The Washington region houses six of the nation’s 10 richest counties, for an increase over last year’s rankings thanks to climbing incomes here as other areas falter in their recovery from the recession.

Northern Virginia added to its hold as the richest area in the nation as Arlington County’s median household income crossed the $100,000 threshold in 2011, according to survey figures released by the U.S. Census Bureau on Thursday. The county became the third-richest in the nation, joining Loudoun and Fairfax, which kept their respective No. 1 and No. 2 ranks from 2010.