HSC Exam Practice

Sample response. Bioaccumulation is the progressive build-up of a contaminant within a single organism over time, so that the internal concentration exceeds the concentration in the surrounding environment. Biomagnification is the increase in contaminant concentration at each successive trophic level: as each predator eats many contaminated prey items, it accumulates more of the contaminant than any individual prey organism carried, leading to the highest concentrations in top predators.

Marking notes. 1 mark — defines bioaccumulation correctly (build-up within one organism). 1 mark — defines biomagnification correctly (increasing concentration up the food chain / across trophic levels). 1 mark — clearly distinguishes the two: bioaccumulation = within an organism; biomagnification = between trophic levels.

Sample response. Lead (Pb) — neurological damage (cognitive impairment, especially in children); Mercury (Hg) — severe neurological toxicity (Minamata disease); Cadmium (Cd) — kidney (renal) damage; Arsenic (As) — chronic toxicity, associated with increased cancer risk; Chromium (Cr) — toxicity depending on speciation (Cr(VI) is carcinogenic).

Marking notes. 1 mark per correctly named metal paired with its primary health effect. Any three from Pb, Hg, Cd, As, Cr accepted.

Sample response. A calibration curve is built by measuring the absorbance of standard solutions of known metal concentration at the target element’s characteristic wavelength. Because the Beer–Lambert law (A = εlc) predicts a linear relationship, the curve allows an unknown sample’s absorbance to be converted directly into concentration [1]. The unknown sample must fall within the calibrated range because Beer–Lambert linearity may not hold at higher concentrations — readings above the highest standard could be extrapolated inaccurately; the sample must be diluted to bring it within the linear range if needed [1].

Sample response. The hollow cathode lamp consists of a cathode made from the target metal element (e.g. Pb). When a current is passed through the lamp, the metal vapour emits light at only the characteristic wavelengths of that element, determined by its unique electron energy levels [1]. In the AAS instrument, only ground-state atoms of that same element can absorb those wavelengths at the same high efficiency [1]. Because no other metal present in the sample has the same electron transition energies, they do not absorb significantly at those wavelengths, minimising spectral interference and making the measurement highly selective for the target metal [1].

Sample response. Reverse osmosis forces water through a semi-permeable membrane under pressure, physically excluding dissolved arsenic ions (and virtually all dissolved salts) from passing through; it is highly effective at lowering dissolved As to below MCL and can be applied at a water treatment plant scale [1]. Its limitation for a small town is the high energy cost and the need to manage a concentrated arsenic-rich brine reject stream safely [1]. Phytoremediation uses plants to take up arsenic from contaminated soil or water into their tissues, reducing the metal burden over time; it is low-cost and may be suitable for managing contaminated source catchments or tailings [1]. Its limitation for a drinking-water supply is that it is very slow (years to decades) and cannot reduce water As concentrations quickly enough for an immediate public health problem, nor is it directly applicable as a drinking-water treatment step [1].

Sample response. Using A = εlc: c = A / (εl) = 0.294 / (4.2 × 10⁴ × 10.0) [1] = 0.294 / 420 000 = 7.0 × 10⁻⁷ mol L⁻¹ [1]. Convert to mg L⁻¹: 7.0 × 10⁻⁷ mol L⁻¹ × 207.2 g mol⁻¹ = 1.45 × 10⁻⁴ g L⁻¹ = 0.000145 mg L⁻¹ [1]. This is well below the ADWG MCL of 0.010 mg L⁻¹, so this sample does not exceed the guideline [1].

Marking notes. 1 mark for substituting into A = εlc correctly; 1 mark for correct mol L⁻¹ value; 1 mark for correct mg L⁻¹ conversion; 1 mark for MCL comparison and conclusion. Accept equivalent working.

Sample response. Bores B-03 (0.014), B-04 (0.018), B-06 (0.022) and B-08 (0.011) all exceed the ADWG MCL of 0.010 mg L⁻¹ [1]. The bore with the highest concentration is B-06 (0.022 mg L⁻¹); this is 2.2× the MCL [1].

Sample response. Deeper bores generally show higher As concentrations (e.g. B-06 at 74 m has the highest As; B-07 at 15 m has one of the lowest) [1]. This pattern is consistent with As being mobilised from arsenic-bearing geological strata (e.g. arsenopyrite minerals) at depth, where reducing conditions in the aquifer can dissolve arsenic from mineral surfaces into groundwater [1]. Additionally, B-06 is a former market garden with legacy pesticide (arsenical pesticide) use, suggesting a combined geological + anthropogenic input that raises As at that site above what geology alone would predict [1].

Sample response. AAS is preferred because it has detection limits in the ppb range, allowing accurate quantification of As at the low concentrations present in most of these bore samples (as low as 0.003 mg L⁻¹ = 3 ppb) [1]. UV-Vis spectrophotometry lacks sufficient sensitivity to measure dissolved arsenic species at these concentrations directly, as As ions do not have strong molar absorptivity in the UV-Vis range in their ionic form — it would require derivatisation to form a coloured complex, adding steps and potential error. AAS also provides element-specific measurement with minimal interference from other dissolved ions in the groundwater matrix [1].

Sample response. The community group’s claim is inadequate for several reasons. While the measured tap water Pb of 0.008 mg L⁻¹ is below the ADWG MCL of 0.010 mg L⁻¹, this single measurement does not capture the full range of exposure pathways for residents in a legacy lead-mining area.

First, bioavailability matters: lead in contaminated soil and dust can be ingested directly (especially by young children engaging in hand-to-mouth behaviour) or inhaled as fine particulates, bypassing the water supply entirely. The MCL only applies to the water route; high soil Pb in the Broken Hill environment creates direct inhalation and ingestion exposure that is entirely separate from tap water.

Second, bioaccumulation means that lead absorbed from any route (water, dust, food) accumulates in bone, blood and soft tissue over years. Even a tap water concentration just below the MCL, combined with additional soil and dust exposure, can drive blood lead levels that exceed health thresholds — particularly in children, where no safe blood lead level has been established. The MCL is set for adults and provides a safety margin, but for chronically exposed children in a legacy site, it is not a guarantee of safety.

Third, a single water sample is insufficient: lead concentrations in tap water vary with time of day (stagnation in pipes overnight concentrates leached Pb), temperature, and whether first-flush samples or running water samples are taken. A single low reading may not represent typical daily exposure.

Two additional monitoring approaches would provide a more complete picture: (1) Blood lead testing of children and pregnant women in the community — this directly measures the body burden from all exposure routes combined and is the most health-relevant indicator; (2) Soil and household dust Pb analysis using AAS — this quantifies the ingestion and inhalation exposure pathway that operates independently of water supply. Additional approaches could include AAS analysis of home-grown vegetables or locally caught fish for dietary Pb, and time-series (first-flush) water sampling to capture pipe-leached Pb peaks.

Marking criteria. 1 mark — acknowledges that water Pb is below MCL but identifies this as an incomplete indicator of total exposure. 1 mark — explains bioavailability: multiple routes (dust, soil, food) contribute to total Pb exposure beyond water. 1 mark — explains bioaccumulation: Pb accumulates in body tissues; blood lead integrates all routes over time. 1 mark — discusses why children face disproportionate risk (developmental vulnerability; hand-to-mouth behaviour; lower safe threshold). 1 mark — identifies limitation of a single water sample (stagnation, temporal variation, first-flush). 1 mark — recommends blood lead monitoring with correct justification. 1 mark — recommends soil/dust AAS analysis with correct justification. 1 mark — reaches an overall evaluative judgement: the claim is inadequate because it conflates one measurement against one exposure route with total safety.