Engineering a Model of the Earth As a Water Filter by Jonathon Kilpatrick, Nanette Marcum-Dietrich, John Wallace, and Carolyn Staudt
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Copyright © 2018, National Science Teachers Association (NSTA). Reprinted with permission from Science and Children, Vol. 56, No. 3, October 2018. Engineering a Model of the Earth as a Water Filter By Jonathon Kilpatrick, Nanette Marcum-Dietrich, John Wallace, and Carolyn Staudt eaching students about the wa- from groundwater. Most is supplied smaller pieces lower down,” offered ter cycle is a staple of elemen- through public drinking water sys- one student. Another replied, “A lot of Ttary science instruction. When tems. But over 15 million families rely sand is mixing with the water though, asked to describe the water cycle, most on private, household wells and use and now it looks really dirty. Maybe students can quickly recite the contin- groundwater as their source of fresh sand should go first.” These conversa- ual cyclical process of condensation, water” (EPA 2016). Noting the im- tions continued throughout the design precipitation, and evaporation. But portance of infiltration in the water process. this simple definition misses the es- cycle and in the supply of essential By the end of the session, the stu- sential process of infiltration. Ground- groundwater led us to develop an engi- dents had constructed three filters. water and the process of infiltration, neering activity in which students are Two of them were stackable filters which cleans that water, are vital to our challenged to build a stackable filter with the water passing in turn through survival, but they are also hidden from using the Earth process of infiltration different layers and materials. The view and too often absent from our sci- as a model. third was a single cup with all of the ence curriculum. materials mixed together. When the Infiltration is the process by which filters were tested, all of them filtered WATER FILTRATION water is naturally filtered by the Earth. the water remarkably well. There was a Rain falls on the Earth’s surface and ACTIVITY noticeable difference in the before and seeps into the ground. The type of soil On a sunny summer afternoon, our after samples of water. particles and vegetation covering the team of teacher researchers met with soil influences infiltration rate. Soil a small group of students and a pile FIGURE 1 pores exist between particle types and of materials (Figure 1). During one of are occupied by water and air. Some of the activities, students were presented Materials (per group). this water soaks deeper into the ground with a large pitcher of “gross” (as they through layers of humus (organic de- described it) water full of dirt and • 6 clear plastic cups bris such as twigs and leaves), surface debris found outside. Make sure stu- soil, subsoil, gravel, sand, and other dents properly wash hands after work- • Pushpins (30 or so) particles, creating a macropore filtra- ing with soil. • Coffee filters tion system and joining the ground- Students were asked, “Can you fil- • Rubber bands (5-10) water storage areas called aquifers that ter water?” Students were instructed are important resources for drinking how to build a stackable filter with • Cheesecloth (1 sq ft., water. cups and pushpins. They were then minimum) An amazing thing happens as left to their own creativity. Using the • Sandwich bag of aquarium this water travels through the layers cups and pushpins plus sand, charcoal, sand of the Earth. During its travels, it is cheesecloth, gravel, and rubber bands, • Sandwich bag of aquarium cleaned. In fact, it is this macropore they experimented until they found charcoal filtration that aids the cleansing pro- what they considered to be the best cess by trapping “dirty” particles in possible way to filter the water. “What • Sandwich bag of aquarium gravel the pores between different soil types. if we pour the water through the gravel According to the Environmental Pro- first to catch the big stuff, then the • “Dirty” water (water mixed tection Agency (EPA), “About half charcoal, then the sand? That would with dirt and debris) of our nation’s drinking water comes work the best because it would catch www.nsta.org/elementaryschool • 73 In the spring of the following year, as we intended for the students to use we set out to complete the development the drawing as a model. of the filtering activity with a select Once the layers were described to group of 15 fifth-grade students and a the students, they were provided with few younger siblings in an afterschool the box of materials and asked to align “Water Science” club. In planning the their materials with the layers of the session, we wondered together what Earth. In small groups of three or four, the students learned from the activity they sorted through their supplies and in the summer. Since all three filters compared their notes to the attributes seemed to clean the water equally well, of each material. The conversations did the students assume that the layers were lively and creative: and order did not matter? While the • “The gravel has to be the activity was a strong engineering activ- fragmented rock!” ity given its focus on designing a solu- tion to a particular problem, it lacked • “Can we mix the sand and the in science content. Students were not charcoal together so that it looks making the connection between how like the topsoil in the terrarium?” the Earth filters water through infil- • “I think charcoal should be first tration and the materials they used to PHOTOS COURTESY OF THE AUTHORS because it looks like topsoil and create their stackable filter. As part of the design process, stu- the sand is harder when it gets wet Our solution was to help students dents test different materials for their and it’s sticky like subsoil.” think as scientists and problem solvers water filters. by seeking inspiration for their filters in • “When we take the cheesecloth one of the most efficient water filtration and fold it over and over, it looks systems that exists: Earth. Instead of • “Kitchen sponges are like ocean all tangled like roots and grass, so merely constructing interesting filter- sponges.” we could use this as our humus.” ing devices, we wanted them to create As the ideas started to dwindle, a Later, one student commented, a filter modeled after Earth processes. new question was posed. “Is there a “When the cheesecloth gets wet it’s natural water filter that you could look all spongy, just like the description of THE EARTH AS A MODEL to in order to help you build a more ef- humus, so this is definitely our humus When the students convened for the fective filter?” Students had few ideas. layer!” afternoon filtering activity, we pre- One student wondered if waterfalls At one point, a student asked how sented them with the same challenge, somehow clean water and another the various materials, which all “look “Can you filter water?” Before asking student thought clouds might help to dirty,” can clean water. He held up the the question, however, we asked the clean the water. The answer students black charcoal as an example. Another students to think of inventions that did offer was the Earth. An undergrad- student replied, “I don’t know if this is were inspired by nature. Their re- uate student assisting with the activity right, but I think we use charcoal fil- sponses were thoughtful and varied: told the students, “The Earth has its ters to clean the water in my fish tank. • “A butterfly sucks nectar through own way of filtering water by allow- So maybe charcoal does a really good a straw-thing on his face, so maybe ing water to infiltrate down through its job of cleaning water.” Another of- straws came from butterflies.” various layers.” We live in a relatively fered that her mother uses soap with The teacher introduced the word rural area, so we asked the students, charcoal in it. proboscis as the “straw-thing” “Where does the drinking water in Although the students would later described by students. your house come from?” Many of the have to research the “why,” they con- students responded that their water cluded that charcoal—for some rea- • “Camouflage clothing might have came from a well. The undergraduate son—was used for cleaning and decid- come from seeing fish and animals university assistant then drew a simple ed that it would take the place of either blend into their surroundings.” diagram on the board that illustrated topsoil or subsoil in their Earth-model • “Didn’t Velcro come from those layers of humus, topsoil, subsoil, frag- water filters. Students were intrigued little burr things that stick to your mented rock, and bedrock. The sim- by the question of how something that clothes in the woods?” plicity of the diagram was intentional, looks so “dirty” can clean water. 74 • • OCTOBER 2018 ous water testing activities that they conducted earlier in the week using a simple chemical water testing kit. A student explained, “There are lots of things in the water we can’t see, like chlorine, nitrogen, phosphate, and there can even be bacteria in water.” Another student added, “You can’t see bacteria but, if you drink it, it can make you really sick!” They concluded that clear was not necessarily clean, depending on the definition used. “As scientists,” one student expressed, “we have to think of all of the possible definitions.” By the end of the session, four groups had created four stackable filters, each unique in the order and amount of materials used. Every group was able to explain how their filter re- created the Earth model. Though this was not a graded activity, the expecta- Students eagerly test their completed water filters.