Fig. 1 shows a river beside a small town. Treated sewage is released from an outfall pipe. Scientists sampled the water 20 m upstream and at several points ...
Question 1 Report
Fig. 1 shows a river beside a small town. Treated sewage is released from an outfall pipe. Scientists sampled the water 20 m upstream and at several points downstream.
| Distance from outfall | Dissolved oxygen / mg per dm3 | Number of fish caught in a standard net |
|---|---|---|
| 20 m upstream | 9.1 | 18 |
| 100 m downstream | 5.4 | 7 |
| 500 m downstream | 7.8 | 14 |
(a) State two substances in untreated sewage that can cause water pollution. [2]
(b) Explain why the dissolved oxygen concentration falls soon after sewage enters the river. [4]
(c) Describe the relationship between dissolved oxygen concentration and the number of fish caught. [3]
(d) Suggest three ways the town could reduce pollution of the river. [3]
Answer Details
(a) Untreated sewage may contain faeces or other organic matter, urine or urea, detergents, pathogens, or nitrate-containing waste. Any two. [2]
(b) Organic material in sewage is food for decomposer microorganisms. Their population can increase, and they respire aerobically. Aerobic respiration uses dissolved oxygen from the river water, so oxygen concentration falls after the outfall. [4]
(c) Higher dissolved oxygen is associated with more fish caught. At 100 m downstream, oxygen is lowest at 5.4 mg dm-3 and only 7 fish are caught. By 500 m downstream, both oxygen concentration, 7.8 mg dm-3, and fish number, 14, have recovered. This is an association in these data; it does not alone prove oxygen is the only cause. [3]
(d) The town could improve sewage treatment, remove organic solids by settlement or filtration, use biological treatment, disinfect treated water, repair leaking sewers, and prevent chemicals being poured into drains. Any three. [3]
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