Septic Systems and the Water Table
Unless your area has the misfortune of being in a drought, odds are, the ground around you is wetter now that we’ve come to spring. Melting snow and springtime rain are valuable in replenishing our groundwater, but when there’s an especially wet spring they can also pose challenges for septic systems by substantially raising the water table. Of course, a big rain event can happen in any season, and some areas tend to always have a high water table — so spring is not the only time when water table difficulties can occur.
What is a high water table, exactly? A high water table is a groundwater level that is very close to or sometimes even above the ground’s surface. There’s a number of potential issues this can cause for a home/property owner, but the high level of soil saturation is a particular issue for septic systems because, in short, it stops them from being able to drain wastewater effectively.
Enough rain and/or snowmelt can create a temporarily high water table nearly anywhere, but a number of areas in the United States also have a naturally high water table. You can generally expect a high water table along the coasts, in low-lying regions, and nearby other major water sources such as the Great Lakes and the Mississippi River.
What Happens to a Septic System When the Water Table Is Too High
If there is too much water in the soil surrounding a septic system, the simple reality is that the flow of wastewater from the home, into the septic, and on into the drainfield will be significantly slowed or even halted. Sometimes, water in the drainfield may even flow back into the septic tank. A waterlogged/flooded tank at best becomes a holding tank, and at worst may cause water to back up into the home. In any case, a property’s drains will run slowly and toilets are less likely to flush properly, hampering day-to-day activities.
It’s also possible for a septic tank to float in the saturated soil if the proper precautions haven’t been taken when installing the system in an area with a high water and/or is prone to flooding, which is another major challenge to fix.
Besides these possible headaches for a property owner, there is a public health risk present when a septic system is unable to drain properly into an oversaturated drainfield, or even when a tank is simply not far enough above the water table. The effectiveness of a septic system as a wastewater management system is dependent on having enough soil for the wastewater to filter through before it becomes a part of the local groundwater supply.
In a heavy rainfall and/or flooding event, the water leaving a septic system and the risen water table will immediately meet and mix, transferring the wastewater’s unfiltered contaminants (including pathogenic organisms like bacteria and viruses, in addition to high concentrations of nitrate, phosphorus, and carbon) into the water supply. This is a direct threat to human and environmental health. Again, this is also an issue for systems that are too close to the top of the water table even if there is no flooding event, as the amount of soil separating the system and the water table may not be adequate to fully filter out these contaminants.
What Can Be Done About a High Water Table During a System’s Installation?
It is essential to pick the best location available on a property for the system where a high water table will be least likely to pose an issue. The first step in that process is simply knowing the water table level. You might consult the state geological or water survey, check with the state environmental agency, or hire a hydrologist to get measurements. Another practice that can be useful to judge water table depth is observing the soil color, which a soil scientist is typically best suited to do. Lower and concave areas of the property should be avoided as the water table will be even higher in these areas because of the natural flow of groundwater moving into these areas.
While it would be easy to simply pick the highest area of elevation on a property, all of the other standard considerations of siting a septic system still apply: setback distances, soil composition, tree and plant growth, and more. All of this must be balanced with elevation and the water table levels when picking a site.
The next critical decision is selecting the right type of septic system to put in to combat the challenges of the water table. A conventional system may still be able to work for some properties, but for others, an alternative system may well be needed. Alternative systems that are appropriate for these situations include pressure distribution systems, mound systems, gravelless systems, and sand filter systems. There is no one size-fits-all alternative system that will work for every property with a high water table, so it is important to take all site factors into consideration and consult with the necessary experts to ensure the best system is selected.
Finally, in some cases, anti-buoyancy measures may be required to be added to the tank during the installation of a system. In an area with a high enough water table and/or that is prone to flooding, it is very much possible for a tank to float when the soil is saturated sufficiently. The first thing to do in such an area is to conduct a buoyancy analysis. Dr. Sara Heger breaks down the process of these calculations in “How to Ensure a Septic Tank Won’t Float in Saturated Soil” for Pumper Magazine.
If it is found that a tank is likely to float, restraint systems such as blocks & straps, restraining collars, and helical anchors are among the options available to prevent flotation.
What Can Homeowners Do When the Water Table Is Higher Than Normal?
When the water table is temporarily high, it is important for homeowners to use common sense water conservation measures to avoid adding any more stress to their septic systems than necessary. The more water that is used on the property while the water table is exceeding its average, the more likely a septic system is to back up into the house, cause severe plumbing issues, or sustain damage itself.
The first thing a homeowner should do is check for leaks in any water-based fixture, such as faucets, showerheads, toilets, sinks, and beyond. If any leak is found, it should be repaired as quickly as possible. Next, if possible, the flow of water from basement sump pumps and roof gutters should be directed away from the septic system’s discharge area.
Then comes the more arduous task of reducing the usage of water in routine activities. This means reducing the number of times the toilet is flushed, the number of showers and baths taken, and operation of the washing machine and dishwasher. The dishwasher should only be run when full, and if feasible, washing clothes at a laundromat should be considered.
If a homeowner notices that their plumbing is still not working correctly after taking steps to reduce the water table height, a licensed septic professional should inspect the system to determine the exact issue.
Sources & More Information
- Best Practices Septic Tank Installation | Valencia Pipe Company
- Choosing an Alternative Septic System for a Homesite with a High Water Table | University of Minnesota Extension
- Hell and High Water: Diminished Septic System Performance in Coastal Regions Due to Climate Change | PLOS One
- How to Ensure a Septic Tank Won’t Float in Saturated Soil | Pumper Magazine
- The Impacts of Soil Color and Depth to Limiting Condition on Septic System Installation | Pumper Magazine
- Navigating the Season: Understanding How Spring Rain Impacts Your Septic System | Helmer Septic
- An Overview of Onsite System Site Plans for Installers | University of Minnesota Water Resources Center
- Septic Systems and High Water Tables | North Dakota State University Extension
- Septic Tank Services in High Water Table Areas: Challenges and Solutions | A1 Tank Service
- Surface Prep for Aboveground Onsite Wastewater Systems | Onsite Installer