Sinkhole risk in Johnson 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: 100%
At or near the surface: 0%
Buried under cover: 100%

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 613 of Johnson County's 613 square miles on soluble rock. With about 160,080 residents (261 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 340 map points about 1.3 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why the 100% under cover still counts, and how these percentages are measured.

Counties around Johnson County

The nearest county rated sinkhole country is Clayton County, IA, about 82 miles north. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Iowa County, IA 24 mi west · 0% at surface
Washington County, IA 25 mi south · 0% at surface
Cedar County, IA 25 mi east · 0% at surface
Muscatine County, IA 28 mi southeast · 0% at surface
Linn County, IA 28 mi north · 0% at surface
Louisa County, IA 36 mi southeast · 0% at surface

The rock underneath

The largest mapped soluble unit here is the Lime Creek Formation (limestone), Late Devonian in age, covering about 44% of the county. Next is the Cedar Valley Limestone at 28%.

Against the rest of Iowa

Johnson County ranks 56 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.