Butte County, CA's soil, for a septic absorption field
Very limited
In Butte County, CA, 92.1% of the surveyed land carries a "Very limited" rating for a standard septic drain field. Practically, that rules out a simple gravity system for most of the county - 92.1% of it sits in USDA's most-constrained bucket. Butte County, CA is 1,636.511 square miles with a population of 208,334.
What Butte County's reading doesn't cover
This is a countywide, area-weighted figure across every soil survey map unit in Butte County, CA - never a reading for one specific parcel. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Butte County will actually do.
What a system would cost here
With 92.1% of Butte 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, Butte County, CA is the 1,024-most-constrained of 3,058 rated counties for septic soil. Within California alone it ranks 25 of 57.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems