Which substance indicates fresh pollution of sanitary significance?

Ammonia is a key indicator of recent pollution that poses sanitary risks. It is a compound that can be found in human waste and agricultural runoff, making its presence in water sources a strong signal that contamination has recently occurred. When ammonia is detected in water, it typically implies the entry of untreated or inadequately treated human effluent, animal waste, or other organic materials.

The presence of nitrites also points toward pollution, particularly related to bacterial action in the nitrogen cycle. However, nitrites indicate a more established process of pollution that has already undergone some level of bacterial transformation, which makes them less indicative of fresh pollution compared to ammonia. Chlorine, while a powerful disinfectant, is used primarily in water treatment and is not a natural indicator of pollution. Phosphates, on the other hand, are more associated with agricultural runoff and can indicate pollution, but they do not provide as direct a link to fresh sanitary contamination as ammonia does.

In summary, ammonia stands out as the most reliable indicator of fresh pollution due to its chemical nature and association with human and animal waste.