Espejo-Herrera (2016) found when diets were high in vitamin C there was no statistically significant increase in risk of colon cancer from elevated nitrate levels in water
As I mentioned before, the cancer link must be related to poor diets lacking in antioxidants
Basically in recent years a bunch of epidemiological studies have found the level at which it affects us is lower than previously thought.
This paper from the Australian and NZ jnl of Public Health is pretty good I think: Nitrate in drinking water and cancer risk: the biological mechanism, epidemiological evidence and future research
As I mentioned before, the cancer link must be related to poor diets lacking in antioxidants