counterintuitive to the fact that clay typically has a higher porosity.

Before the Activity

• You may want to perform this activity in a science classroom or outdoors if weather permits.
• Gather materials and make copies of the Does Media Matter? Worksheet (PDF), one per student.
• Prepare four 5-gallon buckets by placing a different media type into each bucket: sand, soil, gravel, mulch. Fill each bucket with 4 liters of material.
• Have handy a measuring cup or volumetric cylinder for measuring water volume.
• Have stopwatches ready.
• Note that students will need their completed worksheets, coir baskets and media mixes for subsequent activities in this unit.

With the Students

1. Divide the class into groups of two or three students each, depending on the class size and availability of resources. Hand out the worksheets.
2. Present the Introduction/Motivation content to students, covering the following main points:

• Introduce students to the definition of media. In the context of this activity and the unit as a whole, media is defined as a combination of organic and/or inorganic earth materials. Describe the properties of inorganic vs. organic materials. Have students record the definition of media on the worksheet.
• Ask students if they know where water goes once it enters the ground and if they can define the terms used to describe the movement of water. Direct them to record their answers after the worksheet question: "What do we call the movement of water INTO media layers and define percolation?"
• Familiarize students with the different soil properties and have students record on their worksheets the definitions for the following properties: permeability, capillary action, porosity, percolation, storage capacity and field capacity.

3. As a class, calculate the storage capacity of each of the media within the prepared 5-gallon buckets by pouring water into the media layer until the water level reaches the top surface of the material. Keep track of the volume of water being poured into each bucket by first accounting for the water with a measuring cup or volumetric cylinder. This is the storage capacity, have students record these measurements in the worksheet table for each material type.
4. Let each group select a media type that it wishes to investigate (make sure at least one group tests each material type) and place the same volume of media as used in step 3 into its planter/basket. Ask students if they think more, less or the same volume of water will drain from the media in the planter/basket and why. Have groups each place a 5-gallon bucket under the planter/basket and then pour the same volume of water that was added to the 5-gallon bucket in step 3 (that is, the known storage capacity). Measure and record on the worksheets the amount of water that drains from the planter/basket graduated containers to get an accurate measurement of the drained volume of water.
5. Have students subtract the storage capacity (obtained in step 3) from the volume of water drained from the planter/basket (obtained in step 4) to obtain the field capacity, filling in the worksheet table. Also record comparative observations amongst the media types.
6. Continuing on, have groups make sure the media is fully saturated by filling their planters/baskets with water to the top of the media surface. Then have students determine the infiltration rate of the media in their planters/baskets. To determine the infiltration rate, add a known volume of water (such as 1 or 2 liters) and record the time it takes for the water to drain through the planter/basket, making sure to collect, measure and record the volume of water that leaves the planter/basket as well. The volume of water drained from the planter/basket divided by the time it takes for the water to drain is the infiltration rate (ml/sec). Note: It is likely that water will continue to drip from the media after the bulk of the water has passed through. So, make a judgment call to record the time it takes for the bulk of the water to pass through the media.
7. Have groups present the data that they collected on their selected media types. As data is presented on media types other than what each group investigated, have them fill in the data from other groups into their worksheet tables, so that everyone has infiltration rates and observations on all media types.
8. Next, challenge teams to use what they learned from the group presentations to create their own media mix combinations to best meet the design requirements. Inform them of the design objectives: Create a media layer that promotes infiltration, maximizes below-ground water storage and provides an environment for healthy plants and microbial communities. Make and test a media mix amount that has a total volume of between 2-3 liters. Direct students to first brainstorm as a group and then decide on their designs, recording on their worksheets the types of media and volumes or ratio of each material added.
9. Have each group determine the infiltration rate of its media mix combination (ml/sec) by running three experiments on the same media mix, each with different water quantities. Each experiment includes three identical trials, from which average infiltration rates can be calculated. On their worksheets, have students record the measured water volumes and infiltration times, and then calculate the infiltration rates.
10. Have students plot the volume of water vs. time. The slopes of the lines are the infiltration rates.
11. Conclude by leading a class discussion to share, compare and review student results and solutions, as described in the Assessment section. Collect and review the worksheets.

Pre-Activity Assessment

Predictions: Ask students the following questions:

• What do you think happens to water once it enters into the ground? (Answer: Once in the ground, many types of bacteria and the roots of plants and trees can access the water. The water that does not leave the soil through evapotranspiration may then take weeks, months or even years to reach the groundwater table, depending on the properties of the media that the water must pass through. This water plays a key role in recharging the groundwater table and providing fresh drinking water that makes its way into our everyday lives.)
• Imagine you have a plastic bottle full of sand and you fill the bottle to the top with water and place the cap on it. Now imagine poking holes in the top of the bottle and turning it upside down and draining the water from the bottle. Would you expect less, the same or more water to leave the bottle then you put in, why? (Answer: Less water will leave the bottle then you put in; the technical term for this is the field capacity of the media. This is caused by frictional and surface tension forces between particles. The amount of water remaining in the soil after all water was drained is called the field capacity.)

Activity Embedded Assessment

Worksheet: Have students complete the Does Media Matter? Worksheet (PDF) as they conduct the activity by recording definitions, answering questions, recording measurements and observations, making calculations and graphing data. Review their worksheets to gauge their depth of comprehension.

Media Mix: Challenge teams to design their own media mix combinations based on what they learned from all the group presentations. Design objectives: Create a media layer that promotes infiltration, maximizes below-ground water storage and provides an environment for healthy plants and microbial communities. Have students brainstorm as a group and decide on their designs, recording on their worksheets the types of media and volumes or ratio of each material added. Make and test a media mix amount that has a total volume of between 2-3 liters, recording test results on the worksheets.

Post-Activity Assessment

Wrap-Up: Lead a class discussion to share, compare and review student results and solutions, including the worksheet questions and graph. Ask the students:

• How do the infiltration rates compare across groups' media mixes and the individual media alone? (Have students share their results. Include a comparison of graphs. Make sure students understand that the slopes of the lines are the infiltration rates.)
• Which media mixes worked the best to meet our design criteria? (Answers will vary; let students share their logic and results. See if students have an understanding of what characteristics each media brings to the mix. Many different types of media mix combinations meet the criteria of increasing infiltration and below-ground storage while creating an environment conducive to successful plant establishment and growth. One popular media mix recommended for rain gardens consist of topsoil, sand and mulch in a 2:2:1 ratio. This media mix ratio has produced successful growth and storage results for several rain gardens in East Tampa, FL and Prince George County, MD.)
• How would engineers use the information from this activity? What might they be designing that they need information on infiltration rates? And would they want to use materials with high or low infiltration rates in their designs? (Answer: Engineers would use this information when designing stormwater management systems for new housing/office/shopping/school developments or to improve existing infrastructure. Knowing the infiltration rates of different soil materials can be helpful to reduce standing water in areas that are prone to flooding and to design traditional [such as stormwater ponds] and green infrastructure [such as rain gardens] projects. An infiltration rate is a particularly important factor for understanding how long an area will remain flooded after and between storm events.)
• What happens to fine-grain materials (sand, silt or clay) when we add water to them? How does this differ from courser materials such as rocks or gravel? (Answer: The smaller particles create soil friction, causing the particles to stick together when wet and reducing the movement of water passing through, compared to the coarser materials.)
• Will a smaller or larger particle have a greater field capacity? (Answer: Smaller-sized particles hold more water within their macro-pores due to an increased surface area resulting in a greater field capacity.)
• How does size of media affect the infiltration rate? (Answer: The larger the particle size, the greater the infiltration rate.)
• What media would you use if you wanted to maximize below-ground storage? (Answer: Media with a high infiltration and permeability rate.)
• What media would you use if you wanted to assure healthy plants and bacteria community? (Answer: A media layer with a high organic content and above-average field capacity.)

For additional resources on the water cycle, soil properties and particle size, see the Internet links provided in the Additional Multimedia Support section.

Additional resources on the water cycle, soil properties and particle size:

• The USGS Water Science School: http://ga.water.usgs.gov/edu/
• The Water Cycle: http://studyjams.scholastic.com/studyjams/jams/science/ecosystems/water-cycle.htm • Soil Stories—The Whole Story (a 30-minute video): http://www.youtube.com/watch?feature=player_detailpage&v=Ego6LI-IjbY
• How is powdered activated carbon used? (as in a Brita water filter system): http://science.howstuffworks.com/environmental/energy/powdered-activated carbon.htm

Davis, A. P., Shokouhian, M., Sharma, H., and Minami, C. (2006). Water quality improvement through bioretention media: Nitrogen and phosphorus removal. Water Environment Research, 78(3), 284-293. doi: 10.2175/106143005x94376