Quick answer: A mound system is a septic drain field built above the natural ground on a bed of clean sand, with a pump that doses tank effluent into it under pressure. Around Fargo–Moorhead, mounds are used where heavy clay and a high seasonal water table leave too little unsaturated soil for in-ground trenches. Pump the tank on schedule, answer the alarm right away, and keep vehicles, trees and packed snow off the mound. For mound service, call (701) 566-7446.
Why the Red River Valley has so many mounds
A septic drain field works by letting effluent soak down through unsaturated soil, where it gets filtered and broken down before it reaches groundwater. That only works if there is enough dry soil between the bottom of the drain field and the water table. Both states set that distance by rule. Minnesota requires a continuous 3-foot soil treatment zone below the distribution media, above any periodically saturated soil or bedrock (Minn. R. 7080.2150). North Dakota requires at least 24 inches of separation below the bottom of the treatment area, and conventional trenches must sit at least 24 inches above mottled soil that indicates a water table (N.D. Admin. Code 62-03.1-03-03 and 62-03.1-03-09).
Much of the valley floor is old lakebed from glacial Lake Agassiz: heavy clay that takes water slowly, with a water table that rises in spring. On many lots, a trench dug deep enough to work would sit too close to that water. NDSU Extension describes the mound as the design for sites with high water tables (within 2 to 3 feet of the surface) and slowly permeable soils. The mound takes advantage of the surface soil, which accepts water better than the soil deeper down. The idea is local, too. EPA's mound fact sheet notes the design was first developed in North Dakota in the late 1940s as the NODAK system, then refined by the University of Wisconsin in the 1970s into the design many states have since written into their rules.
How a mound system works
- Septic tank. Wastewater from the house goes to a septic tank first, the same as a conventional system. Solids settle out as sludge and grease floats up as scum. Many tanks have an effluent filter on the outlet to hold back fine solids.
- Pump (dosing) chamber. Clear effluent flows into a second tank or compartment that holds a pump, float switches and a high-water alarm. When the level reaches the "on" float, the pump sends a measured dose up to the mound, typically several times a day.
- Pressure distribution. The dose travels through a supply line into small perforated pipes laid level in a bed of rock or other distribution media on top of the mound. Pressure spreads the effluent evenly along the whole bed rather than flooding one end. Both states require pressure distribution in a mound.
- Sand and native soil. Effluent trickles through a layer of clean sand, which does much of the treatment, and then into the roughened native soil under the mound. A cap of soil and topsoil with grass covers it all.
The rules spell out the shape. Minnesota requires at least 12 inches of clean sand under the distribution bed, a bed no wider than 10 feet, side slopes no steeper than 3 to 1, a capped inspection pipe at least 4 inches across reaching down into the bed, and at least 6 inches of topsoil over the entire mound (Minn. R. 7080.2220). North Dakota calls for 12 to 24 inches of basal sand extending at least 5 feet beyond the rock bed in every direction, a rock layer no more than 10 feet wide, and a berm on the uphill side to divert surface water (N.D. Admin. Code 62-03.1-03-12). The result is usually a long, low, grassy hill. University of Minnesota Extension puts the typical height at 18 inches to 5 feet above the surrounding yard. Minnesota's chapter 7081 covers mid-sized systems over 5,000 gallons per day, so a typical house falls under chapter 7080.
Maintenance that keeps a mound working
- Pump the tank on schedule. University of Minnesota Extension recommends cleaning the tank at least every three years, and Minnesota rules require an assessment at least that often (Minn. R. 7080.2450). This matters even more with a mound. Solids that escape the tank can plug the small holes in the pressure pipes and clog the sand. Have the pump chamber checked for sludge at the same visit. See septic tank pumping.
- Clean the effluent filter. If your tank has one, it should be inspected and cleaned regularly, usually when the tank is pumped. A clogged filter can back water up into the house, which is the filter doing its job.
- Have the pump, floats and alarm checked. EPA recommends checking the dosing chamber, pump and floats every year, along with electrical parts for corrosion. Test the alarm so you know it works before you need it.
- Keep traffic off. Nothing heavier than a riding lawn mower should go over any part of the system, according to University of Minnesota Extension. That includes the area just downslope of the mound. Don't park, store equipment or let livestock on it.
- Grass only. Minnesota's rules prohibit trees, shrubs and deep-rooted plants on the system, and North Dakota's mound rule says not to plant trees or shrubs on top. Roots go after the moisture in the bed and the pipes. Keep a healthy grass cover and mow it.
- Keep extra water away. Roof downspouts, sump pump discharge and drainage should go somewhere else. Fix leaking toilets and faucets, and spread laundry across the week. A mound is sized for a set daily flow.
Winter care for a mound
A mound sits above grade in open, windy country, so winter is when it needs the most attention. University of Minnesota Extension's guidance comes down to a few habits:
- Leave the snow on it. Snow over the tank and mound is insulation. Don't shovel, plow or blow it off.
- Don't pack the snow. Compacted snow insulates far less than loose snow and lets frost go deeper. Keep cars, snowmobiles, ATVs, foot traffic and livestock off the system, especially in winter.
- Add loose cover if snow is thin. Let the grass grow a little longer in late summer and fall. If snow is light or the system is new and the grass isn't established, spread 8 to 12 inches of loose straw, leaves or hay over the pipes, tank and mound before frost sets in. Remove it in spring. Don't lay permanent rigid insulation over the mound, because the soil needs air.
- Keep normal use going. Regular warm water use helps, so spread out laundry and keep using the dishwasher. Don't leave water running continuously, which overloads the system. Never pour antifreeze, salt or additives down the drain.
The full fall checklist is in winterizing a septic system. If the drains have already stopped, see frozen septic system: what to do.
Signs of trouble
- The alarm is sounding. The pump chamber level is too high. The cause could be a failed pump, a stuck float, a tripped breaker, or a mound that isn't taking water. Silence the alarm, cut water use, check the breaker on the pump circuit, and call.
- Wet spots or seepage. Spongy ground or seeping water on the sides, top or toe of the mound. Toe seepage is one of the classic mound problems in EPA's fact sheet.
- Odors. A sewage smell near the mound, tank or pump chamber, especially on still, humid days.
- Slow drains or backups in the lowest fixtures, particularly if the alarm hasn't sounded. That points to a problem before the pump, such as a full tank or clogged filter.
- Rising water in the inspection pipe. If you check the mound's inspection pipe through the year and see standing water getting deeper over time, the bed is ponding and may be headed for trouble.
Many mound problems start in the pump chamber, so a good diagnosis tests the pump, floats and alarm circuit before anyone assumes the mound itself has failed. See drain field and mound repair, or emergency service if sewage is backing up now.
How long a mound lasts, and replacing one
A mound doesn't have a fixed expiration date. EPA's fact sheet says a properly installed and maintained mound should last a long time, and it credits Wisconsin's success rate of over 95 percent to careful siting, design, construction and maintenance. In practice, the parts with moving pieces wear out first. Pumps, floats and alarms get replaced several times over a mound's life, and those are repairs, not a new system. What ends a mound's life is usually the sand and soil underneath clogging up, most often from solids that escaped an overdue tank, or water use the mound was never sized for.
Replacing a mound means starting over the right way: a new soil evaluation, a design sized to the house's bedroom count, and a permit through the county. That's Fargo Cass Public Health in Cass County, ND and Clay County Environmental Health in Clay County, MN. Mounds cost more to build than conventional trenches because of the sand, the pump chamber and the controls. The real price depends on the soil, the size of the house, how far the sand has to be hauled, site access and the season. There's no reliable single figure for this area, so get a site-specific quote. Call (701) 566-7446 to get one started.
Frequently asked questions
Why do so many homes around Fargo have mound septic systems?
Much of the Red River Valley is slow-draining lakebed clay with a seasonally high water table, so many lots don't have enough unsaturated soil under an in-ground trench to treat effluent. Minnesota requires a 3-foot treatment zone below the distribution media, and North Dakota requires at least 24 inches of separation. A mound builds that treatment zone above grade with clean sand. The design itself started in North Dakota as the NODAK system in the late 1940s.
How often should a mound septic system be pumped?
The septic tank ahead of a mound should be checked and usually pumped at least every three years; Minnesota rules require an assessment at least that often. Because solids that escape the tank can plug the small holes in the mound's pressure pipes, staying on schedule matters even more with a mound than with a gravity system. Have the pump chamber checked at the same visit, and clean the effluent filter if you have one.
What should I do when my mound system alarm goes off?
The alarm means the water level in the pump chamber is too high. Common causes are a failed pump, a stuck float, a tripped breaker, or a mound that isn't taking water. Silence the alarm, cut water use to a minimum, check the breaker for the pump circuit, and call for service. Don't ignore it or just reset it repeatedly, because the pump chamber's reserve space is limited.
Can I drive on or plant trees on a septic mound?
No. Keep cars, trucks, ATVs, snowmobiles and livestock off the mound and the area just downslope of it, since compaction damages the sand and soil and drives frost deeper. Minnesota's rules prohibit trees, shrubs and deep-rooted plants on the system. North Dakota's mound rule says not to plant trees or shrubs on top. Grass is the right cover.
How do I keep a mound system from freezing in winter?
Leave snow on the mound and keep all traffic off it, because packed snow insulates far less than loose snow. Let the grass grow a little longer in late summer and fall. If snow cover is thin or the system is new, spread 8 to 12 inches of loose straw, leaves or hay over the tank, pipes and mound before frost sets in, then remove it in spring. Fix dripping fixtures, but don't leave water running to keep things warm.
Is a wet spot at the toe of my mound a problem?
It's a warning sign worth a service call. Seepage or spongy areas on the sides, top or toe of a mound can mean the pump is running too long, a pipe is leaking, solids have clogged the distribution system or the soil under the mound, or the house is sending it more water than it was designed for. Caught early, many of those are fixable without rebuilding the mound.