My water has coliform bacteria. This is dangerous because it means harmful bacteria and other disease causing organisms may be in your drinking water. Since coliform bacteria are found in the intestinal tract of humans and other warm blooded animals, it means that the water has been contaminated somehow. It also means the water shouldn't be used for drinking or cooking until the problem is fixed. My water has high nitrates.
Excessive nitrates are dangerous to infants under a year of age.
They can cause methemoglobinemia, a type of blue baby syndrome. They are natural in the environment, but amounts increase because of manure, fertilizers, septic systems, and other organic matter being in the area. Nitrate levels can be lowered using treatment devices or infants and nursing mothers can use another source of water.