Kanawha County, WV's soil, for a septic absorption field
Very limited
Soil across 91.9% of Kanawha County, WV rates "Very limited" in USDA's septic-absorption-field interpretation. Practically, that rules out a simple gravity system for most of the county - 91.9% of it sits in USDA's most-constrained bucket. Kanawha County, WV is 901.738 square miles with a population of 173,906.
What Kanawha County's reading doesn't cover
Kanawha County's rating is an average across the whole county, not a promise about any one lot inside it. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Kanawha County will actually do.
What a system would cost here
With 91.9% of Kanawha 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.
Kanawha County's rank among West Virginia counties
Nationally, Kanawha County, WV is the 1,052-most-constrained of 3,058 rated counties for septic soil. Within West Virginia alone it ranks 48 of 55.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems