Reading a River: Sewage, Fertiliser and a Single-Fix Report
A citizens' group surveyed a 4 km stretch of a river that flows through a town. Site 1 is 500 m upstream of the sewage outfall, Site 2 is 300 m downstream of it, and Site 3 is 3 km further downstream. All samples were collected on the same morning at 6 am; water temperature was 28 °C.
| Measure | Site 1 | Site 2 | Site 3 | Site 4 (tributary draining farmland) |
|---|---|---|---|---|
| Dissolved O₂ (mg/L) | 8.2 | 3.1 | 5.8 | 7.9 |
| BOD (mg/L) | 1.5 | 18.4 | 6.2 | 1.8 |
| Nitrate (mg/L) | 0.4 | 9.8 | 3.1 | 11.2 |
| Faecal coliform (per 100 mL) | 40 | 24 000 | 900 | 60 |
| Algal density | low | very high | moderate | low |
Site 4 has almost the same nitrate level as Site 2 but clean-looking water. The group's draft report says the sewage plant is the only source of pollution and recommends a single remedy: releasing oxygen-rich water from tankers into the river.
1. Explain why dissolved oxygen at Site 2 is only 3.1 mg/L, using both the BOD and the algal data. Why does the 6 am sampling time matter?
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BOD 18.4 mg/L means a large mass of biodegradable organic matter is present. Aerobic bacteria oxidise it and consume dissolved oxygen faster than the river can replace it from the air, so DO falls from 8.2 to 3.1 mg/L. The very high algal density shows a second, linked process: the nitrate and other sewage nutrients fertilise an algal bloom. Algae release oxygen in sunlight but respire all night, and when they die their cells become additional organic matter for bacteria, so the bloom also raises oxygen demand.
Sampling at 6 am matters because it is close to the end of the night, when algal respiration has been removing oxygen for hours and photosynthesis has not yet begun. An afternoon sample would show higher DO and could make the sewage impact look smaller than it is. DO also falls as temperature rises, and at 28 °C the water holds less oxygen, so the same organic load is more damaging than in winter.
2. Site 4 has nitrate 11.2 mg/L, higher than Site 2, yet its water looks clean. Explain the difference between the two sites.
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The two sites carry different kinds of pollution. Site 2 receives raw or partly treated sewage, which is rich in organic matter (high BOD) and in faecal bacteria (24 000 coliforms) as well as nutrients. Site 4 receives fertiliser runoff from fields: nitrate is high, but there is little organic matter to decompose (BOD 1.8) and no faecal source (60 coliforms, about background). Fertiliser alone does not consume oxygen directly, so the water stays clear at the point of entry.
The risk at Site 4 is delayed. The nitrate will travel downstream and feed an algal bloom there, and when that bloom dies and decomposes, BOD and oxygen demand will rise far from the field. Coliform counts are an indicator of faecal contamination, not of nutrient pollution, so a low count cannot be used to call fertiliser runoff harmless. Judging water quality needs several indicators, not one.
3. Criticise the group's report and recommend what should be done. Predict what will be found at Site 3 and explain the improvement there.
Read the worked answer
The report makes two errors. First, it identifies one source when the data show at least two: the sewage outfall (Site 2) and farmland runoff (Site 4) both deliver nitrate, and they require different controls. Second, tanker oxygen treats the symptom for a few hours; it does not remove the organic load, pathogens or nutrients that cause the problem, and it would be very costly to sustain.
Better measures: repair and upgrade the sewage works to secondary treatment with disinfection, stop illegal connections and leaking septic tanks, and control diffuse pollution with reduced fertiliser use, riparian buffer strips and constructed wetlands. Monitoring should include upstream and downstream sites, several seasons and both morning and afternoon samples.
Site 3 shows partial recovery: dilution, reaeration over 3 km and settling of solids have raised DO to 5.8 mg/L and lowered BOD to 6.2 mg/L, algal density to moderate and coliforms to 900. Recovery is real but incomplete; 900 coliforms per 100 mL still makes the water unsafe for drinking or bathing, and nitrate remains above background, especially in the dry season when flow is low.