Septic soil in Reynolds County, MO, by the numbers
Very limited
83.2% of Reynolds County, MO's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. A standard gravity system routinely fails on ground like this; the same share of the county falls in USDA's worst absorption-field bucket outright. Reynolds County covers 808.477 square miles and counts 5,927 residents.
This is a county figure, not a parcel figure
USDA's survey covers the whole of Reynolds County, MO at once, not any single address. A site-specific perc test, usually $750-$1,850, is what a health department will actually require before permitting a lot in Reynolds County.
System cost, conventional vs. engineered
Given Reynolds County's rating, plan for the engineered-system range rather than the conventional one. 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, Reynolds County, MO is the 1,843-most-constrained of 3,058 rated counties for septic soil. Within Missouri alone it ranks 97 of 114.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems