A high value of biochemical oxygen demand in a water sample indicates:

The correct answer is B, a heavy load of organic pollution. Biochemical oxygen demand measures the oxygen that micro-organisms need in order to break down the organic matter present in a sample. The more organic waste, such as sewage, the more oxygen the microbes consume, so a high BOD is a direct sign of heavy organic pollution. Option A is wrong because clean water has a low BOD; a river upstream of a city will show a much lower value than the same river below the sewage outfall. Option C is wrong because high BOD in fact goes with low dissolved oxygen, since the microbes are using the oxygen up; the two move in opposite directions. Option D is wrong because BOD depends on the activity of bacteria, so a high value indicates their presence and vigour rather than their absence. Chemical oxygen demand for the same sample is always higher than BOD.