Clinton County, IL's soil, for a septic absorption field
Very limited
Soil across 87.4% of Clinton County, IL rates "Very limited" in USDA's septic-absorption-field interpretation. Practically, that rules out a simple gravity system for most of the county - 87.4% of it sits in USDA's most-constrained bucket. Clinton County, IL is 473.941 square miles with a population of 37,087.
What Clinton County's reading doesn't cover
Clinton 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 Clinton County will actually do.
What a system would cost here
With 87.4% of Clinton 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.
Clinton County's rank among Illinois counties
Nationally, Clinton County, IL is the 1,518-most-constrained of 3,058 rated counties for septic soil. Within Illinois alone it ranks 53 of 101.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems