Sinkhole risk in Fairfax County, VA: 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: 2%
Buried under cover: 0%

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 7 of Fairfax County's 391 square miles on soluble rock, 7 of them with the rock at or near the surface. With about 1,160,925 residents (2,969 per square mile), most of the county is built up, so a mapped unit is likely to sit under somebody's street. The figures come from 525 map points about 0.9 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 Fairfax County

The nearest county rated sinkhole country is Clarke County, VA, about 43 miles northwest. None of the 6 closest counties land in the same band, so Fairfax County stands out from its surroundings.

Fairfax city, VA 2 mi northwest · 0% at surface
Falls Church city, VA 6 mi northeast · 0% at surface
Arlington County, VA 10 mi east · 0% at surface
Manassas Park city, VA 10 mi southwest · 0% at surface
Alexandria city, VA 10 mi east · 0% at surface
Manassas city, VA 13 mi southwest · under 1% at surface

What Fairfax County is built on

The largest mapped soluble unit here is the Newark Supergroup; Conglomerate, mixed clasts (conglomerate), Triassic in age, covering about 2% of the county.

Against the rest of Virginia

Fairfax County ranks 77 of 133 in Virginia for soluble rock at the surface, and sits above 57% of US counties on that measure.