Water quality can be impacted if we are not careful with how we dispose of unwanted products. Old paints, pesticides, cleaners, pharmaceuticals, etc. should be taken to the appropriate Household Hazardous Waste disposal facility.
If they are disposed of down the drain or toilet, they could directly impact the drinking water of those on well and septic. Septic systems and sewer treatment plants cannot remove all of the chemicals that some household products contain.
Even after going through a treatment plant, some chemicals that don't break down easily can enter the environment in trace amounts, eventually accumulating into amounts large enough to impact aquatic habitat.