Can the soil survey tell me if land will pass a perc test?

Not definitively — only the local health department's site evaluation decides — but it stacks the odds. The USDA SSURGO survey rates every mapped soil for 'septic tank absorption fields' (not limited / somewhat limited / very limited) and reports the properties behind the rating: permeability, depth to seasonal high water table, depth to bedrock or restrictive layer, and slope. A 'very limited' rating with a shallow water table fails perc tests far more often than not; a 'not limited' rating on deep, well-drained soil usually passes. Use it to decide which lots are worth the cost of formal testing.

What hydrologic soil group is best for septic?

Groups A and B (sandy to loamy, moderate-to-high infiltration) are generally favorable. Group C is marginal, and Group D (clays, shallow water tables, or shallow rock) commonly fails conventional systems. Two caveats: extremely fast Group A sands can fail on the other end — insufficient treatment before effluent reaches groundwater — and the group describes runoff behavior, so always read it with the water-table and restrictive-layer depths, not alone.

What is a seasonal high water table and why does it matter for septic?

It's the shallowest depth groundwater reaches during the wet season, recorded in the soil survey from soil color patterns (redoximorphic features) — which means it's detectable even in a dry-season site visit by a soil scientist, but invisible to a casual buyer in August. Most states require a separation of 1 to 4 feet between the drainfield bottom and the seasonal high water table; a water table at 12 inches usually forces an expensive alternative system or kills the site.

What if the lot fails the perc test?

Options, in rising cost order: test another area of the parcel (soil changes over short distances), or move to an alternative system where allowed — shallow-placed/low-pressure pipe, sand mound, drip dispersal, or aerobic treatment. Alternatives commonly run two to five times the cost of a conventional trench system and add maintenance obligations. If no system can be permitted, the lot is a camping spot — which is why septic feasibility should be a purchase contingency on any unsewered land.

How accurate is the soil survey at lot scale?

SSURGO maps polygons, typically with minimum delineations of an acre or more, and every polygon can contain unmapped inclusions of a different soil. On a 5-acre parcel the mapped unit is a strong prior; on a quarter-acre corner of a polygon boundary it can simply be wrong in either direction. That cuts both ways: some 'very limited' lots hide a passing pocket, and some 'not limited' lots hide a wet inclusion. The survey tells you where to spend testing money, not whether to skip it.

Blog · Buyer guide · August 26, 2026

Buying land for a septic system: what the soil survey tells you before the perc test

On unsewered land, the septic question is the buildability question — no approved system, no house. The perc test (or the modern soils evaluation) is the official answer, but it costs money and requires access. The USDA soil survey is free, covers nearly every acre in the country, and predicts the official answer well enough to tell you which lots deserve the testing budget. Here's how to read it like a soil scientist.

Four numbers that predict the verdict

1. The septic rating itself

SSURGO carries an interpretation called "Septic Tank Absorption Fields" for every mapped soil: not limited, somewhat limited, or very limited, with the specific limiting factors listed (slow percolation, wetness, shallow bedrock, slope, flooding). This is the closest thing to a free perc-test forecast that exists. "Very limited — wetness, slow water movement" on a Group D clay is a lot that fails conventional systems far more often than not.

2. Hydrologic soil group

Group	Texture tendency	Septic outlook
A	Sands, gravels	Percs fast — sometimes too fast (treatment concern over shallow aquifers)
B	Loams, sandy loams	The sweet spot for conventional trenches
C	Clay loams, shallow layers	Marginal — expect larger fields or shallow-placed systems
D	Clays, wet or shallow soils	Conventional systems commonly infeasible

A dual rating like B/D means B if drained, D in its natural state — on raw land, read it as D.

3. Depth to seasonal high water table

The regulatory killer. Health codes require vertical separation — commonly 1 to 4 ft depending on state and system type — between the bottom of the drainfield and the seasonal high water table. The survey reports this depth from soil color evidence (gray redoximorphic mottles), which records the wet season permanently: a soil scientist can read a March water table in an August test pit, but a buyer kicking dry dirt in summer cannot. A reported water table at 12–18 inches on the buildable area is the single strongest "walk away or budget for a mound system" signal in the survey.

4. Depth to restrictive layer

Bedrock, dense till, or a cemented pan close to the surface leaves no soil column to treat effluent. The same separation logic applies as for the water table. Mountain lots fail on rock; coastal-plain lots fail on water; Piedmont clay fails on percolation — every region has its signature failure mode, and the survey names it.

What the survey cannot do

SSURGO maps polygons; your drainfield is a point. Soil map units have inclusions — pockets of different soil too small to map — and boundaries drawn at survey scale, not parcel scale. A "very limited" polygon can hide a passing knoll; a "not limited" polygon can hide a wet swale. The survey's correct use is triage: it tells you whether to negotiate a septic contingency, where on the parcel to point the evaluator, and which of five candidate lots deserves the testing money first. The county health department's site evaluation is the only answer that permits a system.

The buyer's sequence on unsewered land

Screen the soil first — rating, group, water table, restrictive depth. Five minutes, free.
Make the offer contingent on septic approval. Standard practice everywhere; a seller who refuses is telling you something.
Point the evaluator at the survey's best ground, not the most convenient corner.
If it fails, get the alternative-system quote before walking — a sand mound or drip system at 2–5× conventional cost may still pencil on the right lot, but it must be priced into the offer.

How SitePrior fits. The free preview pulls the mapped soil series, hydrologic group, and drainage class for any address instantly. The full $29 report adds the septic-suitability reading, depth-to-water-table and restrictive-layer values, slope, flood zone, wetlands screen, and a plain-English summary — the complete step-1 triage, before you spend on the evaluator. Run the free preview →

Sources: USDA-NRCS SSURGO database and Web Soil Survey septic-tank absorption field interpretation; NRCS National Engineering Handbook Part 630 Ch. 7 (hydrologic soil groups); state onsite wastewater rules vary — the local health department's site evaluation governs. Screening guidance only.

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