A mound septic system builds the drainfield above the natural ground in a bed of imported sand, then doses it with a pump so the whole bed loads evenly. It exists for one reason: the site does not have enough clean unsaturated soil between the surface and groundwater or bedrock, so the design manufactures it. Installed cost is $15,000 to $35,000, two to three times a conventional field, and almost all of that difference is sand, engineering and the pump.
Why a county requires a mound
- A seasonal high water table within a few feet of the surface, which most of the upper Midwest has in spring.
- Shallow bedrock, common in New England, the Ozarks and parts of the Appalachians.
- A shallow layer of good soil over heavy clay, where the water has nowhere to go once it reaches the clay.
- Slow perc, where a conventional field would need more area than the lot has.
In every case the missing ingredient is vertical separation: the two to four feet of unsaturated aerobic soil that codes require between the trench bottom and groundwater. A mound supplies it by stacking sand on top of the ground instead of digging into it.
How a mound is built, bottom to top
- The ground is plowed, not scraped. The surface is roughened along the contour so the interface between the sand and the native soil stays permeable.
- One to three feet of specified sand is placed. It has to meet a gradation spec, and sourcing it is a large part of the price.
- A gravel or chamber bed is built on top of the sand with small-orifice pressure laterals through it.
- A pump chamber downstream of the septic tank doses the laterals several times a day so the bed loads evenly instead of all at once.
- Topsoil and seed cover the whole thing, giving the permanent raised area in the yard.
What it costs
| Item | Typical cost |
|---|---|
| Design and engineering | $800 to $3,000 |
| Soil testing and permit | $500 to $2,000 |
| Septic tank | $1,200 to $3,500 |
| Pump chamber, pump and panel | $2,000 to $5,000 |
| Specified sand, delivered and placed | $4,000 to $12,000 |
| Gravel, laterals and fabric | $1,500 to $4,000 |
| Excavation and placement | $3,000 to $8,000 |
| Topsoil, seed and restoration | $1,000 to $3,000 |
| Total | $15,000 to $35,000 |
A mound needs a specific sand gradation, and the nearest pit that produces it may be an hour away. Two identical houses in neighboring counties can be $8,000 apart on sand alone. Ask any bidder where the sand is coming from, because that number is not negotiable once the job starts.
Living with the mound
It is a grassy rise in the yard, typically three to five feet at the crown and shaped to shed water. You mow it, you keep vehicles and livestock off it, you do not plant anything with deep roots on it, and you do not landscape around it with retaining walls or beds that trap water against the toe. Most people find it far less intrusive than they feared, but nobody has ever made one invisible, so site it during design rather than arguing about it afterward.
Maintenance
- Pump the septic tank every 2 to 4 years. Solids reaching a pressure lateral clog orifices, and cleaning them is a real job.
- Clean the outlet filter annually. On a pumped system it is protecting an expensive component, not just the soil.
- Flush the laterals every few years through the cleanout ends, which is why the design includes them.
- Test the alarm. A pump failure means the tank backs up within a day or two.
- Keep water off the toe. Roof runoff and a downspout aimed at the mound will saturate the sand from the wrong end.
How long does a mound last?
Twenty to thirty years is typical for the sand bed, on the shorter side of a conventional field because the whole design runs closer to its limits. The pump is a 5 to 15 year item. What kills a mound early is the same thing that kills any field, solids from an unpumped tank, and because a mound is a shallow bed rather than deep trenches, it is less forgiving about it.