Septic soil in Coffee County, TN, by the numbers
Very limited
Soil across 68.3% of Coffee County, TN rates "Very limited" in USDA's septic-absorption-field interpretation. This is the reading that pushes a project toward an engineered system rather than a conventional one, and it covers 68.3% of Coffee County's land. Coffee County, TN is 428.964 square miles with a population of 61,285.
What Coffee County's reading doesn't cover
USDA's survey covers the whole of Coffee County, TN 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 Coffee County will actually do.
What a system would cost here
With 68.3% of Coffee 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.
Coffee County's rank among Tennessee counties
Nationally, Coffee County, TN is the 2,519-most-constrained of 3,058 rated counties for septic soil. Within Tennessee alone it ranks 82 of 95.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems