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Will it perc?

If there is no sewer, a house needs a septic system, and a septic system needs soil that drains at the right speed. A lot that will not pass a percolation test is a lot you cannot build a conventional home on. Here is how far a free soil map gets you before you pay for the test.

What a perc test is

A licensed soil evaluator or the county health department digs test holes where the drain field would go, fills them with water, and times how fast it drops. Too slow (clay, hardpan, a high water table) and effluent pools at the surface; too fast (coarse sand, gravel) and it reaches groundwater untreated. Most counties also require a minimum depth to bedrock and to the seasonal water table. The test costs a few hundred to a couple of thousand dollars and is usually a contract contingency.

What the soil survey says

The USDA soil survey maps the country into soil map units and rates each one for "septic tank absorption fields": the likelihood that a conventional drain field will work and what would limit it. LandCheck shows that rating on every click under Soil:

Not limited	The soil has no property that would restrict a conventional drain field. Good sign; still test.
Somewhat limited	One or more limitations that design or extra cost can usually overcome, for example slope or moderate permeability.
Very limited	A limitation serious enough that a conventional system probably will not work: slow percolation, a high water table, shallow bedrock, flooding. Expect an alternative system or a failed test.

The limiting features are listed with the rating (for example "depth to saturated zone" or "slow water movement"). Those are the exact things the evaluator will be checking.

What it cannot tell you

• Map units are drawn at survey scale; a 5-acre parcel may straddle two or contain unmapped pockets
• The rating is for a conventional system; mound, drip and aerobic systems have different rules
• County setback rules (wells, streams, property lines) decide where the field can even go
• Lot size minimums for septic are a zoning matter, not a soil matter
• A previous failed test on the lot is a public health record, not a soil map

How to use it

1. Turn on the Soil layer and click the parcel. Read the map unit, the septic rating and its limiting features.
2. Click a few spots across the parcel; the rating can change at a soil boundary, and a "very limited" corner does not doom a lot with a "not limited" building site.
3. Check the flood zone and wetlands layers on the same click. Drain fields are not allowed in wetlands, and a floodplain adds setbacks.
4. Ask the seller for any prior perc test or soil evaluation and check with the county health department for records on the parcel.
5. Write the perc test into the contract as a contingency, with the seller's permission to dig.

Open the map with the soil layer on →

The soil survey is a screening tool. Only a site evaluation by the county or a licensed evaluator determines whether a lot can be permitted for septic.