MnDOT grant generates results as OSTP assesses rest stop septic system environmental safety

Minnesota is well-regarded for plentiful and clean rest stops along its highways. Below the surface however, the Minnesota Department of Transportation (MnDot) wondered how well the 52 rest stops served by subsurface sewage treatment systems (SSTS) were functioning and if the systems met environmental and public health protection standards. With this concern in mind, MnDOT approved a grant for the university’s Onsite Sewage Treatment Program (OSTP) to evaluate rest area septic systems across the state.

The objective of the program is to create a proactive SSTS management plan for each MnDot rest area, pinpointing system shortcomings and then targeting funding to resolve problems before the environment and public health are threatened.

“We developed a risk matrix,” said Sara Heger, OSTP team member and state Extension specialist.  Through an initial assessment of five chosen facilities, OSTP staff inspected septic tanks, sampled the effluent in the tanks and evaluated the soil treatment systems at each site.  Later, the team used this inspection protocol at rest stop septic systems statewide. The researchers focused only on characteristics that most directly affect the environment and human health.

Soil treatment system characteristics included in the risk matrix include system type, waste distribution method, size of the system and amount of separation, surfacing of effluent, ponding, soil compaction and vegetation issues.  The last variable considered was management of the system. Each character value was rated on a scale of 1 to 5, 1 being the highest risk and 5 the lowest risk. Character values were also weighted low, medium or high impact on the septic site and surrounding environment. This weighting system determines the overall risk value and rating of each rest stop facility.

For the future, Heger says that accurate data about each site needs to be maintained and easily available, tracking flow data and wastewater properties for existing systems and to inform MnDOT about the specifics of sites needing a new system. “We will need further research with a broader criteria to fully understand the scope of maintaining rest area septic systems in a cost- effective, environmentally friendly manner, and protects public health.”

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