Heavy Metal Contamination & Analysis

Q1 — Marking criteria (8 marks)

• 1 mark — Correctly identifies which sites and metals exceed ecological triggers: Cd exceeds 0.0002 mg L⁻¹ at RJ-2 (0.0035), RJ-3 (0.0022) and RJ-4 (0.0008) but not at RJ-1; As exceeds 0.024 mg L⁻¹ at no site (all below), but exceeds the drinking-water MCL of 0.01 mg L⁻¹ at RJ-2 (0.018) and RJ-3 (0.012). [Award 1 mark for a fully accurate identification with data; half-credit for partially correct.] Note: the ecological trigger for Cd is exceeded at three downstream sites; As does not exceed its ecological trigger of 0.024 but does exceed the drinking-water MCL at RJ-2 and RJ-3.
• 2 marks — Justifies AAS for this task with specific reference to: (1) detection limits in the ppb range (matching the low concentrations at these sites) meaning AAS is sensitive enough to detect Cd and As at ecologically significant levels; (2) element-specific hollow cathode lamps that distinguish Cd from As and from each other without interference from the mine-drainage matrix, making AAS highly specific for each target metal.
• 1 mark — Evaluates the 1980s remediation program as only partially effective: Cd levels at RJ-2 remain 17× the ecological trigger, indicating that the tailings and acid rock drainage are still mobilising metals during wet seasons. The 40-year gap between the program and current exceedances demonstrates that passive remediation (capping) alone did not prevent ongoing leaching.
• 2 marks — Recommends and justifies two suitable strategies (1 mark each). Acceptable strategies include: (a) active AMD treatment — lime dosing or reactive barriers to neutralise pH and precipitate metals before they enter the river; (b) tailings re-capping with engineered covers to limit water infiltration and oxygen contact, reducing AMD generation at source; (c) ion-exchange resin treatment for point-of-discharge water; (d) phytoremediation planting on the tailings surface to stabilise fine material and reduce runoff; (e) increased wet-season monitoring frequency at additional downstream transects to track plume extent. Must justify why the strategy addresses the ongoing wet-season mobilisation problem.
• 1 mark — Articulates a clear limitation: water-column AAS data only capture dissolved metal concentrations at the moment of sampling; they do not reflect the cumulative internal concentration in aquatic organisms. Because Cd and As can bioaccumulate in freshwater invertebrates, fish and birds (and biomagnify up the food chain), even water concentrations that are intermittently above trigger values may translate to chronically elevated tissue concentrations in resident biota — understating the ecological risk.

Sample Band 6 response (abridged). At the Rum Jungle site, Cd exceeds the ADWG ecological freshwater trigger value (0.0002 mg L⁻¹) at sites RJ-2, RJ-3 and RJ-4 by factors of 17, 11 and 4 respectively. As does not exceed the ecological trigger of 0.024 mg L⁻¹ at any site, but does exceed the drinking-water MCL (0.01 mg L⁻¹) at RJ-2 and RJ-3. AAS is the appropriate analytical choice because its detection limits reach the ppb range, matching the concentrations present, and each hollow cathode lamp is element-specific — the Cd lamp emits only Cd-characteristic wavelengths, so As and other AMD metals in the matrix do not interfere with the Cd measurement. The 1980s remediation is evidently insufficient: Cd exceedances persist 35 years later, indicating that the tailings and surrounding acid rock are still leaching metals each wet season, likely because the original capping did not fully cut off water and oxygen contact with sulfide minerals. Two further strategies are warranted: first, an active lime-dosing or reactive permeable barrier system at the main AMD discharge point to raise pH and precipitate metal hydroxide solids before they reach the river; second, engineering a low-permeability tailings cover to reduce wet-season infiltration and oxygen access at source. Water-column data alone underestimate ecological risk: Cd bioaccumulates in the tissues of benthic invertebrates and fish at concentrations far above the dissolved water concentration, and can biomagnify to levels that are toxic to top predators and to traditional landowners who consume fish from the Finniss River system.

Q2 — Marking criteria (7 marks)

• 1 mark — Correctly refutes “AAS only measures freely floating ions”: AAS does not measure aqueous ions directly. The sample is atomised first (flame or graphite furnace), and AAS measures the absorption of light by free ground-state atoms — it measures atoms after atomisation, not dissolved ions in solution.
• 1 mark — Partially concedes “cannot distinguish between different forms”: standard AAS measures total dissolved concentration for the target element, not oxidation-state speciation (e.g. As(III) vs As(V)). This is a real and acknowledged limitation. However, it is not unique to AAS; speciation requires coupled techniques such as HPLC-ICP-MS regardless of method.
• 1 mark — Addresses the “one element at a time” limitation: this is true for conventional AAS (one hollow cathode lamp per run), but it is a practical trade-off, not an absolute disqualifier. For regulatory monitoring of a small suite of elements (e.g. the five ADWG-regulated heavy metals), running separate lamps sequentially is routine and perfectly adequate. ICP-MS offers multi-element simultaneous analysis and may be preferred for research, but is more expensive.
• 1 mark — Correctly refutes the claim about UV-Vis: UV-Vis spectrophotometry measures absorption by molecules in solution. Most heavy metal ions (Pb²⁺, Cd²⁺, Hg²⁺) do not have strong characteristic UV-Vis absorption in their dissolved ionic form and cannot be quantified directly by UV-Vis. UV-Vis is used for non-metal analytes with strong chromophores (e.g. nitrate, phosphate, phenols) or for coloured metal complexes after derivatisation, but it lacks the ppb sensitivity and element specificity of AAS for Pb, Cd and Hg.
• 1 mark — Correctly connects the Beer–Lambert law: both AAS and UV-Vis are grounded in A = εlc, but the key distinction is that AAS measures absorption by atoms (ground-state, after atomisation) while UV-Vis measures absorption by molecules or ions in solution. The molar absorptivity (ε) for most heavy metal ions in the UV-Vis region is far lower than for the same elements measured as free atoms in AAS, explaining the inferior detection limits of UV-Vis for these analytes.
• 1 mark — Provides a defensible judgement: AAS remains the method of choice for routine regulatory monitoring of Pb, Cd, Hg and As in water because it has ppb detection limits, is element-specific, and is the method referenced in both the ADWG and Australian Standard methods (AS/NZS 8147). AAS is less suitable when simultaneous multi-element analysis at ppt levels is required (use ICP-MS), or when speciation is needed (use HPLC-ICP-MS).
• 1 mark — Reformulates a defensible statement (example): “AAS quantifies total dissolved heavy metal concentration with ppb sensitivity by measuring ground-state atom absorption after atomisation; it is the standard method for Pb, Cd and Hg monitoring under Australian regulations. Its limitation is sequential element analysis and lack of speciation, not its inability to detect these metals at environmentally relevant concentrations. UV-Vis spectrophotometry is not a viable substitute for AAS for Pb, Cd or Hg monitoring because it lacks sufficient sensitivity and element specificity for those analytes.”