Septic soil in Benton County, MO, by the numbers
Very limited
Soil across 80.4% of Benton County, MO rates "Very limited" in USDA's septic-absorption-field interpretation. That reading means a conventional system is a poor bet here - 80.4% of Benton County lands in the strictest "very limited" bucket USDA uses. Benton County, MO is 704.033 square miles with a population of 20,614.
What Benton County's reading doesn't cover
USDA's survey covers the whole of Benton County, MO 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 Benton County will actually do.
What a system would cost here
With 80.4% of Benton 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.
Where this sits nationally
Nationally, Benton County, MO is the 2,023-most-constrained of 3,058 rated counties for septic soil. Within Missouri alone it ranks 101 of 114.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems