We had a similar discussion in Biology a while back regarding drinking water safety, but not with any emphasis on detection of contaminants. You might want to refer back to that discussion for additional information on water treatment and issues regarding compounds formed during that treatment.
As the blood becomes increasingly diluted, water moves out of the bloodstream and into cells, which swell. The swelling of the brain is responsible for the symptoms of severe hyponatremia -- nausea, confusion, seizures and coma. If pressure inside the skull increases enough, the base of the brain is squeezed downward through where connects it to the spinal cord, causing death."