Septic Suitability Checker: Will Your Soil Handle a Drain Field?

What USDA’s rating weighs

Absorption depends on how fast water moves through the soil, the depth to a water table, ponding, flooding, and the depth to bedrock or a cemented pan. Stones, boulders, ice and bedrock or a cemented pan get in the way of installing the system, and on a steep slope effluent can seep sideways and surface downslope. Where loose sand and gravel or fractured bedrock lies less than 4 feet below the distribution lines, the field may not filter the effluent well, especially when the system is new, and groundwater can be contaminated^[1].

The depth it rates

NRCS’s description says only the soil between 24 and 60 inches (61 and 152 cm) deep is evaluated^[1]. Some criteria in the current rule reach deeper: in the example, the ‘Slow water movement’ reason reads ‘Percolation 60 - 180cm (24-72")’^[2].

24 to 60 inches: the depth of soil USDA’s septic rating evaluates^[1].

Rating classes

NRCS defines each class, and scores every reason from 0.00, not a limitation, to 1.00, the greatest negative impact on the use^[1]:

Reading the reasons

Each reason is one of the soil features NRCS lists as affecting absorption, installation or public health^[1]. The common ones:

• Depth to saturated zone: a water table comes close to the drain field.
• Slow water movement: water moves through the soil too slowly to absorb the effluent.
• Depth to bedrock: bedrock lies within the depth the rule checks.
• Seepage, bottom layer, or filtering capacity: the soil lets effluent through too fast to filter it.
• Ponding and flooding: water stands on or floods the site.

National and state ratings

Alongside the national rating, some states publish their own septic ratings in the soil survey, often one per kind of system; in this example, Texas publishes 5^[2]. Here the national rating and Texas's own rating for absorption fields agree: Very limited. Approval is a state and local matter: EPA doesn’t regulate home septic systems, and in most states the local health department issues the permits^[3].

Worked example

Austin silty clay in Bell County, Texas (85% of its map unit) is very limited for absorption fields nationally, for 2 reasons: depth to bedrock (1.00) and slow water movement (1.00)^[2]. The example result above shows each rating with its reasons.

Before you buy or build

Soil surveys are a useful preliminary planning tool, but survey maps are too general to judge one homesite: the soil where the field would go has to be evaluated on site^[4]. A perc test measures how fast clean water soaks into wet soil, in minutes per inch, and K-State notes it doesn’t reveal limiting conditions in the soil profile^[4]. Use this rating to know what to expect before you buy or plan, and see the water table depth tool if a saturated zone is the reason.

Ways people use this

• Buying rural land without sewer.
• Checking a lot before making an offer.
• Planning a new home or a replacement system.

How we calculate this

We read USDA’s ‘ENG - Septic Tank Absorption Fields’ rating and its reasons for the main soil at your point, plus any state septic ratings for the same soil^[2]. Reasons are sorted by score; ‘Not rated’ stays as USDA publishes it. Inches are converted at 2.54 cm per inch^[5].

Frequently asked questions