Washtenaw County, MI's soil, for a septic absorption field
Very limited
Soil across 84.3% of Washtenaw County, MI rates "Very limited" in USDA's septic-absorption-field interpretation. 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. Washtenaw County, MI is 706.015 square miles with a population of 373,875.
What Washtenaw County's reading doesn't cover
USDA's survey covers the whole of Washtenaw County, MI 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 Washtenaw County will actually do.
What a system would cost here
With 84.3% of Washtenaw 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.
Washtenaw County's rank among Michigan counties
Nationally, Washtenaw County, MI is the 1,752-most-constrained of 3,058 rated counties for septic soil. Within Michigan alone it ranks 74 of 83.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems