Clay County, IL's soil, for a septic absorption field

Very limited

In Clay County, IL, 96.1% of the surveyed land carries a "Very limited" rating for a standard septic drain field. Practically, that rules out a simple gravity system for most of the county - 96.1% of it sits in USDA's most-constrained bucket. Clay County covers 468.351 square miles and counts 12,821 residents.

This is a county figure, not a parcel figure

Two lots a mile apart in Clay County can carry different soil, even though they share one countywide rating. A site-specific perc test, usually $750-$1,850, is what a health department will actually require before permitting a lot in Clay County.

System cost, conventional vs. engineered

Given Clay County's rating, plan for the engineered-system range rather than the conventional one. The engineered-system range, $10,000-$40,000 (mound or aerobic treatment), is the more realistic number to plan around, against $3,000-$8,000 for a conventional system on the rare lot that passes.

Where this sits nationally

Nationally, Clay County, IL is the 519-most-constrained of 3,058 rated counties for septic soil. Within Illinois alone it ranks 24 of 101.

Further reading

How this rating is built, and its real limits · Conventional vs. engineered septic systems