Protecting your well from your septic system

Many homes that have a septic system also have a private well, often on the same lot. That's why we cover both on one site. Your wastewater goes into the ground, and your drinking water comes out of the ground. EPA's guidance on septic systems and drinking water explains how to keep the two apart.

How contamination happens

Wastewater contains bacteria, viruses, and nutrients such as nitrate. In a working system, the tank holds back solids and the soil under the drainfield removes most pathogens and some nutrients before the water reaches groundwater. EPA stresses that soil can't remove everything. Medicines, some cleaning products, and other chemicals can pass through.

Problems arise when:

• the system is failing and untreated wastewater surfaces in the yard, where it can enter a well through an unsecured cap or cracks in the casing;
• the well is too close to the drainfield;
• the well is downgradient, meaning groundwater flows from the septic system toward the well;
• the soil is very sandy or porous, so effluent moves quickly with little treatment.

Lower risk, higher risk

EPA sums it up this way. Your well's risk is lower when the well and septic system are far apart, the well is deep (especially in bedrock or below a layer of silt or clay), and the septic system is pumped and serviced regularly. The risk is higher when the well is shallow and in permeable soil, sits downgradient of the septic system, or the system isn't maintained.

Setback distances

EPA notes that most states or local governments require a minimum horizontal distance, or setback, between a septic system and a drinking water well. The numbers vary by state and by the type of well and system, so check with your local health department. EPA adds that where the soil is sandy or porous, you may want more distance than the minimum. Remember that your neighbors' systems count too.

What you can do

• Maintain the septic system. Inspect it at least every three years and pump on schedule. Our pumping calculator gives a starting point.
• Keep chemicals and medicines out of drains. They can pass through the soil into groundwater.
• Protect the wellhead. Keep the cap tight and vented, make sure the casing sticks up above the ground, and slope the ground so water drains away.
• Test the well every year. Coliform and nitrate are the two most likely to show septic influence. Use our test reader to interpret the results.
• Know where everything is. A sketch showing the well, tank, and drainfield with distances helps contractors avoid mistakes. A new drainfield or well should never be placed without checking the setbacks.

Warning signs

EPA lists high nitrate or coliform in drinking water wells among the signs of a failing septic system. If your well test changes suddenly, check the septic system. If your septic system shows signs of failure, such as soggy ground, odors, or backups, test the well. See signs your septic system is failing.

Understanding groundwater flow

EPA explains that groundwater flowing beneath a drainfield picks up any contaminants left in the effluent. A well is at greater risk if it sits in the path of that flow. Groundwater generally moves from higher ground toward lower ground and toward streams, lakes or other discharge points. As a rough guide:

• A well uphill from the septic system is usually better protected.
• A well downhill, or between the drainfield and a stream, is more exposed.
• In fractured bedrock or karst (limestone with caves and sinkholes), water can travel quickly and unpredictably. Extra caution is needed there.

Your local health department or a licensed well contractor can help you judge flow direction on your property.

Signs your well may be affected

Watch for:

• a positive coliform or E. coli test;
• nitrate rising from year to year, even if still under 10 mg/L;
• changes in taste, color or smell, which CDC says can mean the water has been contaminated;
• soapy or sewage odors in the water;
• illness in the household with no other explanation, especially recurring stomach upsets. EPA's testing chart links recurring gastrointestinal illness to coliform bacteria.

If any of these appear, switch to bottled water, have the well tested by a certified lab, and have the septic system inspected.

Maintenance that protects your water

EPA recommends:

• inspecting the septic system at least every three years (alternative systems generally once a year);
• pumping the tank about every three to five years, or when sludge and scum approach the outlet;
• using water efficiently, so the drainfield isn't overloaded;
• disposing of waste properly: only human waste and toilet paper in toilets, and no chemicals, medicines or grease down the drain.

When something's wrong with the septic system

If your septic system is failing, untreated wastewater can surface and reach the well through an unsealed cap or cracks in the casing. Act quickly:

1. Cut water use to reduce the load.
2. Call a septic professional to diagnose the problem.
3. Keep people and pets away from surfacing sewage.
4. Test the well, and use bottled water until results come back clean.
5. Contact your local health department. Repairs usually need a permit.

Planning new construction

If you're adding a bedroom, replacing a drainfield or drilling a new well, the setbacks matter. Check the required distances with your health department before you finalize the design. Many health departments require a site evaluation and permit, and will check well and septic placement together.

Key takeaways

• Keep the well uphill and as far from the drainfield as practical.
• Maintain the septic system on schedule.
• Test the well every year for coliform and nitrate.
• Treat sudden changes in water quality as a reason to check both systems.

Frequently asked questions

Does my neighbor's septic system matter? Yes. Setbacks apply to every system near your well, not just your own.

How deep should my well be? There's no single answer. Deeper wells in protected aquifers are generally safer, so ask a licensed well contractor about local conditions.