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Lesson plan Grade 9 Science

Ravenscarr's Wells and Ellinmere Brook: Where Baseflow Goes

A 55-minute grade 9 lesson where students convert well pumping rates to stream units and trace how pumping a shallow aquifer lowers a brook's late-summer flow.

Objectives

  1. Students will convert a pumping rate among million gallons per day, gallons per minute, and cubic feet per second.
  2. Students will calculate how much of a brook's baseflow a wellfield captures.
  3. Students will explain the feedback that connects groundwater pumping to stream temperature and dissolved oxygen.

Vocabulary

Baseflow
Baseflow is the water that keeps a stream flowing even when it has not rained for a while. It comes from groundwater slowly seeping into the streambed.
Aquifer
An aquifer is an underground layer of rock, sand, or gravel that holds water in the tiny spaces between its grains. Wells reach down into aquifers to bring water up to the surface.
Water table
The water table is the underground level below which the ground is completely soaked with water. Dig a hole deeper than the water table and it will start filling with water.

Also: streamflow depletion, cubic feet per second

Teacher overview

In late summer, a small stream is fed almost entirely by groundwater seeping in through its bed and banks, which hydrologists call baseflow. When a town pumps wells that draw on the same shallow aquifer, some of the water that would have reached the stream is intercepted instead, so the stream runs lower. This lesson uses the fictional town of Ravenscarr and the fictional Ellinmere Brook to make that connection quantitative, then follows the feedback further, because a lower summer flow warms faster and warm water holds less dissolved oxygen. A common misconception is that a well and a nearby stream are separate supplies, so that pumping groundwater leaves surface water untouched; the calculations here are built to correct that directly.

Procedure

Hook (5 min): Ask students where the water in a small brook comes from during a stretch of three weeks with no rain. Take answers without correcting them, then introduce baseflow as groundwater that seeps into the channel. Ask what might happen to that seepage if a town drilled wells into the same shallow aquifer 500 feet away.

Direct instruction (15 min): Put the three conversions on the board: one million gallons per day equals 694 gallons per minute, one million gallons per day equals 1.547 cubic feet per second, and one cubic foot per second equals 448.8 gallons per minute. Explain that stream gauges report cubic feet per second while water utilities report million gallons per day, so comparing a well to a brook always starts with a conversion. Model a worked example that is not on the handout, using a different fictional town that pumps 1.2 MGD. Show 1.2 times 1.547 equals 1.8564, about 1.86 cfs, then explain that a basin study estimated 60 percent of that pumping would otherwise have reached the brook, so the depletion is 0.60 times 1.8564, which is about 1.11 cfs. Sketch the feedback chain on the board: less baseflow, lower summer flow, faster warming, and less dissolved oxygen for fish.

Guided practice (15 min): Distribute the handout for the fictional Ravenscarr wellfield. Work item 1 together, then have pairs complete items 2 through 4. Circulate and check that students multiply by 1.547 rather than 694 when they need cubic feet per second, and that they take 60 percent of the pumping rate rather than 60 percent of the brook's flow.

Independent practice (12 min): Students finish items 5 through 8 on their own, then compare answers with a partner. Item 8 asks them to state the consequence of the expansion in words, so remind them to name the specific flow value they calculated.

Closure and exit ticket (8 min): Formative check. On an index card, each student answers this question: "A fictional wellfield pumps 2.0 MGD. Using 1.547 cubic feet per second per MGD, what is that pumping rate in cubic feet per second?" The answer is 3.094 cfs. Collect the cards. If several students report 1,388 or divide instead of multiplying, open the next class by rebuilding the conversion table together.

Ravenscarr's Wells and Ellinmere Brook: Where Baseflow Goes — Grade 9 Science
Name: ____________________________ Date: ______________

Student Handout

The fictional town of Ravenscarr draws its drinking water from four shallow wells in a sand and gravel aquifer beside Ellinmere Brook. The wells together pump 1.8 MGD. A fictional basin study estimates that 60 percent of the water pumped from these wells would otherwise have discharged into the brook as baseflow. Before the wellfield was built, the brook's August baseflow measured 14 cubic feet per second. A fictional state rule requires that at least 11 cubic feet per second remain in the brook during August.

Use these conversions: 1 MGD equals 694 gpm; 1 MGD equals 1.547 cfs; 1 cfs equals 448.8 gpm. Round each answer to two decimal places unless the question says otherwise.

  1. Convert the wellfield's 1.8 MGD to gallons per minute.
  2. Convert the wellfield's 1.8 MGD to cubic feet per second.
  3. Using your answer to item 2, how many cubic feet per second of baseflow does the wellfield capture, if 60 percent of the pumping would otherwise have reached the brook?
  4. What is the brook's August baseflow with the wellfield running, starting from 14 cubic feet per second?
  5. By what percent has the brook's August baseflow dropped from the original 14 cubic feet per second? Round to the nearest tenth of a percent.
  6. How many cubic feet per second above the fictional 11 cfs minimum is the brook now, using your answer to item 4?
  7. Ravenscarr wants to expand the wellfield to 3.5 MGD. What would the brook's August baseflow be, again starting from 14 cubic feet per second and capturing 60 percent of the pumping?
  8. Would the expanded wellfield meet the fictional 11 cfs minimum? State how many cubic feet per second above or below the minimum the brook would be.

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: 1249.2 gpm. Multiply 1.8 MGD by 694 gallons per minute per MGD, which gives 1,249.2 gallons per minute.
  2. Answer: 2.78 cfs. Multiply 1.8 MGD by 1.547 cubic feet per second per MGD, which gives 2.7846, or about 2.78 cubic feet per second.
  3. Answer: 1.67 cfs. Take 60 percent of 2.7846, so 0.60 times 2.7846 equals 1.67076, or about 1.67 cubic feet per second of captured baseflow.
  4. Answer: 12.33 cfs. Subtract the captured baseflow from the original flow: 14 minus 1.67 equals 12.33 cubic feet per second.
  5. Answer: 11.9 percent. Divide the captured baseflow by the original flow and multiply by 100: 1.67 divided by 14 is 0.1193, which is 11.9 percent.
  6. Answer: 1.33 cfs. The brook now carries 12.33 cubic feet per second, and 12.33 minus 11 equals 1.33 cubic feet per second above the minimum.
  7. Answer: 10.75 cfs. First convert: 3.5 times 1.547 equals 5.4145 cubic feet per second. Then capture 60 percent: 0.60 times 5.4145 equals 3.2487, about 3.25. Then subtract: 14 minus 3.25 equals 10.75 cubic feet per second.
  8. Answer: No, it would be 0.25 cfs below the minimum. The expanded wellfield leaves 10.75 cubic feet per second, and 11 minus 10.75 equals 0.25 cubic feet per second short of the fictional minimum.

Differentiation

Support

Provide a conversion reference card with the three factors and one fully worked example, and give students item 2 already completed so they can begin the depletion calculation from a correct value. Pair students who struggle with decimals so that one partner performs the multiplication and the other checks the unit label on every line.

Extension

Ask students to find the largest pumping rate, in million gallons per day, that would still leave the fictional brook at exactly the 11 cfs minimum, then explain what other information a hydrologist would need before trusting that number. Students can also describe one change to the fictional wellfield, such as moving wells farther from the channel, that might reduce the share of pumping taken from baseflow.

ELL supports

Pre-teach "baseflow," "aquifer," "capture," and "minimum" with a labeled cross-section drawing showing the water table, the brook, and a well. Provide a sentence frame for item 8: "The expanded wellfield leaves ___ cubic feet per second, which is ___ the fictional minimum of 11 cubic feet per second."

Extensions

Have students extend the feedback chain in writing. Starting from the reduced August flow they calculated for the fictional brook, they describe how a shallower, slower stream warms more quickly on a hot day, why warmer water holds less dissolved oxygen, and why trout need dissolved oxygen above 5 mg/L. Ask each student to name one measurement a monitoring crew could take to check whether the predicted change is actually happening.