Jim Anderson
We’ve received a few questions recently dealing with issues around high groundwater, drainage and impacts on current septic systems, and designs for the future.
The first question outlined a situation where presence of redoximorphic features occurred within a foot of the soil surface. Data from piezometers (a tube or pipe installed in the ground that can be used to determine perched or seasonal water levels in soil) indicated that saturated soil was present below 3 feet.
The designer asking the question explained that regional drainage had been conducted in the area 50 to 60 years ago, and his opinion is redoximorphic features were then relics, reflective of the drainage condition 50 years ago and not during the present. I agreed this was a scenario where his conclusion may be accurate. With the drainage changes, soil hydrology has changed and it would take a long time for the features in the soil to break down and disappear.
He indicated system designs would change if the piezometer data were used instead of the soil evaluation, which would indicate a higher level of saturation. I agreed designs would change if the piezometer data were used. I suggested before they moved ahead with designs for systems, they perform additional soil analysis. I am awaiting results to see if professional soil scientists agree the features are relics.
RAIN, RAIN, GO AWAY
The second question came from closer to home. Last year was a year of high precipitation — not excessive, but certainly within the top 10 recorded years. This has resulted in lake levels rising to levels that have not been seen in 20 years or more. Inland lakes as well as the Great Lakes have seen high water tables. Lake Superior levels are near an all-time high.
The small lake I live on — as evidenced by how the water level has stayed above the rock that is just off our boat dock — has maintained the high-water level since fall 2018, due primarily to the high precipitation. Added to the precipitation is the presence of beavers whose natural instinct is to dam any flowing water and increase water levels. At times of low waters, this is just a minor nuisance. Last year, these factors led to shoreline houses on low, level land experiencing septic problems.
Rising water levels in the lake or river also rise in the adjacent soils, even if the area is not flooded. As the depth to soil saturation decreases, so does the separation distance between the saturated soil and the infiltration surface of sewage treatment trenches. This results in less unsaturated soil available for treatment and less oxygen available for the aerobic organisms in the soil that not only assist in treatment, but also break down organic material forming the biomat.
With less oxygen available, the biomat becomes thicker and more resistant to flow, decreasing the infiltration rate into the soil. During wet periods where water is higher in adjacent surface waters, the system is less able to accept the effluent delivered from the house.
If there is adequate capacity in the system, it can make it through this wet period and return to normal when the level goes down. However, if the condition persists or the saturation extends up into the trench itself, sewage will either back up into the house or rise to the soil surface. Neither of these conditions is acceptable for long-term operation.
DESIGN FOR THE WORST CASE
I designed systems in areas where the long-term soil drainage patterns were altered primarily for agricultural production. My experience was when houses are built and septic systems installed, soil drainage patterns were interrupted and drainage conditions reverted to their more natural state. This meant that the presence of redoximorphic features reflected the soil condition present after construction. Those features should be used to estimate the level of soil saturation for design purposes. This probably means designing mounds or at-grade systems instead of excavated trenches for the soil treatment units.
Where flooding or ponding (inundation) is likely, it is important to design and install systems for those conditions — maybe not expecting it every year, but at some frequency. Systems should be designed so the infiltration surface is above the 100-year flood elevation. Again, this means designing and installing aboveground mounds and at-grades in the area.
Inspection ports and system access should be installed to prevent infiltration through these openings during times when water is ponded on top of the system. When the floodwaters recede, any sewage tanks (septic, pretreatment or pump) should be inspected and pumped before the system is used.
For my neighbors, a long-term solution is going to be similar: use of aboveground systems instead of their trenches and beds to avoid recurring problems. Extended periods of higher lake levels are forecast in our area due to the changing climate, which is expected to result in overall higher precipitation and in storms of greater intensity. Now is the time to make these changes to avoid additional problems.
