Microbial Testing

Q1 — Sample Band 6 response (8 marks), annotated

Pattern in the data: E. coli was absent from both sites in Q1 at the building tap (0 CFU/100 mL) and at the bore in Q2, but was consistently detected at high concentrations in Q3 and Q4. At the bore head, counts escalated from 182,000 CFU/100 mL in Q3 to approximately 310,000 CFU/100 mL in Q4 — far exceeding the Australian Drinking Water Guideline of fewer than 1 CFU/100 mL. Building tap counts were lower than bore counts in both contaminated quarters (44,000 and 91,000 CFU/100 mL respectively) but remained vastly above the safe limit. Q1 bore head (7,000 CFU/100 mL) also exceeded the guideline. [1 — pattern with specific CFU values]

CFU calculation for Q4 bore head (10−3 plate, 31 colonies): CFU/mL = 31 ÷ (0.1 × 10−3) = 31 ÷ 0.0001 = 310,000 CFU/mL. Converting to per 100 mL: 310,000 × 100 = 31,000,000 CFU/100 mL. This is the same order of magnitude as the 310,000 figure in the table, which was calculated from the 10−2 plate. The 10−3 plate result (31 colonies) falls within the preferred 30–300 countable range, whereas the 10−2 plate result (312 colonies) exceeds 300, making individual colony distinction difficult and potentially causing undercounting. Therefore the 10−3 plate gives the more reliable count. [2 — correct calculation; dilution comparison with 30–300 rule correctly applied]

Hypothesis: The consistent pattern of higher E. coli concentrations at the bore head than at the building tap may reflect that the pipe network between the bore and the building provides some additional natural filtration or UV light exposure, or that the building’s internal chlorination system reduces bacterial counts. A testable hypothesis: “The chlorination treatment unit between the bore head and the building tap reduces E. coli CFU/100 mL by a factor of at least 2 in Q3 and Q4.” Independent variable: presence of chlorination treatment; dependent variable: CFU E. coli/100 mL measured at each point. [1 — testable hypothesis with named independent and dependent variables]

Validity strengths: (1) Multiple dilutions (10−2 and 10−3) were used, allowing the most appropriate countable plate to be selected for calculation. (2) Three replicate plates per dilution level improve reliability by allowing averaging and identification of outliers. Limitation: Testing was conducted only once per quarter (four occasions per year). Contamination events following rainfall, flooding, or pipe disturbance could be completely missed between collection points; a single sample cannot capture within-quarter variability. [1 — two strengths correctly identified; one limitation with explanation]

Public health actions: (1) Issue an immediate boil-water advisory for the community: the E. coli counts in Q3 and Q4 far exceed the guideline of <1 CFU/100 mL, indicating faecal contamination and the likely presence of other pathogens including Salmonella, Campylobacter, and potentially Cryptosporidium; boiling kills bacterial and protozoal pathogens and is a practical short-term intervention. (2) Investigate and treat the contamination source at the bore head: the bore is the entry point for contamination and counts are higher there than at the tap, suggesting subsurface contamination from surface runoff, animal faeces, or a damaged bore casing. Treating the bore with chlorination and repairing any casing faults would address the root cause rather than only the symptom. [2 — two justified actions, each linked to lesson content]

Marking criteria.

• 1 mark — Describes the trend across all four quarters with specific CFU values (pattern of escalating contamination; bore > tap; seasonal/quarterly pattern).
• 1 mark — Correct CFU/mL calculation from the 10−3 plate result for Q4 bore head.
• 1 mark — Correctly identifies the 10−3 plate as giving a more reliable count (31 colonies is within the 30–300 range; 312 exceeds it) with clear reasoning.
• 1 mark — States a specific, testable hypothesis that names an independent variable, a dependent variable, and is consistent with the data pattern.
• 1 mark — Identifies two validity strengths of the design (e.g. multiple dilutions + replicate plates).
• 1 mark — Identifies at least one significant limitation and explains its effect on reliability or validity (e.g. low sampling frequency; seasonal variation missed).
• 2 marks — Two specific public health actions, each with a justification drawn from lesson content (1 mark each: action + lesson-linked justification).

Q2 — Sample Band 6 source critique (7 marks), annotated

One defensible element: UV light does inactivate many bacteria by damaging their DNA, preventing replication. If a UV purifier produces zero colonies on agar after treatment, this is consistent with a significant reduction in viable bacteria that can grow on nutrient agar under standard conditions. This part of the claim is at least partially correct. [1 — correctly identifies and explains the defensible element]

Error 1 — “completely safe from all pathogens” / “100% free from microorganisms”: Zero colonies on a nutrient agar plate only means no bacteria capable of growing on that specific medium under those specific incubation conditions were detected in the plated volume. Viruses (e.g. norovirus, hepatitis A virus) cannot grow on any agar medium — they require living host cells — and would show zero colonies regardless of whether the UV treatment killed them or not. A zero plate count therefore provides no information about viral safety. [1 — specifically identifies the virus limitation of the plate count method]

Error 2 — plate counts “give a complete picture of all possible health risks in water”: Nutrient agar plate counts detect only bacteria that can grow on that medium under the test conditions. Protozoan parasites such as Cryptosporidium parvum and Giardia lamblia cannot be cultured on agar — they require immunofluorescence microscopy or PCR for detection. A zero colony count says nothing about the presence or absence of these organisms. Additionally, bacteria requiring special culture conditions (e.g. Campylobacter, which requires microaerophilic conditions and selective media) would not appear in a standard nutrient agar count. The claim that plate counts give a “complete picture” is false. [1 — identifies protozoa or specialised bacteria as pathogens not detected by plate count, with specific examples]

Error 3 — families no longer need water tested by health authorities: Even if a UV purifier is effective, UV-treated water can be re-contaminated after the treatment unit if plumbing is compromised, if the UV lamp fails, or if the device is used incorrectly. Ongoing monitoring by qualified environmental health officers using accredited methods — including selective media for specific indicator organisms and testing for viruses and protozoa using appropriate techniques — is required to confirm continued safety over time. Consumer-device testing is not an equivalent replacement for regulatory monitoring. [1 — correctly challenges the claim that consumer testing replaces regulatory monitoring, with lesson-linked reasoning]

Plate count method limitations relevant to UV purifier evaluation: The plate count method only estimates viable bacteria (cells that can grow under the conditions used). Dead bacteria (which a UV device may have inactivated but not removed) still occupy space in the water and are not detected. More importantly, the plate count gives no information about the UV device’s effectiveness against viruses, protozoa, or bacterial species requiring special media. A genuine safety evaluation of such a device would require multiple test methods (membrane filtration with selective media for E. coli, PCR for viruses, immunofluorescence for Cryptosporidium) and independent testing by an accredited laboratory, not a simple plate count. [1 — applies specific plate count limitations to evaluate the adequacy of using it for a UV device claim]

Scientifically defensible reformulation: “Independent laboratory testing shows this UV purifier significantly reduces the colony-forming count of culturable bacteria in tap water under the test conditions used. A zero-colony result on nutrient agar indicates that no bacteria capable of growing under those conditions were detectable in the volumes plated. However, plate counts do not detect viruses, protozoan parasites, or bacteria requiring specialised culture conditions. This device should be used as an additional precaution alongside — not as a replacement for — regular monitoring by qualified environmental health officers using the full range of appropriate testing methods.” [1 — reformulation is biologically accurate, applies the limitations of the plate count method, and correctly scopes what zero colonies means]

Marking criteria.

• 1 mark — Identifies one defensible element (UV does inactivate many bacteria; zero-colony plate count is consistent with reduced viable bacterial count) and explains why it is correct.
• 1 mark — Identifies and explains Error 1: zero colonies does not mean free of viruses (viruses cannot be detected by agar plate counts).
• 1 mark — Identifies and explains Error 2: plate counts do not detect protozoan parasites (e.g. Cryptosporidium, Giardia) or bacteria requiring specialised media — the method does not provide a “complete picture.”
• 1 mark — Identifies and explains Error 3: consumer plate count testing cannot replace ongoing regulatory monitoring by qualified health authorities.
• 1 mark — Applies specific plate count method limitations (detects only culturable bacteria under specific conditions) to explain why a plate count is inadequate for independently verifying UV purifier safety claims.
• 2 marks — Reformulates the claim into a scientifically accurate statement that correctly defines what zero colonies means, scopes the test’s limitations, and does not overstate safety. (1 mark for correct scoping of zero-colony meaning; 1 mark for explicitly retaining regulatory monitoring and identifying non-culturable pathogen classes.)