HSC Exam Practice

Sample response. Turbidity is a measure of the cloudiness of water caused by suspended particles (e.g. sediment, microorganisms, colloidal matter) that scatter or absorb light. The unit of measurement is the nephelometric turbidity unit (NTU).

Marking notes. 1 mark — definition includes suspended particles reducing water clarity / scattering light. 1 mark — unit identified as NTU (or nephelometric turbidity unit).

Sample response. Dissolved oxygen (DO) is the concentration of oxygen gas dissolved in water; a high DO value (>6 mg L⁻¹) indicates the water can support aerobic aquatic life. BOD (biochemical oxygen demand) is the amount of oxygen consumed by microorganisms decomposing organic matter; a high BOD value indicates a high organic pollution load, which will deplete DO and stress or kill aquatic organisms.

Marking notes. 1 mark — correct definition of DO. 1 mark — correct definition of BOD. 1 mark — correct interpretation of high values: high DO = healthy; high BOD = polluted / oxygen-depleting.

Sample response. The ADWG (Australian Drinking Water Guidelines) are Australia’s national framework for safe drinking water, setting maximum permissible levels for physical, chemical and microbiological contaminants to protect human health and aesthetic quality of water. Two parameters with ADWG limits include: lead (Pb <10 µg L⁻¹) and nitrate (NO₃⁻ <50 mg L⁻¹). Accept also: turbidity, pH, arsenic, mercury, cadmium, TDS, coliform bacteria.

Marking notes. 1 mark — purpose: sets maximum limits to protect human health. 1 mark — first named parameter with value or category. 1 mark — second named parameter with value or category.

Sample response. Conductivity (EC) measures the ability of water to carry an electrical current, which is proportional to the concentration of dissolved ions. It indirectly measures total dissolved ion load (TDS surrogate). EC is valued as a rapid field test because it can be measured instantly with a handheld probe without any reagents or laboratory processing, allowing continuous or near-continuous monitoring of river systems such as the Hunter River Salinity Trading Scheme.

Marking notes. 1 mark — EC measures ability to conduct electricity ∝ dissolved ion concentration. 1 mark — indirect measure of dissolved ion load / TDS. 1 mark — rapid field measurement; probe-based; continuous monitoring possible.

Sample response. Excess nitrate and phosphate, typically from agricultural fertiliser runoff or urban stormwater, enter the water body and act as nutrients for algae and aquatic plants, stimulating rapid algal blooms (eutrophication). The algal biomass shades out submerged plants. When algae die, their decomposition by bacteria consumes large amounts of dissolved oxygen in a process driven by high BOD. This oxygen depletion (hypoxia) can cause fish and invertebrate death, collapsing the ecosystem.

Marking notes. 1 mark — identifies nitrate and phosphate as nutrients; likely source is agricultural fertiliser runoff / urban stormwater. 1 mark — algal bloom / eutrophication occurs. 1 mark — decomposition by microorganisms consumes oxygen (high BOD). 1 mark — dissolved oxygen falls (hypoxia), harming or killing aquatic life.

Sample response. Heavy metals such as Pb, As and Cd are chemically stable and do not degrade in the environment; once dissolved in water they persist. Biologically, they are bioaccumulated up food chains and are cumulative toxins — they bind irreversibly to enzyme active sites and accumulate in organs such as the liver and kidneys. Even small daily doses over years can cause serious harm (e.g. Pb causes neurological damage; As is carcinogenic; Cd causes kidney disease). This is why ADWG limits are set at the µg L⁻¹ level rather than mg L⁻¹.

Marking notes. 1 mark — chemical persistence / do not degrade. 1 mark — bioaccumulate and are cumulative toxins. 1 mark — specific biological effect mentioned (enzyme inhibition / neurological / carcinogenic / kidney) justifying strict limits.

Part (a) — 2 marks. Darling Lower has the highest EC under low-flow conditions at approximately 2640 µS cm⁻¹ (accept 2500–2700). Reaches exceeding the ADWG 800 µS cm⁻¹ limit during low-flow are: Darling Upper (~1850), Darling Lower (~2640) and Murray Lower (~1120). Murray Mid (~680) remains below the limit. [1 mark — identifies Darling Lower as highest; 1 mark — correctly identifies 3 reaches exceeding ADWG 800]

Part (b) — 3 marks. EC is consistently higher during low-flow (summer/autumn) than high-flow (winter/spring) across all four reaches. During low-flow, reduced river volume concentrates dissolved ions and saline groundwater discharge (draining salt-rich soils) contributes proportionally more to river flow. During high-flow, dilution by large volumes of relatively fresh rainfall and runoff reduces the ionic concentration. The Darling reaches show the most extreme values because they drain the most extensively cleared (and hence most salinised) agricultural catchments in the Basin. [1 mark — all reaches show lower EC in high-flow than low-flow; 1 mark — low-flow concentration effect explained (smaller volume, groundwater dominates); 1 mark — high-flow dilution by fresh rainfall runoff explained]

Part (c) — 3 marks. EC alone only indicates total dissolved ion load; it does not reveal which specific ions are present, their chemical nature, or their biological toxicity. Darling Lower water at 2640 µS cm⁻¹ clearly exceeds the ADWG drinking limit, but EC does not distinguish between harmless sodium chloride and toxic dissolved heavy metals (e.g. As, Pb), microbiological contamination, or pH anomalies that would independently render water unsafe. Two additional parameters that should be measured: (i) pH (to check acidity/alkalinity); (ii) heavy metal concentrations e.g. arsenic or lead (to check specific toxicants). Accept also: coliform bacteria count; turbidity; nitrate; DO. [1 mark — EC does not identify which ions; 1 mark — does not capture microbiological risk or pH or toxicant identity; 1 mark — two valid additional parameters named]

Sample response. The claim is largely incorrect. The statement that pH alone determines drinking water safety is wrong: pH is one of many parameters assessed by the ADWG. A sample could have pH 7.0 and still contain toxic heavy metals (e.g. Pb or As above ADWG limits), dangerous microbial contamination (coliforms), or excessive turbidity — none of which are detected by pH alone. The ADWG range of 6.5–8.5 (not 6–9 as stated) is also narrower than claimed. The assertion that dissolved ions and trace metals do not need routine monitoring is also incorrect: the Murray–Darling Basin salinity crisis demonstrates that dissolved ions (EC and TDS) can reach levels harmful to agriculture and aquatic life; heavy metals from industrial discharge and mine drainage do occur in Australian waterways at levels requiring monitoring. A defensible reformulation: “Drinking water safety is assessed against multiple parameters including pH, turbidity, dissolved oxygen, conductivity, heavy metal concentrations and microbiological indicators, each monitored against the ADWG because no single parameter captures all chemical and biological risks to human health.”

Marking notes. 1 mark — states overall evaluative judgement (largely incorrect / multiple errors). 1 mark — refutes “pH alone” — names at least one other parameter pH does not detect. 1 mark — refutes incorrect pH range (correct ADWG is 6.5–8.5). 1 mark — refutes “dissolved ions negligible” with Australian evidence (Murray–Darling salinity, heavy metals in industrial/mine drainage). 1 mark — provides a scientifically defensible reformulation that identifies multiple parameters and/or the ADWG multi-parameter framework.

Sample Band 6 response. The statement is false. Water quality is an inherently multi-parameter concept: no single measurement captures all the chemical, physical and biological dimensions that determine safety for drinking or ecosystem health. The Australian Drinking Water Guidelines set limits for pH, turbidity, heavy metals, coliform bacteria, TDS/conductivity, nitrate, phosphate and many other parameters independently because each captures a different aspect of risk that the others do not. Consider conductivity (EC): in the Murray–Darling Basin, EC is the primary parameter monitored under salinity trading schemes, and elevated EC in the Darling reaches (exceeding 2500 µS cm⁻¹ during low flow) correctly signals an ionic load problem for irrigation and aquatic life. However, EC tells us nothing about whether the water contains microbial contamination from feedlot runoff, heavy metals from mine drainage, or whether pH is in range for pipe infrastructure. A sample could have acceptable EC but contain arsenic at 50 µg L⁻¹ (5× the ADWG limit) — dangerously toxic, yet invisible to EC monitoring. Similarly, for ecological health: dissolved oxygen is the most direct measure of whether aquatic organisms can survive, but DO alone will not signal if turbidity from sediment runoff is blocking light to seagrass beds in GBRMPA coastal waterways, a separate and independent risk. In the Sydney Water context, pH, turbidity and coliform bacteria are all monitored because each represents a distinct failure mode: pH outside 6.5–8.5 causes pipe corrosion or taste issues; turbidity above 5 NTU indicates pathogen removal is compromised; coliform above zero indicates treatment failure. All three can be simultaneously managed, only one can be acceptable while others fail, or all three can be out of range at once. The statement that any single parameter is sufficient is therefore not only factually wrong but also dangerous from a public health perspective: it would lead to monitoring gaps that allow safe-looking water to reach consumers carrying undetected hazards. Safe water assessment requires the concurrent evaluation of physical, chemical and biological parameters, each judged against evidence-based guidelines, because water quality risks are multi-dimensional and no parameter substitutes for the others.

Marking notes. Award marks for: (1) Explicit statement that the claim is false — no single parameter sufficient. (2) Defines or implicitly applies water quality as multi-parameter. (3) Names and correctly applies conductivity / EC in a specific Australian context (e.g. Murray–Darling salinity, Hunter River Trading Scheme). (4) Explains a specific limitation of EC alone (cannot detect microbiological risk, heavy metals, pH etc). (5) Names a second Australian water management context (e.g. GBRMPA coastal waterway monitoring; Sydney Water ADWG compliance). (6) Applies a second parameter with a distinct, non-overlapping risk (e.g. DO for ecosystem health; coliform for microbiological safety; turbidity for light penetration / pathogen risk). (7) Makes the multi-parameter case explicitly: different parameters capture different failure modes; failure of one does not imply failure of others. (8) Reaches an overall evaluative judgement that connects back to the original statement with precision. [1 mark each, max 8]