Septic System Flooding, Key Factors to Know During Heavy Rainfall and Melting Snow


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Septic System Flooding, Key Factors to Know During Heavy Rainfall and Melting Snow                                                                         

In this article, we’ll learn about how flooding impacts homes with private septic systems, steps that can be taken before, during and after a flood to minimize damage to septic systems, and how to ensure that your system is safe to use at full capacity again.

The winter of 2017-2018 was a remarkable skiing season in western Canada. Some ski areas reported over 12 metres of snow accumulation during this time. Notably, below-average temperatures and wet weather in the spring of 2018 added volume to this already extraordinary snowpack. All that snow represented an enormous volume of melt-water when rain and warmer temperatures finally arrived in May, and the result was record flooding in many parts of British Columbia and Alberta.

saturated septic drain field

Flooding – which in many areas of western Canada is occurring with greater frequency and greater intensity – presents a host of concerns and costs along with the destruction it brings. One of these is damage to home septic systems, an inability to use them during and after the flood and subsequent sewage contamination of the flooded area.

Sewage, whether from public wastewater treatment facilities or private septic systems, are a significant source of contamination during flooding and in its aftermath. Flood waters carry raw sewage throughout the flood zone, spreading pathogens widely and contaminating homes, commercial and public buildings, and drinking water supplies.

Private septic system owners know that they are responsible for the care and maintenance of what is essentially their very own wastewater treatment plant. When flooding is predicted, homeowners with septic systems have several major concerns to consider:

Can we continue to use our toilets, sinks and showers?

At what point should we reduce or stop sending waste down the drains? And how do we know when we’ve reached that point?

What can we do to protect our system from damage?

How do we minimize the system from contaminating our property and beyond?

How do we assess any damage done to the system, and whether it is safe to use?

It’s tempting to think that once floodwaters have receded, the absence of standing water over the septic system means that it is ready for use again. Maybe we’ve temporarily evacuated during the flood or stayed put to ride it out, and suffered through not flushing or sending anything down the drain for a number of days. And now that the sun is out again, and the water seems to be gone, everything can get back to normal. Unfortunately, that is not always the case, since the effects of flooding on a septic system can sometimes be hidden from sight.

As I’ve often noted in these blog posts, a septic system is more than a tank and a series of pipes. Tanks and pipes are simply components of a system that is designed and installed for a particular site and its unique location, topography and soil makeup. The design and installation of a septic system take into consideration proximity to water bodies and the vertical separation of the leaching field from the underlying groundwater. It also considers the type of soil and its porosity, that is, how quickly or slowly the effluent will travel downwards toward the water table. Vegetation above the leaching field also influences the treatment (nitrogen removal, for example) and movement of effluent.

Recalling all these physical, location-specific components of a septic system, it’s easy to see how flooding damage can occur in ways that are obvious in some cases, and hidden from sight in others.

The Impact of Flooded Septic Systems

The sheer force of floodwaters creates the most obvious flood damage when buildings and vehicles are simply carried downstream with the massive volumes of water, and land and roadways are washed away through violent erosion. Septic tanks and the network of leaching field pipes are routinely installed relatively close to the surface; anywhere from two to four feet below grade is typical. Therefore, septic systems are vulnerable to floodwater erosion that can break system components and release untreated sewage and effluent in the process. In extreme cases, septic tanks and effluent distribution pipes may be torn completely out of the ground.

septic tank backing up during rainfall

Sometimes, flooding occurs more slowly, more implacably, and the damage arises in a more subtle way. Even when not as dramatic as tearing up septic components, floodwater can wash away the soil covering the tank and drain pipe network, cause tank covers to pop off and short out electric components such as pumps or filtering systems. Soil, silt and debris will enter system components, clogging pipes and inlet and outlet ports. Floodwater infiltration into the septic tank can disrupt the scum layer and cause it to clog inlet and outlet ports as well.

septic tank filling with groundwater

Flooding in populated areas also distributes a variety of toxic agents throughout the flood zone. Gasoline, oil, cleaning agents, pesticides, paints and solvents, and other chemicals become waterborne when floodwater infiltrates buildings and vehicles. When these chemicals enter septic tanks, they harm the beneficial bacteria in the tank that work to break down sewage, meaning that the effluent leaving the tank towards the leaching field is not effectively treated.

Saturated Septic Drain Field

The leaching field is normally unsaturated, giving a place to where tank effluent can flow. When the soil in the leaching field becomes saturated with floodwater, effluent really has nowhere to go. It’s a bit like when a parking lot is full, and cars are left circling around, unable to park anywhere. That means that the tank isn’t able to release effluent efficiently, and starts to overfill. You would notice this in the house when drains and toilets are moving slowly, or – disaster! – backing up into the house. While the septic tank can provide a limited amount of storage capacity, it’s important to keep in mind that when effluent flow to the leaching field is inhibited by saturated soil for an extended period of time, the system is in effect not working.

saturated septic drain field

Another consequence of flooding: your septic system was designed and installed to create a suitable vertical separation between the leaching field and the water table, so that effluent can be further treated by soil bacteria before reaching and mixing with groundwater. Flooding has the effect of raising the water table and thereby decreasing the vertical separation. Flooding also creates anaerobic conditions in the soil as stagnant floodwater prevents oxygen from reaching the soil. These conditions dramatically decrease the aerobic soil bacteria responsible for treating the effluent. These two factors, the decrease in vertical separation and the inhibition of aerobic soil bacteria, results in the ineffective treatment of effluent before it mixes with groundwater.

Flooding also has the effect of compacting soils, so that even when the soil has dried up – by floodwater flowing to surface water bodies, evaporation and moving downwards to groundwater – the soil structure is left permanently altered, with lower porosity. Since soil porosity is a critical factor in the design and installation of each particular septic system, soil compaction can compromise the overall efficiency of the system.

Septic Tank & Drain Field Flooding, Helpful Tips

All told, there are numerous ways in which a septic system is negatively impacted by flooding. Fortunately, there are steps that can be taken before, during and after a flooding event to ensure that damage is minimized, and to evaluate the actions needed to bring the system back to full function.

septic tank backs up during rainfall

Before the flood. The proper design, installation and maintenance of a septic system can be accomplished to lend an element of resiliency to flooding. Most simply, stormwater drainage should always be directed away from the septic tank and leaching field. This means paying attention to gutters and downspouts on the house and drainage from impervious surfaces like paved driveways. Soil over the tank and leaching field should be appropriately mounded or sloped to move water away from the site, and maintaining proper vegetation cover over septic system components will help to mitigate erosion caused by floodwater when the floods come. If you are living in a flood-prone area, a backflow preventer is also a sound idea. Flooding can force wastewater from the septic tank out through all of its pipe connections, including the line that carries waste from the house. The one-way backflow valve works to prevent reverse movement of house drainage, helping to keep sewage contamination out of the house.

When flooding is predicted or imminent, it may be tempting to consider pumping the tank, with the thought of creating more storage capacity until floodwaters recede. DON’T PUMP THE TANK! Although you are indeed creating storage space, you are also making the tank more buoyant, and in flood-saturated soil this could cause the tank to literally pop out of the ground, breaking plumbing connections and incapacitating the whole system. This can be a problem for newly installed systems too, as they haven’t been used enough and so don’t have sufficiently filled tanks. Ideally, the tank is at least half-full to prevent buoyancy issues.

Septic Tank Filling With Groundwater

Often older tanks or deteriorating concrete tanks can allow groundwater to penetrate back into the tank itself.  This can be caused by loose baffles at the inlet or outlets of the septic tank.  Septic tank risers and lids may also allow groundwater to seep into the tank if not properly sealed.  Cracks in the septic tank also not only pose a contamination problem with wastewater leaking in the surrounding soils, but it also has the reverse effect of flood waters or heavy rainfall events penetrating back into the tank.  This can quickly lead your septic tank to become filled with groundwater, which can, unfortunately, lead to back up problems in the home.

During the flood: The most important thing to keep in mind about your septic system during a flood is, use it as little as possible, or not at all! This means minimizing or halting toilet flushing, using the shower, or doing dishes or the laundry. Consider paper plates and going to a Laundromat for a few days or weeks, and possibly arranging for a portable toilet. If there is a basement, plugging all the drains will prevent seepage of sewage or contaminated floodwater.

After the flood. There are two tasks to tackle regarding your septic system when the flooding is over and while you’re waiting for floodwater to recede. The first should be assuring the health and safety of everyone on the property, and the other is assessing any damage to the system and what will be needed to make it operational again.

Regarding health and safety, don’t use well water until it has been tested and shown to be free of contamination by pathogens. Floodwater can enter wells, and flood-saturated soil may allow septic effluent to mix with well-water. Septic systems and well water systems are designed to have adequate separation from each other, but flooding can disrupt this very important separation, and as we’ve just learned, flooding also compromises the effective treatment of the sewage in the septic system. Therefore the potential for untreated sewage to contaminate well-water supplies during a flood is very real. This could mean days or weeks of bottled water and boiling water for drinking, cooking and washing up, but this expense and inconvenience is a far better alternative to the serious illnesses that may result from drinking contaminated water.

The septic system should not be used unless and until water near the tank and leaching field is lower than the water around the house. Water seeks its own level, so ensuring that septic waste flows in the right direction will prevent continuing contamination of the house with sewage. Once the flooding is over and as water levels recede, professional inspection and maintenance should be engaged. This would include having the tank pumped (after the potential for buoyancy problems are over), cleaning filters and drains, and inspecting electrical connections, the manhole cover, inspection ports and the leaching field. It may also involve new soil testing to assess the impact of the flood on soil structure.

Physical damage to septic system components are the most obvious issues to address, but keep in mind the soil where the system is located, because the septic system includes not only the tank and pipes but the soil and vegetation around them! During clean-up, minimize foot traffic and don’t allow heavy equipment to drive over the septic area, or allow clean-up debris to be piled there. Both will further compact the soil and impair the future function of the system. Do repair any soil erosion over and around the system with new topsoil, and reseed or replant vegetation as necessary.

The B.C. Government has prepared a helpful flooding checklist for septic systems that outlines what to do with your septic system before, during and after a flood. Here are the highlights:

When flooding is expected:

  • Make sure the septic tank is near full.
  • Plug floor drains.

When flooding is happening:

  • Discontinue use of the septic system
  • Shut off power to electrical components, if any.
  • Consider silt barriers to prevent clogging of the system.
  • Don’t drink well water.

After the flood:

  • Don’t drink well water.
  • Don’t use the system until floodwater has receded to below the level of the house.
  • Have the system professionally inspected.

Flooding can be catastrophic in many ways. As with all forms of emergency preparedness, attention to your home septic system before, during and after a flood will go a long way towards minimizing the impact of a flood and getting your home back to normal as quickly as possible.

groundstone wastewater

Septic System Installer, Designer, Repairing

Contact us today for a no obligation overview of your septic system. Should you require design work for building permit applications, installation of your septic system or repair to your existing system, we have you covered.
250-768-0056 Office
778-363-0828 Cell

Luis Goncalves, ROWP, IN, PL

I fell into this amazing trade quite by accident, I have always loved being around heavy machinery and moving earth and in contrast love working with designing things.  This trade in the septic field has led me down a wonderful path of exploring all of my skills and passions while working hard at achieving success.

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