Why Delaware County septic systems are often mounds or drip

Drive the back roads between Delaware, Ostrander, and Sunbury and you will pass plenty of grassy humps behind newer houses. They are septic mounds, and there is a good reason so many homes in the county have one, or a drip field, or an aerobic unit, instead of the plain gravel trenches of older farmhouses. The reason is written in the soil survey.

What the Delaware County soil survey shows

The Soil Survey of Delaware County, issued by the USDA's Natural Resources Conservation Service in 2001, groups the county into soil associations. Three of them describe most of its farmland and exurban lots:

• Glynwood-Blount-Pewamo, about 26 percent of the county
• Blount-Pewamo-Glynwood, about 31 percent
• Bennington-Cardington-Pewamo, about 15 percent

Together that is roughly seven acres in ten dominated by the same handful of glacial till soils. The survey's own table of limitations rates Blount, Bennington, Cardington, Centerburg, and Glynwood as severe for septic tank absorption fields because of wetness and slow percolation, and Pewamo as severe because of ponding and slow percolation. Its foreword puts it plainly: clayey or wet soils may be poorly suited to septic absorption fields.

Meet the soils

These are home-county names. Two of them, Bennington and Cardington, have their official type locations in Delaware County. What they share is water sitting close to the surface for part of every year:

• Blount: somewhat poorly drained, with a perched water table about half a foot to two feet down in the wet season.
• Pewamo: very poorly drained, the dark soil of depressions and swales, with water from a foot above the surface to a foot below it between December and May.
• Glynwood and Cardington: moderately well drained, yet still perched within one to two feet through late winter and spring.
• Bennington: somewhat poorly drained, perched within about half a foot to a foot from November into May.

Perched means the water is resting on dense till below, not rising from a deep aquifer. That is why the same yard can be bone dry in August and squishy in April.

Why that rules out so many trenches

Ohio's current septic rules, in force since 2015, judge a site by its limiting conditions and require vertical separation between the bottom of the soil absorption component and the first of them. A seasonal water table is one of those conditions. Where water perches within a foot or two of the surface, there is simply not enough dry soil under a conventional trench to meet the separation, before the dense till itself is even considered.

Designers work around that in three main ways:

• Build up. A mound adds selected sand above the natural soil so effluent gets the treatment depth the site lacks. Ohio State University Extension describes mounds as suited to sites with a limiting layer 12 to 36 inches below the surface, which fits much of this county.
• Stay shallow and spread out. Drip distribution places small doses of filtered effluent in the top several inches of soil across a wide area, using the thin layer that does drain.
• Clean it first. Aerobic and other pretreatment units earn credit toward the required separation, so less soil depth is needed.

Delaware Public Health District's packet for new system owners lists leachfield, mound, and drip systems as the types it allows, notes there is no countywide ban on any type, and says the soil conditions on each lot determine what will serve best.

What it means for owners

Cost. Mounds and drip systems need a dosing tank, a pump, controls, and more material than trenches, so a new or replacement system on these soils costs more to build than a trench system on deep, well-drained ground would.

Upkeep. A pump and floats mean an alarm and yearly attention. The district's permit-length chart reflects that: mounds, drip systems, and aeration units carry one-year operation permits, compared with five years for a gravity septic-to-leach-field system.

Timing. A soil evaluation reads the profile rather than the weather, looking for the gray colors and mottles that record years of wetness, so a dry summer does not make a wet lot look better on paper. Installers do avoid shaping a mound site or cutting trenches in saturated ground, because working wet clay smears it and seals the very surface that is supposed to accept effluent.

Older homes. Farmhouses and early subdivisions often have gravel trenches dug before today's separation rules. Many still work, but when they fail, the replacement is usually one of the designs above rather than new trenches in the same wet ground.

Reading your own yard

You can learn a good deal before any soil professional arrives. Walk the lot in March or April after a wet week: standing water in low spots, dark and spongy ground, and crayfish chimneys poking up through the lawn all point to Pewamo-type depressions. Note where the ground stays firm and sits a little higher; those rises are where designers look first. Pull the county soil survey map for the parcel, which shows the mapped series. And if the home already has a system, notice what kind it is. A mound or drip field on a newer house tells you an earlier evaluation found shallow limiting conditions, and a future replacement will probably face the same answer.

If you are buying or building

• Ask for the permit file and the soil evaluation behind the existing design; the district's sewage records are searchable online.
• For vacant land, get the site and soil evaluation before closing, not after.
• Budget for a mound or drip design unless the evaluation shows otherwise.
• Count the one-year operation permit and yearly service in the cost of ownership.

For the full permit sequence, see the installation page, and for what a mound owner does from year to year, read our guide to living with a septic mound.