Septic soil in Thomas County, KS, by the numbers
Somewhat limited
87.1% of Thomas County, KS's surveyed soil comes back "Somewhat limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. It's a soil condition contractors work around routinely, with a bigger field or added fill, rather than one that forces an engineered system outright. Thomas County, KS is 1,074.687 square miles with a population of 7,802.
What Thomas County's reading doesn't cover
Two lots a mile apart in Thomas 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 Thomas County will actually do.
What a system would cost here
With 87.1% of Thomas County rating "Somewhat 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, Thomas County, KS is the 3,053-most-constrained of 3,058 rated counties for septic soil. Within Kansas alone it ranks 103 of 105.
Further reading
How this rating is built, and its real limits · How a percolation test actually works