Septic soil in Mahoning County, OH, by the numbers
Very limited
In Mahoning County, OH, 91% 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 - 91% of it sits in USDA's most-constrained bucket. Mahoning County, OH is 411.521 square miles with a population of 225,786.
What Mahoning County's reading doesn't cover
USDA's survey covers the whole of Mahoning County, OH 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 Mahoning County will actually do.
What a system would cost here
With 91% of Mahoning 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, Mahoning County, OH is the 1,158-most-constrained of 3,058 rated counties for septic soil. Within Ohio alone it ranks 73 of 87.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems