McDonough County's SSURGO septic rating
Very limited
McDonough County, IL: USDA's soil survey calls 75.7% of the county "Very limited" for a septic leach field. This is the reading that pushes a project toward an engineered system rather than a conventional one, and it covers 75.7% of McDonough County's land. The county spans 589.305 square miles, home to 26,662 people.
What this number does and doesn't tell you
McDonough County's rating is an average across the whole county, not a promise about any one lot inside it. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
McDonough County: conventional vs. engineered cost
75.7% of McDonough County carrying a "Very limited" reading changes the realistic budget here. Conventional systems run $3,000-$8,000 where they pass; where they don't, engineered alternatives (mound or aerobic treatment, whichever the health department requires) run $10,000-$40,000 - budget toward the higher figure until a perc test says otherwise.
McDonough County's rank among Illinois counties
Among the 3,058 rated counties on this site, McDonough County, IL ranks 2,249 most-constrained nationally, and 73 of 101 within Illinois.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems