Chemistry • Year 12 • Module 8 • Lesson 6
Water Quality Parameters & Standards
Apply water quality parameters to multi-site field data, interpret a real-data graph of dissolved oxygen vs temperature, and reason about contamination sources using parameter patterns.
1. Interpret multi-site river monitoring data
The table below shows water quality measurements taken at four sites along a river in the Murray–Darling Basin during summer. Site 1 is upstream of all agricultural and urban activity. Sites 2–4 are progressively downstream. ADWG and ecosystem limits are shown in the header row. 9 marks
| Parameter | Guideline / limit | Site 1 (upstream) | Site 2 (irrigation district) | Site 3 (feedlot runoff zone) | Site 4 (urban discharge) |
|---|---|---|---|---|---|
| Temperature (°C) | — | 18 | 22 | 24 | 26 |
| pH | 6.5–8.5 | 7.4 | 7.1 | 6.2 | 8.9 |
| Turbidity (NTU) | <5 | 2 | 8 | 31 | 19 |
| DO (mg L−1) | >6 | 9.1 | 7.4 | 3.8 | 4.2 |
| BOD (mg L−1) | <2 (background) | 1.0 | 2.3 | 11.4 | 7.6 |
| EC (µS cm−1) | <800 | 210 | 1420 | 890 | 1050 |
| Nitrate (mg L−1) | <50 | 0.4 | 14.2 | 38.7 | 9.1 |
| Coliform (cfu/100 mL) | <1 (drinking) | 0 | 12 | 1840 | 620 |
1.1 Identify the two parameters at Site 2 that most strongly suggest the influence of agricultural irrigation. Explain your reasoning for each. 4 marks
1.2 At Site 3, three parameters simultaneously exceed their guideline values. Using lesson content, explain the likely contamination source and how each of the three exceedances is connected to it. 3 marks
1.3 At Site 4, pH is 8.9 (above ADWG range) and DO is only 4.2 mg L−1. Suggest a process that could simultaneously raise pH and lower DO in an urban waterway, and name one parameter you would also expect to be elevated as a result. 2 marks
2. Interpret graph — dissolved oxygen solubility vs temperature
The figure below shows the solubility of dissolved oxygen (DO) in freshwater at atmospheric pressure as a function of water temperature, based on standard saturation data (after APHA Standard Methods, 23rd ed.). Three field samples from a Queensland coastal waterway are plotted. 9 marks
Figure 2.1. Dissolved oxygen solubility in freshwater vs temperature (1 atm). Field sample data from GBRMPA coastal waterway monitoring (illustrative values after APHA Standard Methods, 23rd ed.).
2.1 Describe the trend in DO solubility as temperature increases. Estimate the DO solubility at 20°C from the curve. 2 marks
2.2 Sample Beta (β) sits below the solubility curve and below the 6 mg L−1 ecosystem guideline. Identify two possible reasons why the measured DO is lower than the solubility value at that temperature. 2 marks
2.3 Sample Gamma (γ) sits above the equilibrium solubility curve at 28°C. Suggest a biological process that could cause DO to exceed the solubility curve in a shallow sunlit waterway. 2 marks
2.4 The GBRMPA monitors Queensland coastal waterways partly to protect coral reefs downstream. Explain why low dissolved oxygen in freshwater runoff entering coastal systems is a concern for reef ecosystems, referring to at least one other water quality parameter you would also monitor. 3 marks
3. Cause-and-effect chain — Murray–Darling Basin salinity crisis
The Murray–Darling Basin salinity crisis is linked to rising groundwater salinisation after broad-scale land clearing. Complete the cause-and-effect chain below by filling in the empty effect boxes. A filled-in starter is shown. 5 marks
4. Predict and justify — Great Artesian Basin bore water
The Great Artesian Basin (GAB) is one of the world’s largest underground freshwater reservoirs. Bore water from the GAB is used for stock and domestic supply in remote western NSW and Queensland. However, bore water is characterised by: very high TDS (often 1500–3000 mg L−1), elevated pH (~8.5–9.2), elevated fluoride (~1.5–3.0 mg L−1), and near-zero coliform bacteria counts. 4 marks
4.1 Predict whether GAB bore water would meet ADWG standards for pH and TDS, and explain why the chemical composition of the water leads to these measurements. 2 marks
4.2 Although bore water does not meet ADWG standards for TDS or pH, it is used in remote communities without significant microbiological treatment. Which ADWG parameter makes bore water safer than surface water in this respect, and why? 2 marks
Q1.1 — Site 2 agricultural signals (4 marks)
Elevated EC (1420 µS cm−1, exceeds <800 guideline): Agricultural irrigation water typically carries dissolved salts; irrigation return flows and fertiliser dissolution increase the ionic load of the river, raising conductivity directly. [1 parameter + 1 reasoning]
Elevated nitrate (14.2 mg L−1): Nitrate is a primary component of agricultural fertilisers; runoff from irrigated paddocks leaches NO3− directly into the river, serving as a direct chemical fingerprint of fertiliser use. [1 parameter + 1 reasoning]
Q1.2 — Site 3 feedlot contamination (3 marks)
The three exceedances are turbidity (31 NTU), BOD (11.4 mg L−1) and coliform bacteria (1840 cfu/100 mL). The likely source is feedlot or intensive livestock runoff. High turbidity results from soil disturbance and suspended faecal particles in runoff. High BOD indicates large amounts of organic matter (manure) being decomposed by microorganisms, depleting oxygen. High coliform counts directly reflect faecal contamination from livestock. [1 mark source; 1 mark for two parameter explanations; 1 mark for linking all three to the same source coherently]
Q1.3 — Site 4 process (2 marks)
An algal bloom (or phytoplankton/macroalgae bloom driven by eutrophication) can simultaneously raise pH through photosynthetic CO2 uptake during the day and lower DO at night or after die-off due to respiratory and decomposition oxygen demand. [1 mark for identifying algal blooms / eutrophication]. An additional parameter expected to be elevated is turbidity (green colouration from algae) or nitrate/phosphate (the nutrient driving the bloom). [1 mark]
Q2.1 — DO vs temperature trend (2 marks)
As temperature increases, DO solubility decreases (inverse relationship). [1 mark] From the curve, DO at 20°C is approximately 9.1 mg L−1 (accept 8.8–9.3 mg L−1). [1 mark]
Q2.2 — Sample Beta below solubility (2 marks)
Any two of: (1) High BOD from organic pollution — microorganisms are consuming oxygen faster than atmospheric re-aeration can replace it. (2) Algal bloom die-off — decomposition of dead algae consuming dissolved oxygen. (3) Reduced mixing / stagnant water — less surface aeration to replenish oxygen. (4) High temperature stress already reduces DO capacity, and any additional oxygen-consuming process compounds the deficit. [1 mark each, max 2]
Q2.3 — Sample Gamma above curve (2 marks)
Photosynthesis by aquatic algae or aquatic plants in sunlit shallow water produces oxygen as a by-product, releasing it directly into the water. [1 mark] This can cause supersaturation, where DO exceeds the equilibrium solubility at that temperature, because the photosynthetic oxygen input rate exceeds the rate at which gas escapes to the atmosphere. [1 mark]
Q2.4 — GBRMPA coastal concern (3 marks)
Low DO in freshwater runoff entering coastal zones can extend hypoxic conditions into nearshore reef environments, stressing or killing coral organisms and invertebrates that require adequate dissolved oxygen for respiration. [1 mark] In addition, freshwater runoff from eutrophic inland waterways carries elevated nitrate and phosphate levels which stimulate macroalgae and phytoplankton growth on reefs, competing with corals for light and space. [1 mark] An additional parameter to monitor is turbidity (sediment-laden runoff reduces light reaching coral symbiotic algae, impairing photosynthesis and bleaching corals) or nitrate/phosphate (eutrophication trigger). [1 mark]
Q3 — Murray–Darling cause-and-effect chain (5 marks)
Effect 1: Water table (groundwater) rises because deep-rooted native vegetation is no longer drawing down soil moisture; rainfall infiltration exceeds evapotranspiration. [1]
Effect 2: (Provided) Groundwater rises, dissolving naturally occurring salts in the soil profile.
Effect 3: Salt-laden groundwater discharges into rivers, raising EC / conductivity and TDS in the Murray–Darling river system to levels that exceed ADWG and irrigation limits. [1]
Effect 4: High salinity kills salt-sensitive aquatic organisms (e.g. freshwater fish, invertebrates, riparian vegetation) and renders water unsuitable for irrigation of salt-sensitive crops and for stock drinking. [1]
Effect 5 (overall): The salinity crisis reduces biodiversity, agricultural productivity and the availability of safe drinking and irrigation water across the entire Basin, requiring costly engineering and water trading schemes (e.g. Hunter River Salinity Trading Scheme approach extended to Murray–Darling) to manage dissolved ion loads. [1 per well-reasoned effect, max 5; Effect 2 is given free]
Q4.1 — GAB bore water ADWG compliance (2 marks)
GAB bore water would exceed the ADWG TDS guideline (<600 mg L−1) because long residence times deep underground allow extensive dissolution of minerals (silicates, carbonates, fluorides) from the host rock. [1 mark] pH would typically exceed the ADWG upper limit of 8.5 because carbonate dissolution and geothermal processes raise alkalinity. [1 mark]
Q4.2 — Microbiologically safer (2 marks)
The near-zero coliform bacteria count makes bore water microbiologically safer than typical surface water. [1 mark] This is because deep subsurface aquifers are isolated from surface contamination sources (animals, agriculture, sewage); the geological filtration through rock layers removes pathogenic microorganisms, meaning the primary human health risk from surface water (microbial contamination) is absent in bore water. [1 mark]