Designing a Fair Water-Quality Comparison
Two students compare turbidity in a creek above and below a construction site. They take one 500 mL sample above the site at 9 am on a dry Monday and one below the site at 4 pm the same day after a rain shower. Turbidity reads 12 NTU above and 85 NTU below. They conclude the construction site causes an eightfold increase in turbidity. Their teacher notes the samples were taken at different times, different flow conditions, and only once each. The class has a turbidity meter, sample bottles, a flow meter, and access to the creek over four weeks.
1. Identify the design flaws in the students' investigation and explain how each could distort the conclusion.
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Flaws:
- Different times and flow conditions: the lower sample followed rain, so elevated turbidity may reflect runoff and higher flow, not the site. Flow and turbidity are strongly linked.
- Single sample per site: no replication means no estimate of natural variability or uncertainty; one reading cannot establish a difference.
- No control for upstream baseline: comparing upstream and downstream at different moments confounds location with time.
- No measurement of flow: without discharge data, turbidity differences cannot be attributed to sediment load.
- No blanks or calibration checks: instrument drift or handling could bias readings.
Effect on conclusion: each flaw provides an alternative explanation (rain, flow, chance) for the eightfold difference, so causation is not established.
2. Outline an improved sampling design using the available equipment and four-week timeframe.
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Design:
- Paired sampling: collect upstream and downstream samples within minutes of each other at each visit, so both sites experience the same weather and flow.
- Replication: take 3 replicate samples per site per visit, and repeat visits at least twice weekly for four weeks, covering both dry and post-rain conditions.
- Stratify by flow: record discharge with the flow meter at each visit; group results into low-flow and high-flow conditions and compare within each group.
- Standardise method: same depth, same bottle rinse procedure, calibrate the turbidity meter, and record time, weather, and recent rainfall.
- Analysis: compare paired upstream–downstream differences, report means with variability, and test whether the difference persists after accounting for flow.
Interpretation: a consistent downstream increase under matched flow conditions, repeated over weeks, supports a site effect; a difference only after rain points to runoff rather than continuous discharge.