Water for Schools
Menu

Worksheet Grade 10 Science

Wexmoor River Oxygen Sag: Reading a Dissolved-Oxygen Profile

A grade 10 science worksheet where students read a dissolved-oxygen profile downstream of a discharge, find the sag point, and compare it to the fish threshold.

Objectives

  1. Students will read a dissolved-oxygen profile and locate the sag point downstream of a discharge.
  2. Students will calculate the oxygen drop, the recovery rate, and the distance below the fish threshold.
  3. Students will explain why oxygen falls after an organic discharge and recovers farther downstream.

Vocabulary

Dissolved oxygen
Dissolved oxygen is the oxygen gas mixed into water that fish and other underwater animals breathe. Cold, moving water holds more oxygen than warm, still water.
Biochemical oxygen demand (BOD)
Biochemical oxygen demand, or BOD, measures how much oxygen tiny microbes will use up as they eat the waste in water. Dirtier water has a higher BOD.

Also: oxygen sag, reaeration

Teacher overview

When a river receives water rich in organic matter, bacteria downstream multiply and consume oxygen as they break that material down, so dissolved oxygen falls, reaches a low point, and then recovers as the river reabsorbs oxygen from the air. This pattern is called an oxygen sag. This worksheet uses a fictional stretch of the Wexmoor River below a food-processing discharge to practice reading such a profile. The oxygen readings are invented for this scenario; the 5 mg/L fish benchmark is a standard water-quality value.

Wexmoor River Oxygen Sag: Reading a Dissolved-Oxygen Profile — Grade 10 Science
Name: ____________________________ Date: ______________

Student Worksheet

Suppose the fictional Wexmoor River flows past a food-processing plant that discharges organic-rich wastewater. A field team measured dissolved oxygen (DO) at several stations downstream and recorded the results below. Most fish need dissolved oxygen above 5 mg/L.

Station (km downstream of the discharge) Dissolved oxygen (mg/L)
0 8.0
2 6.0
4 4.0
6 3.0
8 4.5
10 6.5
12 7.5
  1. At which station is the dissolved oxygen lowest, and what is the reading there? This low point is called the sag point.
  2. Calculate the drop in dissolved oxygen from the discharge point (station 0) to the sag point.
  3. Between which stations is the dissolved oxygen below the 5 mg/L level that most fish need? List the stations that fall below it.
  4. From the sag point to station 12, calculate the recovery rate of dissolved oxygen in mg/L per kilometer.
  5. Explain why the dissolved oxygen drops in the first few kilometers below the discharge.
  6. Explain why the dissolved oxygen recovers again farther downstream.
  7. Name one treatment change the plant could make to reduce the oxygen sag, and explain how it would help. (Consider lowering the organic load before discharge.)
  8. Predict whether the sag point would be higher or lower if the plant doubled its organic load, and explain your reasoning.
  9. Using only the measured stations, calculate the distance, in kilometers, from the first station below the 5 mg/L threshold to the last one.
  10. At the sag point, how far below the 5 mg/L fish threshold is the dissolved oxygen?

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: 3.0 mg/L at station 6. The 6 km station has the lowest reading in the table, which is the sag point of the profile.
  2. Answer: 5.0 mg/L. The drop is 8.0 mg/L at station 0 minus 3.0 mg/L at the sag point: 8.0 - 3.0 = 5.0 mg/L.
  3. Answer: Stations 4, 6, and 8 km. Their readings (4.0, 3.0, and 4.5 mg/L) are all below 5 mg/L, while stations 2 and 10 are above it.
  4. Answer: 0.75 mg/L per km. From the sag point (3.0 mg/L at 6 km) to station 12 (7.5 mg/L), the rise is 7.5 - 3.0 = 4.5 mg/L over 12 - 6 = 6 km, so 4.5 / 6 = 0.75 mg/L per km.
  5. Answer: The discharge adds organic matter, and bacteria use dissolved oxygen to decompose it. Because the bacteria multiply and keep consuming oxygen faster than the river can replace it, the oxygen level falls.
  6. Answer: Farther downstream the organic load has mostly been used up, so bacteria consume less oxygen, while the river keeps absorbing oxygen from the air (reaeration). The balance shifts back toward gaining oxygen, so the level recovers.
  7. Answer: The plant could improve secondary treatment to lower its BOD, aiming for a BOD at or below the 30 mg/L level that municipal secondary treatment must meet. Sending out water with far less organic matter means downstream bacteria consume less oxygen, so the sag would be shallower. Accept any answer naming a load-reducing treatment with a sound explanation.
  8. Answer: Lower. Doubling the organic load feeds more bacteria, which consume more oxygen before the river can recover, so the sag point would drop to an even lower dissolved-oxygen reading.
  9. Answer: 4 km. Dissolved oxygen is below 5 mg/L from station 4 to station 8, a stretch of 8 - 4 = 4 km.
  10. Answer: 2.0 mg/L. The fish threshold is 5 mg/L and the sag reading is 3.0 mg/L, so 5.0 - 3.0 = 2.0 mg/L below the threshold.

Differentiation

Support

Provide the profile as a pre-plotted line graph as well as the table, and mark the 5 mg/L threshold as a dashed line so students can see which stations fall below it before they calculate item 9.

Extension

Ask students to sketch how the profile would change if reaeration were slower (for example, in a warm, still pool), and explain how a shallower, faster riffle would shift the recovery rate they calculated in item 4.

ELL supports

Pre-teach "dissolved oxygen," "sag," and "recover" with a simple up-and-down curve labeled in English and, where possible, students' home languages. Allow students to answer the explanation items with a labeled arrow diagram plus a short phrase.