What Franklin County, IL's soil rates for a septic system
Very limited
90.3% of Franklin County, IL's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. 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. Franklin County, IL is 408.935 square miles with a population of 37,033.
What Franklin County's reading doesn't cover
Franklin County's rating is an average across the whole county, not a promise about any one lot inside it. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Franklin County will actually do.
What a system would cost here
With 90.3% of Franklin 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.
How Franklin County, IL compares
Nationally, Franklin County, IL is the 1,232-most-constrained of 3,058 rated counties for septic soil. Within Illinois alone it ranks 47 of 101.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems