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Protecting Drinking Water From Septic Systems

Where Does Septic System Water Come From?

About 25 percent of U.S. households rely on septic systems to treat and dispose of sanitary waste that includes wastewater from kitchens, clothes washing machines, and bathrooms.

Septic systems are primarily located in rural areas not served by sanitary sewers. A typical household septic system consists of a septic tank, a distribution box, and a drain field.

The septic tank is a rectangular or cylindrical container made of concrete, fiberglass, or polyethylene. Wastewater flows into the tank, where it is held for a period of time to allow suspended solids to separate out.

The heavier solids collect in the bottom of the tank and are partially decomposed by microbial activity. Grease, oil, and fat, along with some digested solids, float to the surface to form a scum layer. (Note: Some septic tanks have a second compartment for additional effluent clarification.)

The partially clarified wastewater that remains between the layers of scum and sludge flows to the distribution box, which distributes it evenly through the drain field. The drain field is a network of perforated pipes laid in gravel-filled trenches or beds. Wastewater flows out of the pipes, through the gravel, and into the surrounding soil.

As the wastewater effluent percolates down through the soil, chemical and biological processes remove some of the contaminants before they reach ground water. Large capacity septic systems are essentially larger versions (with larger capacities and flow rates) of single family residential septic systems, but they may have more than one septic tank or drain field for additional treatment capacity.

In some cases, an effluent filter may be added at the outlet of the large capacity septic tank to achieve further removal of solids. Many large systems rely on pumps rather than gravity to provide an even flow distribution into the drain field.

Protection of Drinking Water Sources

Septic systems can be a substantial source of water contamination and pose a risk to public health. Among the dangers include gastrointestinal illness, cholera, hepatitis A and typhoid due to bacteria, protozoa and viruses present in (sanitary) wastewater.

When it comes to nitrogen – a component of urine, feces, food waste and cleaning products – ingestion of high levels can lead to methemoglobinemia (“blue baby syndrome”), which reduces blood’s oxygen-carrying capacity and may even be fatal for infants.

Regulatory bodies have therefore set a Drinking Water Maximum Contaminant Level (MCL) of 10 mg/l or parts per million (ppm) for nitrates measured as nitrogen in drinking water sources. To minimize risks associated with septic system discharges into these areas, responsible stewardship is required to ensure proper functioning of these systems so that the MCL standard would not be exceeded.


Prevention measures are an important way of stopping septic systems from contaminating source water. Different states and localities have different regulations regarding how to site, design, construct, and operate and maintain large capacity septic systems.

Examples of prevention measures include siting the system away from bodies of water, proper design and construction techniques, adequate operation and maintenance routines, using appropriate soil and infrastructure conditions, creating a scheme of incentives to discourage irresponsible behavior, conducting regular inspections of the system, and more.

Each individual measure may be useful on its own to help prevent contamination of source waters but combining multiple prevention measures in an overall prevention approach is often the most effective way for reducing risks in any given community.


When determining where to site a septic system, local health and sanitation officials must consider several factors. The system should be set back from buildings and drinking water sources like wells to avoid potential contamination.

Additionally, areas with high water tables and shallow impermeable layers are generally not suitable as there isn’t an adequate unsaturated soil thickness for treatment. Furthermore, the soil permeability must be within an appropriate range – if it’s too low the drain field may not be able to handle the wastewater; if it’s too high, the effluent could reach ground water before it is adequately treated.

Loamy soils that are well-drained tend to work best for septic systems, and officials should also evaluate the combined loadings of all systems in the area to see if it will stress the soils or receiving waters.

Design and Construction

When designing and constructing a septic tank and drain field, there are several factors to consider. The size of the tank and field should be appropriate for the amount of wastewater it will receive.

Additionally, properties such as topography, permeability of the soil, and other environmental aspects should be taken into account. It is also important to make sure that wastewater does not contain too much fats, grease or oils which can lead to clogging and premature failure.

Construction must be done by a licensed septic system installer in accordance with regulations. Care must also be taken not to compact or drive over the soil during installation. Heavy vehicles should not operate from below the drain field area and work should not be performed on wet ground as this can cause excessive smearing.

Operation and Maintenance

Properly operating and maintaining a septic system is key to preventing contamination from happening. Homeowner’s Associations and tenant associations should educate their members on how to take care of their systems while commercial establishments like strip malls should also do the same. To monitor any signs of failure, it’s important to continuously watch out for odors, surfacing sewage, and lush vegetation in the drain field area, as well as to inspect the septic tank annually.

In order to extend your drain field’s life and reduce hydraulic overload, be sure to conserve water through using less water fixture models, taking shorter showers and washing only full loads of dishes or laundry. Additionally, keep runoff from roofs, driveways, or patios away from your drainfield by diverting direct rainwater away with a landscaping blade or swale.

Septic tanks need to be pumped every two-five years depending on tank size and amount of wastewater used, but generally speaking avoid introducing solvents such as drainage cleaners, pesticides or pharmaceuticals into your system as this can interfere with its operation technology.

You should also refrain from using additives containing yeast/bacteria/solvents or petrochemicals as there has been no proof that these products improve performance. Moreover, use your system sensibly – cars and heavy equipment should not be traversing over it at all – nor should you plant trees nearby due to potential root obstruction/infiltrations – while construction shouldn’t occur over the drainfield either.

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