Septic soil in St. Louis County, MO, by the numbers
Very limited
In St. Louis County, MO, 56.9% 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 - 56.9% of it sits in USDA's most-constrained bucket. St. Louis County, MO is 508.525 square miles with a population of 992,929.
What St. Louis County's reading doesn't cover
Two lots a mile apart in St. Louis 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 St. Louis County will actually do.
What a system would cost here
With 56.9% of St. Louis 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.
How St. Louis County, MO compares
Nationally, St. Louis County, MO is the 2,782-most-constrained of 3,058 rated counties for septic soil. Within Missouri alone it ranks 112 of 114.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems