How is surface water contaminated?
Contamination of surface water means pollution of the water in a biological way. Therefore some kind of fecal pollution must be happenning.
This may happen when excessive raining happen and the sewer system cannot hold that much water. Water can get out of the sewer pipes and get mixed with the surface runoff and carried to the surface water resources. Also if a city dumping wastewater to the surface water resources without any treatment contamination will happen. Also if you are a farmer and let your cows drink and get washed by river, this can also cause contamination.