THere has been studies done showing Nitrogen levels in water...

THere has been studies done showing Nitrogen levels in water previously considered low enough to prevent blue baby syndrome are still 10 times higher than needed to prevent colon and bowel cancer which from memory is around 3 mg per liter
IT looks like this Chinese Class 1v ( <1.5 gm liter ) is under that needed to prevent bowel cancer and if so is stricter than much of the nitrate levels considered safe in the USA .
Many aquifers are being contaminated by nitrates from agriculture ie the Canterbury plains in NZ , where the sandy soils with low cation capacity and high percolation act like a low quality sieve , but it can take years for the poisoned water to get into the aquifer but once in the outcome is irreversible also for many years probably similar around the world