Colusa County's SSURGO septic rating
Very limited
USDA puts Colusa County, CA at "Very limited" for septic absorption fields, the dominant reading across 94.6% of the county's soil. Practically, that rules out a simple gravity system for most of the county - 94.6% of it sits in USDA's most-constrained bucket. The county spans 1,150.639 square miles, home to 22,074 people.
What this number does and doesn't tell you
Nothing about Colusa County, CA's SSURGO figure applies to a specific address - it's a county-wide average. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
Colusa County: conventional vs. engineered cost
94.6% of Colusa 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.
Colusa County's rank among California counties
Among the 3,058 rated counties on this site, Colusa County, CA ranks 708 most-constrained nationally, and 15 of 57 within California.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems