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Lesson plan Grade 10 Science

Langmere Reservoir Turnover: Choosing an Intake Depth

A grade 10 lesson using an August depth profile of temperature, oxygen, and manganese to decide which intake a reservoir plant should draw from before fall turnover.

Objectives

  1. Students will identify the thermocline in a reservoir depth profile and describe how it separates the water column.
  2. Students will connect the loss of dissolved oxygen at depth to the release of manganese from lake sediments.
  3. Students will recommend an intake depth and a turnover response using stated water quality criteria.

Vocabulary

Key words: thermocline, epilimnion, hypolimnion, turnover, anoxic, multi-level intake

Teacher overview

A deep reservoir in late summer holds three layers rather than one uniform body of water, and a plant that draws without regard to that layering can face taste, odor, and discolored water complaints. Warm surface water floats on cold bottom water, the middle layer resists mixing, and once the bottom layer runs out of oxygen the sediments begin releasing dissolved manganese and iron. Deep water is often taken to be the cleanest choice because it sits farthest from runoff and boats, which is the reasoning the profile data is meant to test. The lesson supplies an August depth profile from the fictional Langmere Reservoir and asks students to defend an intake choice, then predict what fall turnover will do to it.

Procedure

Prediction and framing (5 min): Show a simple side view of the fictional Langmere Reservoir with intakes marked at 4 meters, 8 meters, and 12 meters below the surface. Ask students to vote for the intake they would use in August and to give a one-sentence reason. Tally the votes on the board without correcting anyone, since the vote is the comparison point at the end of the lesson.

How a reservoir layers (10 min): Explain that summer sun warms only the top few meters, and warm water is less dense, so it floats. Wind mixes that top layer, called the epilimnion, but it does not have the energy to mix the whole column. The sharp temperature drop below it is the thermocline, and the cold, isolated bottom layer is the hypolimnion. Add the biological piece: organic matter sinks into the hypolimnion, bacteria consume it and use up the oxygen, and no new oxygen arrives because the layer is cut off from the surface. When the oxygen runs out, chemistry at the sediment surface changes and manganese and iron that were locked in solid form dissolve into the water. Name the consequence for the plant, which is that dissolved manganese passes straight through a filter unless it is oxidized first.

Profile analysis (22 min): Distribute the handout and have students work in pairs through items 1 through 9. Circulate and watch for two predictable errors: reading the largest single temperature value instead of the largest change between depths, and treating the manganese column as a percentage rather than a concentration. Ask pairs who finish early to sketch all three profiles on one set of axes with depth on the vertical axis, which is how limnologists and operators usually see the data.

Comparing recommendations (8 min): Ask three pairs with different intake choices to defend them, then return to the opening vote. Draw out the trade-off explicitly: the 4-meter intake has good oxygen and almost no manganese but carries warmer water with more algae, while the 12-meter intake is cold and clear of algae but loaded with manganese that the plant must oxidize. Close by asking what a plant without a multi-level intake would have to do instead, which is treat whatever the single intake gives it.

Exit ticket (5 min): Have each student complete handout item 10 and turn it in. Look for two elements in the answer: that turnover mixes the manganese and low-oxygen water throughout the reservoir, and that the plant should be watching for it rather than reacting after complaints arrive. If fewer than four out of five students include both, revisit the turnover sequence at the start of the next class.

Langmere Reservoir Turnover: Choosing an Intake Depth — Grade 10 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose the fictional Langmere Reservoir was profiled on an August morning. Samples were collected every two meters from the surface down to 16 meters. The plant draws 6.0 MGD from the reservoir and can select any one of three intakes, at 4 meters, 8 meters, or 12 meters. Most fish need dissolved oxygen above 5 mg/L.

Depth (m) Temperature (degrees C) Dissolved oxygen (mg/L) Manganese (mg/L)
0 26.0 8.6 0.01
2 25.8 8.5 0.01
4 25.2 8.2 0.02
6 21.0 6.4 0.05
8 15.5 3.1 0.28
10 12.0 1.2 0.95
12 10.5 0.4 1.60
14 9.8 0.2 2.10
16 9.5 0.1 2.40
  1. Using the temperature column, find the two-meter interval with the largest temperature change and calculate that change in degrees Celsius per meter.
  2. Using the dissolved oxygen column, state the shallowest depth in the table at which oxygen has fallen below 5 mg/L.
  3. Using the surface temperature of 26.0 degrees Celsius and the 16-meter temperature of 9.5 degrees Celsius, calculate the total temperature difference across the profile.
  4. Manganese measures 0.02 mg/L at 4 meters and 1.60 mg/L at 12 meters. Calculate how many times greater the 12-meter concentration is.
  5. Choose the intake depth of 4, 8, or 12 meters that you would use in August, and defend it in two sentences using the oxygen and manganese data.
  6. The plant draws 6.0 MGD. Using a manganese concentration of 1.60 mg/L and the formula pounds per day equals mg/L times MGD times 8.34, calculate the manganese load the plant would take in each day from the 12-meter intake.
  7. Using manganese concentrations of 1.60 mg/L at 12 meters and 0.02 mg/L at 4 meters, calculate the percent reduction in manganese the plant gains by switching intakes. Round to the nearest tenth.
  8. Explain why the bottom layer stays cold and separate all summer even though the surface is heated every day. Use the words density and mixing in your answer.
  9. In October the surface water cools to about 10 degrees Celsius. Predict what happens to the temperature, oxygen, and manganese profiles when that happens, and name the process.
  10. Write two or three sentences telling the plant operator what to expect during the week of turnover and what to do about it. Explain both what happens in the reservoir and one specific action the plant should take.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 2.75 degrees Celsius per meter.
    The largest change falls between 6 meters and 8 meters, where temperature drops from 21.0 to 15.5 degrees Celsius. That is 5.5 degrees across 2 meters, which is 2.75 degrees per meter. The next largest interval, 4 to 6 meters, gives only 2.1 degrees per meter. This interval is the thermocline.
  2. Answer: 8 meters.
    Oxygen measures 6.4 mg/L at 6 meters and 3.1 mg/L at 8 meters, so 8 meters is the shallowest listed depth below the 5 mg/L benchmark. Everything deeper is lower still, reaching 0.1 mg/L at the bottom.
  3. Answer: 16.5 degrees Celsius.
    Subtract the deep temperature from the surface temperature: 26.0 minus 9.5 = 16.5 degrees Celsius across 16 meters.
  4. Answer: 80 times.
    Divide the deep value by the shallow value: 1.60 / 0.02 = 80, so manganese at 12 meters is eighty times the concentration at 4 meters.
  5. Answer: The 4-meter intake.
    At 4 meters the water carries 8.2 mg/L of dissolved oxygen and only 0.02 mg/L of manganese, so the plant avoids the manganese problem entirely. Accept an argument for the 8-meter intake if the student acknowledges the 0.28 mg/L of manganese and names the added oxidation step. A choice of 12 meters should be marked down unless the student explicitly plans for 1.60 mg/L of manganese.
  6. Answer: 80.1 lb/day.
    Apply the loading formula: 1.60 mg/L x 6.0 MGD x 8.34 = 80.06 pounds per day, or about 80.1 pounds per day of manganese entering the plant.
  7. Answer: 98.8 percent.
    Subtract and divide by the larger value: 1.60 minus 0.02 = 1.58, and 1.58 / 1.60 = 0.9875, so the switch removes 98.8 percent of the incoming manganese before any treatment chemical is added.
  8. Answer: Warm water is less dense and floats, and wind mixes only the top layer.
    Sunlight warms the surface, and warm water expands slightly, so it is less dense than the cold water beneath it. That density difference resists mixing. Wind supplies energy to stir the epilimnion, but not enough to overturn the whole column against the density step at the thermocline. The hypolimnion therefore stays cold and cut off from the atmosphere, which is also why its oxygen is never replaced once bacteria consume it.
  9. Answer: Turnover, which mixes the column.
    As the surface cools toward the temperature of the deep water, the density difference disappears and wind can mix the entire reservoir. Temperature becomes nearly uniform from top to bottom, oxygen from the surface is carried downward, and the manganese that had built up near the sediments is spread through the whole water column. For a few days to a few weeks the reservoir looks the same at every depth.
  10. Answer: Manganese spreads to every intake depth during turnover, so the plant should monitor and pre-oxidize.
    Students should predict that switching to a shallow intake stops helping, because the mixed water carries manganese at every depth, and that taste, odor, and discolored water complaints often follow. Acceptable actions include sampling raw water manganese daily through the turnover window, starting or increasing pre-oxidation ahead of filtration, adjusting filter run times, and notifying customers before complaints arrive. Answers that only describe the mixing without naming an action are incomplete.

Differentiation

Support

Give students the profile already plotted as three graphs with depth on the vertical axis, so items 1 through 4 become a matter of reading the curves rather than scanning a table. Work one interval of item 1 aloud, using the 0 to 2 meter interval, so the idea of change per meter is modeled before students search for the largest one.

Extension

Have students calculate the manganese load the plant would take in at each of the three intake depths at 6.0 MGD, then determine how much oxidant the fictional Langmere plant would need for each choice if it uses 1.9 milligrams of potassium permanganate per milligram of manganese. Ask them to convert each result to pounds per day and rank the three intakes by chemical cost.

ELL supports

Provide a labeled cross-section of the reservoir with the three layers colored and named in both English and the student's home language where possible, and post the terms "thermocline," "anoxic," and "turnover" with small diagrams. Offer the sentence frame "I would choose the ___ meter intake because the oxygen is ___ and the manganese is ___" for item 5, and allow item 9 to be answered as a labeled before-and-after sketch.

Extensions

Ask students to design a monitoring schedule for the fictional Langmere Reservoir that would tell operators when turnover has begun. The schedule should state which depths to sample, which three measurements to take, how often to take them between September and November, and what change in the readings would count as the signal that mixing has started.