The two factors which can be used to find whether the river water has been contaminated are:

i) The presence of coliform bacteria in rainwater.- Coliform is a harmless bacteria found in human intestine. However, if coliform is present in water it is a sign that the water may contain other harmful bacteria.

(ii) Measurement of pH of rainwater.- pH level below 7 indicates high levels of acid and hence confirms the contamination of water.