Septic soil in Platte County, WY, by the numbers
Very limited
Soil across 50.4% of Platte County, WY rates "Very limited" in USDA's septic-absorption-field interpretation. 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. Platte County, WY is 2,081.459 square miles with a population of 8,512.
What Platte County's reading doesn't cover
Platte 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 Platte County will actually do.
What a system would cost here
With 50.4% of Platte 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, Platte County, WY is the 2,877-most-constrained of 3,058 rated counties for septic soil. Within Wyoming alone it ranks 18 of 22.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems