Percolation & Aquifers (Steve Soth)

Title: Getting Clean: An Investigation of Aquifers

Principle(s) Investigated: Gravity, Soil Porosity, Filtration

Standards: MS-ESS 2-4; develop a model to describe the cycling of water through Earth's systems driven by energy from the sun and the force of gravity

Materials:

Water (tap water); 16 oz (US) = 473.18 ml, we'll use a volume of 400ml (can be a reduced volume, but make sure this is a constant for all students / teams)

Measuring (graduated) beaker or flask and container to hold water (water bottle with cap)

coloring / food coloring (grocery store; I selected green)

Substrate materials: rock; gravel; sand; cotton balls or potting soil (these last 2 simulate organic layer. Cotton balls are found in the first-aid isle in stores while garden soil can be purchased at garden centers & home improvement stores) NOTE: potting soil can be very dusty and may need to be "washed" up to 5 or more times to remove excess sediment, peat moss, or wood fibers. I have found cotton balls or cotton filling are easier to use than potting soil in this activity.

2-litre bottles, cut in 2 pieces (soda bottles can be bought at a grocery store, empty and cut these yourself w/ exacto knife, razor, or scissors)

nylon window screen / mesh; zip ties (purchased at hardware stores & discount stores; a 2' square piece of screen is a reasonable size, cut this into 3" squares)

stopwatch (google, smartphone app)

Pen (permanent marker)

Procedure:

Overview: We'll be running 3 trials for this activity; the first 2 will be plain water, the third with have a few drops of food coloring added. Each trial will filter 400 ml (or less, depending on your available time)

1. (Students) Work in teams of 2 (no more than 3 per group). Gather materials you and your team need (from those provided) and return to your desks to assemble the containment vessel (the 2-litre bottle is in 2 pieces). The base of the bottle remains empty, the top portion is inverted. (The teacher can assist as needed)

2. Use a piece of screen mesh to cover the outer surface of the small bottle opening. (Tightly seal the screen mesh around the threaded portion of the bottle.) Secure the mesh with a zip-tie. This assembly will serve as a model of an unconfined aquifer. Water will be able to drain down through the layer(s) of sediment / gravel and out the screen into the storage area below the aquifer.

3. Once the screen mesh is secured in place, have students fill the model aquifer with the materials needed (rock, gravel, sand, cotton balls or organic material), be sure not to mix (if multiple materials are used). These should be poured into the chamber separately to create one to three layers shown in the videos (below). If asked to use three layers, these should be arranged with the organic layer (cotton balls) on the bottom, then sand, and finally the gravel / rock layer on top.

4. Students now begin to prepare for data collection. Locate the tab labeled "data table" on the Aquifer Labsheet. (the link is immediately above).

5. A stopwatch is now necessary. You can use the link I included next to the data on the "datatable" tab or you may use your own stopwatch. When your team is ready, one person will start the stopwatch / timer immediately as the second team member begins to pour water onto the top of the "aquifer". Continue timing and allow the water to percolate through the layers, a stream of water should be flowing into the lower chamber. When water slows down and begins to drip at approximately 1 drop per second, stop time and record this value in the data table. (Step #6, below) Watch this closely, this may occur rapidly! It will only take seconds for some teams!

6. Record the elapsed time in the Aquifer Labsheet (unit of time = seconds). The data sheet is found on the tab labeled "data table" (link is above).

7. Measure the volume of water which percolated through the "aquifer" and repeat a second time. The third trial we'll use water with a few drops of food coloring dye added to determine the effective filtration from different materials or a combination of them.

8. (OPTIONAL) Bring up your final "colored" samples to the front desk to compare filtration effectiveness by arranging in an order of color intensity.

9. Review a graph of the data on the tab labeled "graph" (this is fairly simple to insert and edit by utilizing the spreadsheet toolbar)

Student prior knowledge:

Surface Tension (Water)

Capillary Action (Water)

Gravity (Earth Gravitational Pull)

Porosity (Soil Particles)

Permeability (Soil & Substrate)

Filtration (Soil Profile & Substrate)

Contaminants in water and their possible origins

Types of Aquifers (confined and unconfined)

Explanation:

In this activity, we investigate the force of gravity acting on water, and we visually demonstrate the movement of water through permeable material (a simulation of the earth's surface). We compare the composition / structure of aquifers -- these natural geological formations are part of the water cycle. We use a small (portable) model to help us understand how layers of sediment, rock, and sand (possibly organic layers) come together and act as a natural filtration mechanism. We should be able to construct a graph that correlates how particle sizes determine how rapidly gravity will draw water below the surface into underground water storage areas. An alternate graph can be constructed to show a comparison of water lost through the filtration process.

I include the use of food coloring to test if some materials are more effective at filtration than others. This insight allows us to understand that porosity and filtration work together simultaneously.

Follow-up discussions can be developed for the classroom:

One discussion can be applied to water pollution. There are superfund cleanup sites located above aquifers -- based on data collected from sampling wells. Some contaminants include jet fuel and form chemical plumes underground. Some terms to discuss are carcinochemicals, carcinogens, pesticides, fertilizers, stormwater, etc.

Another topic can be the effectiveness of water filtration plants. There is an entire network of fitration plants throughout the LA area (Palmdale to Carson), and new engineering techniques will be able to refine runoff stormwater (wastewater) into domestic (drinkable / potable) water.

A third topic is the rate at which our water sources are being depleted on a global scale. NASA's GRACE (Gravity Recovery and Climate Experiment) Satellite Program has been mapping the status of underground water sources in a worldwide effort. The U.S. Ogallala Aquifer (just East of the Rocky Mountains) is expected to be exhausted by the end of this century. Our understanding of the length of time it took for aquifers to become what they are can be discussed.

Questions & Answers: Predicted results: I have posted these questions on the Aquifer Labsheet, please go to the first tab "predictions". Hide row #2, these are the answers and can be reviewed in class.

How does porosity affect H2O movement?

Soil porosity allows water to be drawn down by the force of gravity; a larger pore size allows water to move faster through layers of substrate.

What material will have the fastest rate of permeability? The slowest?

Faster - rock or sand; slower - organics

What can be a benefit of multiple layers of substrate for an aquifer?

Benefits can vary. Examples: Increased filtration; less expense to treat drinking water; removal of contaminants found in stormwater

Applications to Everyday Life: Geologic surveys are an important aspect of urban development, landfills need to be permitted to avoid contamination of groundwater.

Permeability of soil is an important consideration for property owners when creating a septic tank; this must be permitted through the State Water Control Board to safeguard groundwater, but can be installed by the homeowner or licensed contractor.

Water replenishment programs (domestic drinking water and agriculture use) are reliant on recharge zones. Typically these zones are located in/near riverbed drainage areas or even in (flat) flood plains such as near Los Angeles; Las Vegas; Fresno (Fresno recently created up to 100+ acres of settling basins prior to the anticipated 2016-2017 El Nino event).

Photographs: