Septic soil in Richland County, IL, by the numbers
Very limited
Soil across 95.5% of Richland County, IL rates "Very limited" in USDA's septic-absorption-field interpretation. That reading means a conventional system is a poor bet here - 95.5% of Richland County lands in the strictest "very limited" bucket USDA uses. Richland County, IL is 360.047 square miles with a population of 15,362.
What Richland County's reading doesn't cover
USDA's survey covers the whole of Richland County, IL at once, not any single address. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Richland County will actually do.
What a system would cost here
With 95.5% of Richland 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 Richland County, IL compares
Nationally, Richland County, IL is the 601-most-constrained of 3,058 rated counties for septic soil. Within Illinois alone it ranks 30 of 101.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems