What Valley County, MT's soil rates for a septic system
Very limited
81.8% of Valley County, MT's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. A standard gravity system routinely fails on ground like this; the same share of the county falls in USDA's worst absorption-field bucket outright. Valley County, MT is 4,926.115 square miles with a population of 7,434.
What Valley County's reading doesn't cover
USDA's survey covers the whole of Valley County, MT 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 Valley County will actually do.
What a system would cost here
With 81.8% of Valley 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, Valley County, MT is the 1,946-most-constrained of 3,058 rated counties for septic soil. Within Montana alone it ranks 25 of 54.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems