SinkholeOdds

Sinkhole risk in Union County, IA: 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
70%
At or near the surface
0%
Buried under cover
70%

The next step for one property

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

That works out to roughly 298 of Union County's 424 square miles on soluble rock. With about 11,955 residents (28 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 390 map points about 1.0 miles apart, so a pocket smaller than that can be missed or overstated.

Read next: why gypsum and salt move faster than limestone, and why the 70% under cover still counts.

Counties around Union County

The nearest county rated sinkhole country is Jackson County, MO, about 140 miles south. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

The rock underneath

The largest mapped soluble unit here is the Kansas City Group (limestone), Late Pennsylvanian-Missouri Series in age, covering about 41% of the county. Next is the Lansing Group at 24%.

The county also lies over the Forest City Basin, a deep salt or gypsum basin. Deep beds rarely cause surface collapse on their own, but wells and mining into them have.

Against the rest of Iowa

Union County ranks 89 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.