Sinkhole risk in Washington 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: 96%
At or near the surface: 0%
Buried under cover: 96%

What to do about a house in Washington County

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

That works out to roughly 547 of Washington County's 569 square miles on soluble rock. With about 22,631 residents (40 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 388 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Washington County

The nearest county rated sinkhole country is Jo Daviess County, IL, about 106 miles northeast. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Keokuk County, IA 23 mi west · 0% at surface
Johnson County, IA 25 mi north · 0% at surface
Louisa County, IA 25 mi east · 0% at surface
Henry County, IA 26 mi south · 0% at surface
Jefferson County, IA 26 mi southwest · 0% at surface
Iowa County, IA 30 mi northwest · 0% at surface

What Washington County is built on

The largest mapped soluble unit here is the Osage Series (dolostone (dolomite)), Early Mississippian in age, covering about 43% of the county. Next is the Yellow Spring Group at 22%.

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

Washington County ranks 93 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.