Septic soil in Gilliam County, OR, by the numbers
Very limited
79.6% of Gilliam County, OR'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. Gilliam County, OR is 1,204.721 square miles with a population of 1,971.
What Gilliam County's reading doesn't cover
USDA's survey covers the whole of Gilliam County, OR 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 Gilliam County will actually do.
What a system would cost here
With 79.6% of Gilliam 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 Gilliam County, OR compares
Nationally, Gilliam County, OR is the 2,060-most-constrained of 3,058 rated counties for septic soil. Within Oregon alone it ranks 30 of 36.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems