Heavy rain can oversaturate the soil around your septic tank and cause flooding which can lead to serious and potentially disastrous results. When rainwater infiltrates the system, causing the tank to take in more liquid than it can handle, water cannot flow out of the septic system properly. With nowhere else to go, water can flow out of the septic tank and pool in your yard. Worse yet, it can back up into the drains and toilets of your home.