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Septic Systems

Septic Systems and Surface Water

Many homeowners rely on septic systems for safe and effective treatment of their wastewater. Household wastewater is treated by a septic system before it filters into the soil. Recycled water from a septic system can help replenish groundwater supplies; however, if the system is not working properly, it can contaminate nearby waterbodies. 

Use the diagram below to learn how nutrients and pathogens from your septic system may impact streams, lakes, or other waterbodies near your home.

Click numbers on the graphic below to display related information beneath the graphic.

An alternative text version that does not require use of a mouse is available on the table tab.

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1. Bathrooms and Kitchens Wastewater from toilets, sinks, showers, and other appliances contains harmful bacteria, viruses, and nutrients that could contaminate nearby surface water sources. You can help reduce the amount of nutrients in your wastewater by limiting use of the garbage disposal and using phosphate-free detergents. Avoid flushing other chemicals or medications down the drain or toilet since they could also contaminate surface water sources.  
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. Learn more about maintaining your drainfield.
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, cleaning products, other potentially harmful chemicals). If untreated wastewater surfaces in the yard, wastewater may contaminate the streams, lakes, or coastal waters near your home. Avoid putting chemicals or medications down the drain or toilet since they could end up in surface waters too.

Learn more about sources of and solutions to nutrient pollution.

Learn more about preventing eutrophication.

5. Water Table The water table is 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 stream, lake, or coastal water is at greater risk of becoming contaminated if it is in the path of groundwater flow beneath the septic system. Learn more about getting up to speed with protecting groundwater.
7. Nutrients in Surface Water (Nitrogen, Phosphorus)

When there are too many nutrients in surface water, they act as a fertilizer for fast-growing bacteria and algae. This rapid growth can cause algal blooms that can reduce water quality, kill aquatic animals and plants, and form toxins in the water. This process is called eutrophication. Harmful algal blooms (HABs) in lakes and streams can be toxic to humans and animals.

Phosphorus: Depending on your soil type, phosphorus from wastewater can be absorbed and retained in the soil. Unabsorbed phosphorus can travel in groundwater toward a waterbody and become a source of contamination. Freshwater is more vulnerable to phosphorus pollution.

Nitrogen: Some nitrogen may be removed as wastewater flows through the septic system and soil. But the remaining nitrogen can enter the underlying groundwater and flow towards a surface water body. If there are many septic systems in a small area, the nitrogen flowing through groundwater could overload a waterbody, causing eutrophication. Saltwater is more vulnerable to nitrogen pollution.

Learn more about harmful algal blooms and cyanobacteria.
8. Setback Distance Most states or local governments require a specific horizontal distance (or setback) between a septic system and surface water bodies. If the soil where you live is sandy, or porous, you may want to place your septic system farther away than the minimum required distance. Contamination is less likely the farther away your septic system is from a body of water. Consult your local health department about required setback distances in your area.
9. Streams, Lakes and Coastal Waters

Groundwater and surface water runoff flows into streams, lakes, and coastal waters. If this water contains contaminants, they can make their way into surface waters, causing eutrophication (see #7). It’s important to keep surface waters healthy to use for recreation, fishing, and as a drinking water source.

Learn more about the environmental problem of nutrient pollution.

Learn more about the effects of nutrient pollution.