Septic on Chattanooga's rock, slopes and sinkholes

If you've ever wondered why the neighbor's septic system looks nothing like yours, or why a repair quote involved a pump and a mound of soil instead of a few new trenches, the answer is usually under your feet. The Chattanooga area packs a lot of different ground into a small space, and Tennessee's septic rule is strict about where a drain field can go.

The lay of the land

East Tennessee geologists describe the Chattanooga area as classic Valley and Ridge country: long folded and faulted ridges like Missionary Ridge with valleys between them, and the Cumberland Plateau's edge (Walden Ridge and Signal Mountain, capped by Pennsylvanian-age rock) rising to the west. Lookout Mountain straddles the Georgia line. According to the U.S. Geological Survey, the Valley and Ridge in Tennessee is underlain by carbonate rock, and roughly 43.5 percent of the state sits on karst or rock that can form karst. Karst is what you get when limestone and dolomite slowly dissolve: sinkholes, caves, springs, and water that can travel quickly underground.

Sinkholes and septic

Tennessee's rule defines a "sink" as a closed depression in karst country, formed when surface limestone dissolves or an underlying cave collapses. Sinkholes, caves, springs, wells, gullies and natural drainageways are all excluded from consideration as drain field area. The rule keeps a septic tank at least 15 feet from a sinkhole, stream, gully or drainageway, and the disposal field at least 25 feet. A broad, gentle sink can be used only if all of these are true:

• Rock is at least four feet below the surface.
• Trenches are no deeper than 30 inches.
• The slope is 30 percent or less.
• The area doesn't flood.

Those limits exist because in karst, poorly treated wastewater can reach groundwater through openings in the rock with little soil to filter it.

Slopes

A lot on the side of a ridge has a different problem: effluent can move sideways through the soil and come out on the hillside below. Tennessee's rule treats slopes steeper than 30 percent as unsuitable unless the soil will prevent that sideways seepage, and anything over 50 percent as unsuitable, period. On a mountain or ridge lot, the drain field often ends up in the one flatter shelf on the property, which leaves little room for a second try if the first field fails.

The setbacks that shrink the usable yard

Source: Tennessee rule 0400-48-01-.11. The rule notes that local conditions may require more distance. Fill dirt doesn't count as usable area unless it actually filters well, and the approved field area isn't supposed to be graded or disturbed without a re-evaluation.

What this means for homeowners

• Protect the field you have. Keep cars, trailers and construction equipment off it, don't pave or build over it, and keep roof drains and sump pump discharge away from it. That's straight from the EPA's care guidance, and it matters more where there's no easy spot for a replacement.
• Pump on schedule. Solids escaping the tank are the most common way a field gets clogged. The pumping guide helps you set the interval.
• Know your reserve area. The University of Tennessee describes a septic system as a tank, a drain field and a reserve area set aside for a replacement field. If your permit shows one, keep it clear.
• Expect engineered options on hard lots. When conventional trenches won't fit, permits here can call for alternative systems such as low-pressure pipe or mounds, which have pumps that need service.

Planning a repair or a new system on a tough lot? Start with the Hamilton County permit guide, then call to talk it through with a local septic contractor.

Good questions

Can I put a septic system on a lot with a sinkhole?

Possibly, but not in the sinkhole. Tennessee's rule keeps drain fields 25 feet from sinkholes and allows a gentle sink to be used only with at least four feet to rock, trenches no deeper than 30 inches, slopes of 30 percent or less, and no flooding. A site evaluation decides.

Why does my system have a pump?

A pump is often used when the only suitable soil is uphill from the house or when the design calls for pressure distribution, like low-pressure pipe systems. Pumps, floats and alarms need periodic checks.