Septic soil in Hillsborough County, NH, by the numbers
Very limited
88.5% of Hillsborough County, NH's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. That reading means a conventional system is a poor bet here - 88.5% of Hillsborough County lands in the strictest "very limited" bucket USDA uses. Hillsborough County, NH is 876.53 square miles with a population of 430,462.
What Hillsborough County's reading doesn't cover
USDA's survey covers the whole of Hillsborough County, NH 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 Hillsborough County will actually do.
What a system would cost here
With 88.5% of Hillsborough 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, Hillsborough County, NH is the 1,416-most-constrained of 3,058 rated counties for septic soil. Within New Hampshire alone it ranks 4 of 10.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems