Hill County, MT's soil, for a septic absorption field
Very limited
In Hill County, MT, 86.6% of the surveyed land carries a "Very limited" rating for a standard septic drain field. Expect the ground itself to work against a plain gravity system; USDA's strictest bucket alone covers 86.6% of the county. Hill County, MT is 2,899.396 square miles with a population of 16,065.
What Hill County's reading doesn't cover
Two lots a mile apart in Hill 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 Hill County will actually do.
What a system would cost here
With 86.6% of Hill 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 Hill County, MT compares
Nationally, Hill County, MT is the 1,572-most-constrained of 3,058 rated counties for septic soil. Within Montana alone it ranks 9 of 54.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems