On ledge-heavy or wet Lakes Region lots, the leach field can't go down — so it goes up. Here's how raised systems work and how to live with one.
Conventional leach fields need several feet of workable soil between the trenches and whatever's below — ledge, hardpan, or seasonal groundwater. Much of the Lakes Region doesn't have it: granite runs shallow in the hill towns, and lakeside lots often sit close to the water table. NH design rules respond by requiring the treatment zone to be built where nature didn't provide one — engineered sand fill, raised above grade, with effluent pumped up into it and dosed evenly. That's the mound in the side yard.
A mound system adds two components a gravity system doesn't have: a pump chamber after the tank, and the dosing pump inside it that pushes effluent uphill on a float schedule. The mound itself contains a pressurized distribution network in gravel atop the sand bed. More parts, more engineering — and one more thing that can fail: the pump. A dead dosing pump presents exactly like a full tank (backups, alarm if equipped), and pumps last 8–12 years.
Mounds occupy the upper half of NH replacement pricing — commonly $25,000–$50,000+ installed, driven by engineered fill, pumping equipment, and design complexity. Which is the strongest possible argument for protecting an existing one: on a lot that required a mound once, the replacement will be a mound too.
Shallow ledge, hardpan, or high groundwater — where soil depth can't provide the required treatment zone, NH design rules require building one above grade from engineered sand.
High water in the pump chamber — usually a failed or unpowered dosing pump. Minimize water use and call same-day; the chamber's reserve capacity is limited.
Typically 8–12 years. Having the pump, floats, and chamber checked at every tank pumping catches most failures before they become backups.
Grass only. Trees and shrubs send roots into the distribution network, and regrading the mound destroys the engineered geometry that makes treatment work.
Alarms, dead pumps, and chamber service — handled before the reserve runs out.