Hamilton County, IA's soil, for a septic absorption field
Very limited
93.9% of Hamilton County, IA's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. Expect the ground itself to work against a plain gravity system; USDA's strictest bucket alone covers 93.9% of the county. Hamilton County, IA is 576.731 square miles with a population of 14,856.
What Hamilton County's reading doesn't cover
This is a countywide, area-weighted figure across every soil survey map unit in Hamilton County, IA - 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 Hamilton County will actually do.
What a system would cost here
With 93.9% of Hamilton 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, Hamilton County, IA is the 805-most-constrained of 3,058 rated counties for septic soil. Within Iowa alone it ranks 58 of 99.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems