Mercer County's SSURGO septic rating
Very limited
USDA puts Mercer County, PA at "Very limited" for septic absorption fields, the dominant reading across 97% of the county's soil. 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. The county spans 672.538 square miles, home to 108,140 people.
What this number does and doesn't tell you
USDA's survey covers the whole of Mercer County, PA at once, not any single address. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
Mercer County: conventional vs. engineered cost
97% of Mercer County carrying a "Very limited" reading changes the realistic budget here. Conventional systems run $3,000-$8,000 where they pass; where they don't, engineered alternatives (mound or aerobic treatment, whichever the health department requires) run $10,000-$40,000 - budget toward the higher figure until a perc test says otherwise.
Where this sits nationally
Among the 3,058 rated counties on this site, Mercer County, PA ranks 373 most-constrained nationally, and 14 of 66 within Pennsylvania.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems