Inadequately treated sewage can contribute to both ground and surface water contamination. Many lakes and rivers are used to supply drinking water as well as recreational sources. Improperly treated sewage contains infectious bacteria and viruses. If it reaches surface water, it increases the chance of swimmers contracting a variety of diseases such as ear infections, gastrointestinal illness or hepatitis. Household wastewater also contains elevated levels of nitrogen and phosphorus, which can quickly lead to water quality degradation. If a septic system is working properly, it will effectively remove most of these pollutants.

Sometimes a septic system can fail because the amount of water entering the system is more than the system can handle. This could cause the water to back up into the house, the basement and the yard. Efficient water use means less water entering the septic system and less opportunity for system failure.