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Worksheet Grade 9 Science

Marrow Creek's Constructed Wetland: Nitrate Removal and Detention Time

A grade 9 worksheet where students calculate nitrate removal efficiency, loading, and detention time for a fictional constructed wetland treating farm runoff.

Objectives

  1. Students will calculate a treatment system's removal efficiency from influent and effluent concentrations.
  2. Students will calculate nutrient loading in pounds per day using the pounds formula.
  3. Students will calculate detention time and explain its effect on treatment performance.

Vocabulary

Detention time
Detention time is how long water stays inside a tank or basin before flowing out. Water must stay long enough for each treatment step, like settling or disinfecting, to finish its job.
Influent
Influent is the incoming dirty water that arrives at a treatment plant, ready to be cleaned. Operators test it to see how much work the plant has to do.
Effluent
Effluent is the cleaned water that leaves a treatment plant at the end of the process. It is tested carefully before being released to a river, lake, or the ocean.
Nitrate
Nitrate is a form of nitrogen that plants use as food. It gets into water from fertilizer, animal waste, and septic systems, and too much of it is unhealthy for both people and streams.

Also: constructed wetland, removal efficiency

Teacher overview

The town of Marrow Creek built a constructed wetland to treat nitrate-rich runoff from surrounding farmland before that runoff reaches the creek the town is named for. A constructed wetland is an engineered version of a natural marsh: water moves slowly through shallow ponds and dense plantings, and microbes in the sediment and on plant roots convert much of the dissolved nitrate to nitrogen gas (denitrification), while the plants take up some as they grow, lowering the nitrate concentration before the water leaves the system. This worksheet uses Marrow Creek's wetland to connect removal efficiency, nutrient loading, and detention time.

A common misconception in this math is treating removal efficiency and detention time as unrelated numbers. They are directly connected: nitrate removal in a constructed wetland is a biological process that takes time, so water that moves through the wetland faster (a shorter detention time) gives the plants and microbes less contact time and lower removal efficiency. Watch for students who calculate detention time correctly in isolation without connecting it back to why efficiency changes during a storm.

Marrow Creek's Constructed Wetland: Nitrate Removal and Detention Time — Grade 9 Science
Name: ____________________________ Date: ______________

Student Worksheet

The town of Marrow Creek's constructed wetland normally treats farm runoff at a flow of 200,000 gallons per day (gpd). All figures in this worksheet describe Marrow Creek's fictional wetland. Use these facts for all items:

  • The wetland pond holds a total volume of 100,000 gallons.
  • Under normal flow, the wetland's influent (incoming) nitrate concentration measures 18 mg/L, and its effluent (outgoing) concentration measures 4 mg/L.
  • One gallon of water weighs 8.34 pounds.
  1. Calculate the wetland's nitrate removal efficiency as a percentage, using the formula: removal efficiency (%) = (influent − effluent) ÷ influent × 100. Round to the nearest tenth of a percent.
  2. Convert the wetland's normal flow of 200,000 gallons per day to million gallons per day (MGD).
  3. Using the pounds formula (pounds/day = concentration in mg/L × flow in MGD × 8.34), calculate the nitrate loading entering the wetland each day, in pounds.
  4. Using the same formula, calculate the nitrate loading leaving the wetland each day, in pounds.
  5. Calculate the mass of nitrate the wetland removes each day, in pounds, using your answers to items 3 and 4.
  6. Calculate the wetland's detention time under normal flow, using the formula: detention time = volume ÷ flow. Give your answer in days, then convert it to hours.
  7. During a heavy storm, runoff temporarily doubles to 400,000 gallons per day. Calculate the wetland's detention time at this storm flow, in days and in hours.
  8. Explain in two or three sentences why a shorter detention time, like the one you calculated in item 7, would likely lower the wetland's nitrate removal efficiency.
  9. Design item. If the storm flow in item 7 lowers removal efficiency to 60 percent while influent nitrate stays at 18 mg/L, what would the effluent nitrate concentration be during the storm? Then, in two or three sentences, propose one design change to Marrow Creek's wetland that could help it keep a higher removal efficiency during storms.

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: 77.8 percent.
    (18 − 4) ÷ 18 × 100 = 14 ÷ 18 × 100 = 77.77..., which rounds to 77.8 percent.

  2. Answer: 0.2 MGD.
    200,000 gallons ÷ 1,000,000 = 0.2 MGD.

  3. Answer: 30.024 lb.
    Pounds/day = 18 mg/L × 0.2 MGD × 8.34 = 30.024 pounds of nitrate entering the wetland each day.

  4. Answer: 6.672 lb.
    Pounds/day = 4 mg/L × 0.2 MGD × 8.34 = 6.672 pounds of nitrate leaving the wetland each day.

  5. Answer: 23.352 lb.
    30.024 − 6.672 = 23.352 pounds of nitrate removed per day. (This matches calculating directly from the concentration difference: 14 mg/L × 0.2 MGD × 8.34 = 23.352 pounds, confirming the mass balance is consistent.)

  6. Answer: 0.5 day (12 hours).
    Detention time = 100,000 gallons ÷ 200,000 gpd = 0.5 day. Converting to hours: 0.5 × 24 = 12 hours.

  7. Answer: 0.25 day (6 hours).
    Detention time = 100,000 gallons ÷ 400,000 gpd = 0.25 day. Converting to hours: 0.25 × 24 = 6 hours, half the normal detention time.

  8. Answer: short explanation.
    Nitrate removal depends mostly on microbes in the sediment and root zone converting nitrate to nitrogen gas, with some plant uptake, a biological process that takes time. With detention time cut from 12 hours to 6 hours during the storm, water passes through before as much of that conversion and uptake can happen, so a smaller share of the incoming nitrate is removed.

  9. Answer: 7.2 mg/L, plus a design proposal.
    At 60 percent removal efficiency, effluent = influent × (1 − 0.60) = 18 × 0.4 = 7.2 mg/L. A reasonable design proposal might include adding a second wetland cell so storm flow can spread across more surface area and maintain detention time, or building a stormwater basin upstream that holds back peak flow and releases it into the wetland gradually.

Differentiation

Support (below grade level)

Provide the removal efficiency and detention time formulas already written with labeled blanks, and complete item 1 as a worked example together as a class before students attempt items 2 through 9 independently.

Extension (above grade level)

Ask students to calculate what wetland pond volume Marrow Creek would need to keep detention time at a full 12 hours even during the 400,000 gpd storm flow, working backward from the detention-time formula.

ELL supports

Pre-teach "influent," "effluent," and "detention time" with a simple diagram of water flowing into one side of a shallow pond planted with reeds and out the other side, labeled "in" and "out" in English and students' home languages where possible. Allow item 8 to be answered with a labeled diagram plus one sentence instead of a full paragraph.