Septic soil in Henry County, TN, by the numbers
Very limited
86% of Henry County, TN's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. Practically, that rules out a simple gravity system for most of the county - 86% of it sits in USDA's most-constrained bucket. Henry County, TN is 561.902 square miles with a population of 32,883.
What Henry County's reading doesn't cover
Two lots a mile apart in Henry County can carry different soil, even though they share one countywide rating. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Henry County will actually do.
What a system would cost here
With 86% of Henry 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, Henry County, TN is the 1,619-most-constrained of 3,058 rated counties for septic soil. Within Tennessee alone it ranks 31 of 95.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems