Triassic Basin Soils and Septic Near Durham

Where is the Triassic Basin around Durham?

The Triassic Basin runs as a long trough of sedimentary rock through southern Durham County and neighboring parts of Chatham, Orange, Wake and Granville. Geologists call it the Deep River basin, and its northern part is the Durham sub-basin.

The NC Geological Survey says the Deep River basin "includes portions of 11 counties": Anson, Chatham, Durham, Granville, Lee, Montgomery, Moore, Orange, Richmond, Union and Wake. Person County is not among them. The USGS describes the Durham basin as reaching north from the Lee-Chatham line for 83 km through Orange, Wake, Durham and Granville, up to 27 km wide and about 1,700 square km in area. NC State Extension sums up the whole basin as 90 miles long, from central Moore County to southern Granville.

Its edges do not follow roads or subdivision lines. A lot on one side of a creek can sit on Triassic siltstone and the next on granite-derived red clay, so the county soil map for the specific parcel matters more than a regional map.

Why are Triassic Basin soils hard for septic systems?

They are hard because the rock underneath is clay-rich and tight, and the soil formed from it inherits that. The NC Geological Survey says the basin rocks are "very clay rich and poorly sorted," and that "the soil formed from these rocks does not readily transmit or soak up water."

The same agency puts it bluntly: "At many Triassic sites, the soil does not perform adequately during percolation tests for septic drain fields." North Carolina does not actually permit by perc test; it uses a soil and site evaluation under 18E .0501-.0509. Triassic soils tend to fail it for three reasons the evaluator can see in a pit: gray wetness colors close to the surface, slow-moving clay subsoil, and expansive clay that cracks when dry.

The USGS lists the Durham basin rocks as siltstone, sandstone, conglomerate, claystone and shale, cut by diabase dikes and sills of Early Jurassic age. Diabase weathers into its own sticky clays, which adds a second problem soil inside the basin.

Which Triassic Basin soil series show up on Durham lots?

The series that come up most in the basin are White Store, Creedmoor, Mayodan and Pinkston, with Iredell where diabase lies below. Their NRCS descriptions differ enough that the series name on a soil map is a useful first clue.

Mayodan is the bright spot: the NRCS description calls it very deep and well drained, and its Durham units mostly rate one step better than the others. Pinkston drains well but is too thin over rock. In Chatham County the same survey adds Polkton, Brickhaven and Carbonton, all rated very limited. For contrast, the granite-derived red clays outside the basin are covered on the red clay soil page.

What does shrink-swell clay do to a drainfield?

Shrink-swell clay swells shut when wet and cracks open when dry, so it neither accepts effluent steadily nor treats it reliably. North Carolina rules treat it as a deal-breaker: in 18E Table VI, a moist consistence of very firm or extremely firm, or a wet feel of very sticky or very plastic, marks expansive clay as unsuitable.

When the field test is unclear, a lab can settle it. Table VII rates clay with an apparent cation exchange capacity of 16.3 cmol/kg or less as slightly expansive and suitable, and anything above that as expansive and unsuitable. The White Store description in the NRCS series data notes slickensides, the polished faces left when clay swells and shears, and the Iredell profile reads "very firm, very sticky, very plastic," matching the rule's language almost word for word.

In wet months the swollen clay holds water above it, which is why basin lots so often combine shrink-swell with a high seasonal water table. Effluent that cannot move down moves sideways or up, and that is how a drain field on these soils ends up with soggy ground over the trenches.

Can you get a septic permit on Triassic Basin soil?

Yes, many basin lots are permitted, but the answer depends on the specific pit, not the soil name. Under 18E .0509, any single unsuitable parameter makes the site unsuitable, and no Improvement Permit can be issued for an unsuitable site.

A failing first look is not always final. The rule lets a site be reclassified through site modifications, an interceptor drain, advanced pretreatment, or a system approved under the provisional, innovative and accepted program. Wetness can be re-examined by monitoring or modeling under .0504(c). An owner may also hire a licensed soil scientist under G.S. 130A-335(a2) instead of waiting for the county. If the county says no, 18E .0203(c) requires a signed written report naming the unsuitable features and the rules behind them.

Designs that show up on basin lots include low-pressure pipe, drip dispersal and pretreatment units; see septic system types for what each requires and who must maintain it. Before buying land in the basin, the NC Geological Survey's advice is to make sure of "acceptable water and septic performance." In practice that means an Improvement Permit in hand, or at least a soil evaluation, before closing; the new septic system page walks through the permit steps.

How do Triassic soils affect wells too?

The same tight rock that slows drainfields also holds little water. The NC Geological Survey warns that "water well yields are typically very low, and many dry holes have been drilled," and NC State Extension gives the Triassic basins an average of 11 to 12 gpm, against 18 to 21 gpm for the Piedmont generally.

The USGS treats the Durham, Sanford and Wadesboro basins as too compact and tightly cemented to count as important aquifers. The Survey's practical tip is to look for diabase: the dikes "make good targets for drilling of water wells," and because diabase is magnetic, a geologist may use a magnetic survey to find one.

A lot in the basin therefore has two questions to answer, and the well location and septic field compete for the same ground. Current rules keep a single-family private well 50 ft from the tank and field; older permits may show 100 ft. Well siting and permits are covered under well construction and permits.

Questions worth asking

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