Columbia County's SSURGO septic rating
Very limited
Columbia County, OR's SSURGO dominant class comes back "Very limited", covering 98.4% of the county. Practically, that rules out a simple gravity system for most of the county - 98.4% of it sits in USDA's most-constrained bucket. The county spans 658.686 square miles, home to 54,063 people.
What this number does and doesn't tell you
USDA's survey covers the whole of Columbia County, OR at once, not any single address. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
Columbia County: conventional vs. engineered cost
98.4% of Columbia County carrying a "Very limited" reading changes the realistic budget here. Conventional systems run $3,000-$8,000 where they pass; where they don't, engineered alternatives (mound or aerobic treatment, whichever the health department requires) run $10,000-$40,000 - budget toward the higher figure until a perc test says otherwise.
Where this sits nationally
Among the 3,058 rated counties on this site, Columbia County, OR ranks 184 most-constrained nationally, and 3 of 36 within Oregon.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems