Septic soil in Warren County, IL, by the numbers
Very limited
Soil across 54.9% of Warren County, IL rates "Very limited" in USDA's septic-absorption-field interpretation. A standard gravity system routinely fails on ground like this; the same share of the county falls in USDA's worst absorption-field bucket outright. Warren County, IL is 542.36 square miles with a population of 16,217.
What Warren County's reading doesn't cover
Nothing about Warren County, IL's SSURGO figure applies to a specific address - it's a county-wide average. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Warren County will actually do.
What a system would cost here
With 54.9% of Warren County rating "Very limited", a standard gravity system is not a safe assumption here. 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, Warren County, IL is the 2,824-most-constrained of 3,058 rated counties for septic soil. Within Illinois alone it ranks 99 of 101.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems