What McDowell County, NC's soil rates for a septic system
Very limited
Soil across 95.4% of McDowell County, NC rates "Very limited" in USDA's septic-absorption-field interpretation. 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. McDowell County, NC is 439.954 square miles with a population of 45,269.
What McDowell County's reading doesn't cover
USDA's survey covers the whole of McDowell County, NC 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 McDowell County will actually do.
What a system would cost here
With 95.4% of McDowell 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.
How McDowell County, NC compares
Nationally, McDowell County, NC is the 610-most-constrained of 3,058 rated counties for septic soil. Within North Carolina alone it ranks 5 of 97.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems