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Septic Systems

Septic Systems and Drinking Water

Septic systems provide wastewater treatment for many homeowners who also often get their drinking water from private wells. If a septic system is not working properly or is located too close to a drinking water well, contaminants from the wastewater can end up in drinking water. Learn how to locate, operate, and maintain your septic system to protect nearby wells.

Use the diagram below to learn about the typical components of a septic system, the potential for contamination of a well by a septic system, and the associated risk factors.

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1. Bathrooms and Kitchens Water from toilets, sinks, showers, and other appliances is called wastewater and can be harmful to human health. Wastewater contains harmful bacteria, viruses, and nutrients that could make you sick if it comes in contact with your drinking water well. Make sure the wastewater is properly treated by your septic system and that your drinking water well is located at the appropriate distance (set back) from your and your neighbor’s system. Avoid flushing other chemicals or medications down the drain or toilet since they could also contaminate your drinking water well.  
2. Septic Tank Wastewater generated in your home exits through a drainage pipe and into a septic tank. The septic tank is a buried, water-tight container that holds wastewater for separation and treatment. The solids settle to the bottom (sludge) and fats, oil and grease float to the top (scum). Microorganisms act to break down the sludge and destroy some of the contaminants in the wastewater. Your septic tank should be serviced and pumped on a regular basis to make sure it’s working properly. Learn more about how your septic system works.
3. Drainfield The drainfield is a shallow, covered trench made in the soil in your yard. Partially treated wastewater from the septic tank flows out through the drainfield, filters down through the soil and enters the groundwater. If the drainfield is overloaded with too much liquid or clogged with solids, it will flood and cause sewage to surface in your yard or back up into your home.  
4. Wastewater Treatment in Soil Filtering wastewater through the soil removes most bacteria and viruses (also known as pathogens) and some nutrients. While soil can treat many contaminants, it cannot remove all of them (e.g., medicines, some cleaning products, other potentially harmful chemicals). If untreated wastewater surfaces in the yard, wastewater may contaminate your drinking water through an unsecured well cap or cracks in the well casing. It’s important to avoid flushing medication and chemicals into your wastewater since it could contaminate your drinking water.  
5. Water Table The water table is found where you first hit water if you dig a hole into the ground.  
6. Groundwater The water below the water table is called groundwater. Groundwater flowing underneath a drainfield captures any remaining contaminants released from the septic system. A drinking water well is at greater risk of becoming contaminated if it is in the path of groundwater flow beneath a septic system.  
7. Drinking Water Well A drinking water well is drilled or dug into the groundwater so water can be pumped to the surface. Deep wells located farther away from a septic system and not in the path of the groundwater flow from the septic system are least likely to be contaminated. Drinking water wells should be regularly tested to ensure your home’s water is safe to drink. Learn about private water wells.
8. Setback Distance Most states or local governments require a specific horizontal distance (or setback) between a septic system and a drinking water well. If the soil where you live is sandy, or porous, you may want to place your well farther away than the minimum required distance. Contamination is less likely the farther apart a well is from a septic system. Consult your local health department about required setback distances in your area.
9. Could my well be affected?

Your septic system could contaminate your drinking water well or a nearby well under certain conditions. Remember to test the drinking water from your well regularly and take corrective action as needed.

The contamination risk to your well is LOWER: 
  • the farther apart the well and septic system are located;
  • the deeper the well is placed and if it is in bedrock or below a defined layer of silt or clay; or,
  • when your septic system is pumped, and serviced on a regular basis.
The contamination risk to your well is HIGHER:
  • if the well is at a shallow depth and in permeable soil;
  • if the well is downgradient of the septic system (i.e., if the groundwater flows from the septic system towards the well);
  • if there are many homes on septic systems near the well; or,
  • if there is poor construction or maintenance of the well and/or septic system (i.e., contaminants can enter a cracked drinking well casing from ground or surface water).
Learn other ways to keep your private well safe from possible sources of contamination.