Prime places besides the smattering of isolated counties and the mentioning of Appalachia in the article: Mountain and shoreside Maine: Probably because there are lots of cabins in those parts. Southwest and East Alabama: Poor rural African Americans. The counties with major cities are the palest. Southern Texas: Poor illegal immigrants. The legal immigrants can likely afford plumbing. The Southwest, a line down the middle of Wisconsin, all of Alaska, NE Washington, south central Oregon, and bits of Montana: Probably a lot of poor Native Americans. Big Island Hawaii: I got nothing. Possibly a mixture of cabins and poor Hawaiians (they have an issue with poverty there, I think). A lot of these are also in places where it's very hard to establish plumbing.