St. Francois County, MO: the USDA septic-suitability picture
Very limited
St. Francois County, MO's SSURGO dominant class comes back "Very limited", covering 84.4% of the county. That reading means a conventional system is a poor bet here - 84.4% of St. Francois County lands in the strictest "very limited" bucket USDA uses. The county spans 451.886 square miles, home to 67,503 people.
What this number does and doesn't tell you
Two lots a mile apart in St. Francois County can carry different soil, even though they share one countywide rating. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
St. Francois County: conventional vs. engineered cost
84.4% of St. Francois 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.
Where this sits nationally
Among the 3,058 rated counties on this site, St. Francois County, MO ranks 1,743 most-constrained nationally, and 93 of 114 within Missouri.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems