Septic Systems and High Water Tables
By Tom Scherer, Irrigation and Water Resources Specialist, Home Septic Systems
NDSU Extension Service

With high local ground water levels due to above-normal rainfall last fall, many home septic systems have become waterlogged or temporarily flooded. As a result drains in the house may run slow, toilets may not flush properly and water may back up into floor drains in the basement.

A septic system has two main components: a septic tank which traps and biologically degrades solid waste and a drainfield which provides additional biological treatment as well as infiltrate the water into the ground. Household water flows from the house sewer system into the septic tank then out to
the drainfield. Any situation that prevents or slows down the flow of water through the septic system can cause problems.

When ground water inundates the septic tank, water will leak in through any opening such as the manhole cover, the inlet/outlet pipes or the tank cover and fill the tank with groundwater instead of waste water from the house. In addition, the high water table may saturate the drainfield.  When this happens the waste water coming from the house cannot move through the septic system easily. Water may actually flow from the drainfield back into the septic tank.

When high water table conditions occur, you may have to treat your septic tank as a holding tank and have it commercially pumped periodically. Remember, don't pump out more than half the volume of the tank. Removing more than half the contents could cause the tank to try to float out of the ground and damage the inlet or outlet pipes.

It is a common practice to pump the excess water from the septic tank onto the ground but this violates the North Dakota state plumbing code. Raw sewage on the ground (or in the snow) can present a health hazard because children and pets can run through it or it can flow into a water course. Water borne diseases are lethal and spread from person to person quickly.

Here are some suggestions to help your septic system deal with high water table:

1.  Reduce water use in the house. Make sure there are no leaking fixtures in the house. A drop of water every 15 seconds can add up to a lot of additional water added to the septic system.

2.  Check faucets, shower heads, toilets, sinks and any other water using device for leaks and repair them as soon as possible.

3.  Don't direct water from a basement sump pump into the septic system. Don't let water from roof gutters or from the sump pump discharge into the drainfield area.

4.  Reduce the number of times you flush the toilet. Wash clothes at a laundromat. Reduce the number of showers and baths each day. Run the dishwasher only when it is full.

Common sense is the key to reducing water use in the house and helping your septic system. Remember the drainfield was designed to infiltrate the amount of water normally discharged from the house. When additional water is added to the drainfield, the ability to handle household water becomes limited.

If household plumbing does not work correctly after the water table has gone down, the drainfield or septic tank may have been damaged. High ground water can cause shifting or settling of soil or septic system components which can affect both the septic tank and the distribution system in drainfield. The shifting can cause the inlet and outlets from the septic tank to become partially blocked. Also, the inlet
or outlet pipes could be blocked due to solids from the tank. Have a licensed septic tank pumper or septic system installer examine the situation.