Beach water quality after a storm overflow
In late October a seaside town in Malta records 48 mm of rain in six hours. During the storm a nearby pumping station discharges diluted wastewater for about 90 minutes. Two days later a school has booked a non-refundable bus for 30 students to swim at the bay. The science teacher samples water at three points: A, 5 m from the outfall; B, 100 m along the beach where the group will swim; C, 300 m offshore. Samples are taken at 08:00 and 15:00 on the same day. Faecal indicator results per 100 mL are: A 08:00 1200, A 15:00 900; B 08:00 320, B 15:00 180; C 08:00 40, C 15:00 30. The school's guideline is 'below 200 per 100 mL'. The swim is planned for 10:00 the next day at B. One lifeguard is available. The sea is calm and looks clear. The school asks for a decision: cancel, move the swim to C, or go ahead with precautions.
1. What variables does this sampling design control or fail to control? Does it support a fair comparison between A, B and C? Give one practical improvement.
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The design holds the sampling day and method constant, and it compares three distances at two times, so it is a start. But it does not control tide, current, wind, wave height, water depth or the exact sampling depth, and there is only one sample per point and time. A and B are near the outfall and shallow; C is offshore and probably deeper, so C differs in more than distance. There is no pre-rain baseline, so the normal level is unknown. The 08:00 versus 15:00 difference could be sunlight, tide or dilution, not only decay. A fairer comparison would take three replicates per point, record tide and current direction, sample at a fixed depth, and add a control beach without an outfall. The improvement I would choose first is repeated sampling with tide data, because a single 180 count at B cannot show a trend.
2. Using the numbers and the timing, what advice should the teacher give the school? Explain the main uncertainty.
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At 15:00 B was 180 per 100 mL, just under the school's 200 guideline, and the 08:00 value of 320 was above it. The direction is downward, but one pair of samples is weak evidence. The planned swim is 19 hours later. Sunlight and dilution may lower counts further, but rain can also resuspend sediment, and a new tide or current can move the plume back. The guideline is a management threshold, not a guarantee that no pathogen is present. My advice is to postpone the swim for 24 to 48 hours and retest B at the intended time and depth. If postponement is impossible, move to C only if the lifeguard confirms safe access and depth, keep students from swallowing water, cover cuts, and rinse after. The non-refundable bus is a financial loss, not a safety reason. A clear appearance is not evidence of low faecal indicators.
3. A parent argues that because C is below 200 and the sea is clear, the group should swim at C. Evaluate this argument.
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The parent is right that C's counts of 40 and 30 are well below 200, and dilution offshore is plausible. But the comparison is not like-for-like: C is 300 m offshore, possibly deeper, with different currents and access, and the lifeguard's coverage may not be adequate. The guideline is written for bathing areas, not for an offshore school group. One sample per time cannot rule out a patchy plume, and a low count does not prove absence of viruses or other pathogens. The decision should weigh exposure: 30 students, one lifeguard, no changing facilities offshore. A better use of C is as supporting evidence that the plume is diluting, not as the main swimming site. If the group must go, B after a retest is more defensible than C without a safety plan.