QUESTION:

Do I need to worry if water ponds over my system?

ANSWER:

This question was posted recently at a homeowner website, and elaborating on the topic will help septic system professionals educate their customers.

The simple answer to the question is yes, you do need to worry. Every year a number of areas experience 100- or 500-year-interval precipitation events where onsite systems can be inundated. And any additional water added to a system has the potential to cause problems with the long-term operation.

Two situations immediately come to mind where water can pond over an onsite system: when the house or system is located in or near a flood plain and when the system is situated in a lower, flat area where surface water can collect. Regardless of the cause of the system being under water, the concerns are the same.

Excess water can hydraulically overload the system. The movement of water over and through the system can bring solids that plug the drainfield and fill the tanks, causing problems with pumps and any other mechanical or electrical connections within the system. In addition, water can flow back from the sewage tanks into the lower level of the house.

FOLLOW FLOOD PLAIN RULES

As with any other potential septic system problems, it’s best to address flooding concerns during design and installation. All local and state flood plain regulations must be followed. Most often in a designated flood plain, systems are not allowed in the floodway portion, while they may be allowed in the flood fringe. The system should be installed at the highest possible elevation. Often the location of the bottom of the distribution media must be at or above the 10-year-interval flood elevation. To avoid surface water ponding, it is important to make sure water from hard surfaces (roofs, driveways, patios, etc.) is routed away from the system. It may be necessary to construct berms or waterways to ensure runoff does not collect over any part of the system.

There are usually additional or somewhat different requirements to ensure a system that experiences flooding from time to time will continue to operate after the flood subsides. For instance, inspection ports should not be installed running from the bottom of the distribution media to the soil surface. Counter to other areas where such inspection ports are encouraged or required, this change prevents a direct connection between floodwaters that may be high in sediment and the bottom of the drainfield.

Often in systems installed in a low, flood-prone area, a pump will be utilized to lift the effluent to a higher elevation, either on the landscape or to an elevated mound treatment system. If a pump is installed, there should be a way to measure flow and pump run times. This will help determine if, as floodwaters rise or surface water ponds near the tanks, it is not infiltrating the tanks and being delivered to the drainfield. If flooding is anticipated, the pump should be shut off and ideally removed before water goes over the top of the tank.

If you can anticipate that tanks will be inundated with water, a method should be provided to prevent backflow into the residence.

In Minnesota, we have a number of mounds installed to provide separation distance in low-lying areas subject to flooding or ponding. In flood plain areas, the bottom of the absorption bed has to be at least 6 inches above the 10-year-flood elevation. Inspection pipes should not be installed unless the top of the mound is above the 100-year-flood elevation. In areas that may be subject to surface runoff, inspection ports may or may not be required.

AFTER THE FLOOD

After the floodwaters recede, a comprehensive system inspection and assessment should be conducted before putting it back in use. This means opening all parts of the system – sewage tanks, drop boxes, anywhere there is access to system components – and assessing whether sediment or vegetative debris have entered the system. All sewage tanks should be pumped and cleaned out.

The tanks should be evaluated for watertightness and structural defects due to the flooding. Debris in the drop boxes should be removed. If there are pumps and a pressure distribution system, the distribution laterals should be jetted and cleaned. Pumps and controls should be reinstalled, recalibrated and tested. The evaluation should include making sure wastewater moves between the parts of the system as intended. This may involve running a hydraulic load test on the soil treatment part of the system.

About a month after the system is restarted, the service provider should schedule a follow-up visit to check for proper operation. Any pumps and controls should be checked and the pump calibrations re-evaluated to make sure they are delivering the correct amount of effluent.