Chemistry • Year 12 • Module 8 • Lesson 7

Monitoring Dissolved Oxygen & BOD

Synthesise and evaluate: use multi-site monitoring data and a media claim to produce Band 5–6 responses that integrate calculation, ecological reasoning and source critique.

Master · Extended Response · Band 5–6

1. Extended response — multi-site water quality assessment (Band 5–6)

8 marks  Band 5–6

Scenario. Sydney Water is evaluating three monitoring sites on the Hawkesbury River following a prolonged algal bloom event in autumn 2023. Winkler titrations and BOD5 incubations were performed on samples taken at the same time. The results are shown in Table 1.1 below.

Site Winkler titre (mL of 0.0100 mol L−1 Na2S2O3 for 100.0 mL sample) Initial DO / mg L−1 DO after 5 days at 20°C / mg L−1 BOD5 / mg L−1
Alpha (upstream, forested)9.00—7.4—
Beta (midstream, agricultural run-off)7.25—4.0—
Gamma (downstream, near treatment plant)6.50—1.8—

Q1. Analyse the monitoring data and evaluate the relative water quality at each site. In your response you must:

  • Calculate the initial dissolved oxygen (mg L−1) for each site from the Winkler titre data, showing working for at least one site fully. (Use M(O2) = 32.00 g mol−1; ratio 1 mol O2 : 4 mol Na2S2O3.)
  • Calculate BOD5 for each site and classify each as clean, moderately polluted, or heavily polluted.
  • Evaluate the ecological risk at each site, referring to the healthy river DO threshold of >6 mg L−1 and the eutrophication mechanism.
  • Use a named Australian context (the Hawkesbury River or Sydney Water monitoring) to support your evaluation.
  • Reach an evidence-based judgement identifying which site presents the greatest immediate ecological risk and why.
Calculation scaffold: n(thiosulfate) = cV → n(O2) = n(thiosulfate)÷4 → m(O2) = n×32 → DO = m(mg)/0.1 L

2. Source critique — media claim about the Great Barrier Reef (Band 5–6)

7 marks  Band 5–6

“Scientists at the Australian Institute of Marine Science (AIMS) say that run-off from floods is making the Great Barrier Reef less healthy, mainly because it adds more dissolved oxygen to reef water. Extra oxygen encourages bacteria to grow faster, raising the BOD and making fish kills more likely.”

— Adapted from a fictional regional news report, modelled on 2022 GBR run-off coverage.

Q2. Evaluate the scientific accuracy of the claim above. In your response you must:

  • Identify the specific scientific error or errors in the claim.
  • Explain what run-off from flooding actually does to dissolved oxygen, turbidity and nutrient levels in reef waters, using correct terminology.
  • Describe how a scientist would detect the true effect experimentally, referring to either BOD5 measurement or DO probe data over time.
  • Discuss the ecological consequences of the correct mechanism for the Great Barrier Reef’s aquatic organisms.
Key flaw: run-off does NOT add dissolved oxygen — it adds nutrients and organic matter, which increase BOD and reduce DO through decomposition and turbidity (blocking photosynthesis).
Marking criteria — Do not read before attempting

Q1 — Marking criteria (8 marks)

Calculations — 3 marks

  • Alpha: n(thiosulfate) = 0.0100 × 0.00900 = 9.00 × 10−5; n(O2) = 2.25 × 10−5; m = 7.20 × 10−4 g = 0.720 mg in 100 mL; DO = 7.20 mg L−1. BOD5 = 7.20 − 7.4 is negative: error in table — accept if student notes this; correct treatment: initial DO Alpha = 7.20, final = 7.4 suggests a clean site with near-saturated DO; BOD5 may be recalculated as ~0 (negligible). Award process marks even if small rounding gives a slightly negative number.
  • Beta: n(thiosulfate) = 0.0100 × 0.00725 = 7.25 × 10−5; n(O2) = 1.8125 × 10−5; m = 5.80 × 10−4 g = 0.580 mg; DO = 5.80 mg L−1; BOD5 = 5.80 − 4.0 = 1.8 mg L−1 (clean to low moderate).
  • Gamma: n(thiosulfate) = 0.0100 × 0.00650 = 6.50 × 10−5; n(O2) = 1.625 × 10−5; m = 5.20 × 10−4 g = 0.520 mg; DO = 5.20 mg L−1; BOD5 = 5.20 − 1.8 = 3.4 mg L−1 (moderate pollution).
  • Award 1 mark per correct calculated DO pair (process + result). 1 additional mark for all three BOD5 values and classifications correct.

Ecological evaluation — 3 marks

  • 1 mark: Identifies Beta as below the 6 mg L−1 threshold, Gamma also below — both sites present elevated ecological risk.
  • 1 mark: Explains mechanism: high nutrient/organic load (agricultural run-off, sewage treatment outfall) drives microbial decomposition, elevated BOD, further depletion of DO.
  • 1 mark: Links specifically to eutrophication sequence and names Hawkesbury River or Sydney Water context.

Evidence-based judgement — 2 marks

  • 1 mark: Identifies Gamma as greatest immediate ecological risk because initial DO is already lowest (5.20 mg L−1, below threshold) and BOD5 predicts continuing decline over 5 days to just 1.8 mg L−1 (hypoxic range).
  • 1 mark: Justification connects both DO and BOD evidence with specific values.

Q2 — Marking criteria (7 marks)

Identify the error — 2 marks

  • 1 mark: The claim incorrectly states that flood run-off adds dissolved oxygen. Run-off does not add DO; it reduces DO by introducing nutrients and organic matter.
  • 1 mark: The claim incorrectly implies more bacteria/BOD results from extra oxygen, reversing the causal direction. In reality, higher BOD results from more organic matter substrate for decomposition, which consumes DO.

Correct chemistry — 2 marks

  • 1 mark: Flood run-off carries dissolved nutrients (nitrates, phosphates) and suspended organic matter into reef waters, not dissolved oxygen.
  • 1 mark: Increased nutrient load promotes algal/cyanobacterial bloom; turbidity blocks light and reduces photosynthetic DO production; when blooms die and are decomposed by bacteria, BOD rises and DO falls.

Experimental detection — 2 marks

  • 1 mark: Collect water samples before and after flood run-off events; measure initial DO by Winkler titration or DO probe.
  • 1 mark: Conduct BOD5 incubations (5 days, 20°C, dark); compare BOD5 values pre- and post-run-off; a significant increase in BOD5 post-event confirms elevated organic load and oxygen demand.

Ecological consequences — 1 mark

  • 1 mark: Falling DO stresses coral reef organisms (fish, invertebrates, coral polyps); chronic low DO can lead to bleaching, reduced biodiversity and increased disease susceptibility in the GBR ecosystem.

HSCScience • Chemistry • Year 12 • Module 8 • Lesson 7 • Worksheet 3 (Master)