Sinkhole risk in Union County, SD: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock: 2%
At or near the surface: 0%
Buried under cover: 2%

What this means before you buy

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 9 of Union County's 461 square miles on soluble rock. With about 17,402 residents (38 per square mile), much of the land is open, and the mapped rock may sit far from where people actually live. The figures come from 251 map points about 1.4 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: how these percentages are measured, and the warning signs that apply anywhere.

Counties around Union County

The nearest county rated sinkhole country is Knox County, NE, about 64 miles west. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Clay County, SD 18 mi west · 0% at surface
Plymouth County, IA 23 mi east · 0% at surface
Dixon County, NE 26 mi southwest · 0% at surface
Sioux County, IA 30 mi northeast · 0% at surface
Dakota County, NE 31 mi south · 0% at surface
Lincoln County, SD 31 mi north · 0% at surface

What Union County is built on

The largest mapped soluble unit here is the Niobrara Formation (limestone), Late Cretaceous in age, covering about 2% of the county.

Against the rest of South Dakota

Union County ranks 63 of 66 in South Dakota for soluble rock at the surface, and sits above 0% of US counties on that measure.