Septic soil in Erie County, PA, by the numbers
Very limited
In Erie County, PA, 94.4% 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 - 94.4% of it sits in USDA's most-constrained bucket. Erie County, PA is 798.904 square miles with a population of 267,750.
What Erie County's reading doesn't cover
Erie County's rating is an average across the whole county, not a promise about any one lot inside it. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Erie County will actually do.
What a system would cost here
With 94.4% of Erie 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, Erie County, PA is the 719-most-constrained of 3,058 rated counties for septic soil. Within Pennsylvania alone it ranks 29 of 66.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems