When you pour hazardous household products down the sink or flush them down the toilet the hazardous materials enter either a septic system or a municipal sewer system. If you have a septic system, wastewater from your house goes into a tank buried underground. The solids settle out and partially decompose. The remaining wastewater then goes into a drain field where the natural processes ongoing in the soil help to further break down the wastewater. Toxic materials in that wastewater can kill the helpful bacteria and the system will not operate properly.
Some toxic materials move through the soil untreated or unchanged.
When this happens ground water or surface waters may become contaminated. For example, many paint removers and aerosol paint products contain the chemical methylene chloride. This chemical can pass directly through a septic system without breaking down at all. Chlorine bleach can also pass through a septic system without breaking down. Also the chlorine can react with organic matter to form new toxic chemicals.