What Mason County, MI's soil rates for a septic system
Very limited
Soil across 89.6% of Mason County, MI rates "Very limited" in USDA's septic-absorption-field interpretation. That reading means a conventional system is a poor bet here - 89.6% of Mason County lands in the strictest "very limited" bucket USDA uses. Mason County, MI is 494.969 square miles with a population of 29,093.
What Mason County's reading doesn't cover
Two lots a mile apart in Mason County can carry different soil, even though they share one countywide rating. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Mason County will actually do.
What a system would cost here
With 89.6% of Mason 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.
Mason County's rank among Michigan counties
Nationally, Mason County, MI is the 1,309-most-constrained of 3,058 rated counties for septic soil. Within Michigan alone it ranks 60 of 83.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems