 Clearly a wake-up call. Michael Nardolilli with the Interstate Commission on the Potomac River Basin. All the other local water systems have backup water sources, but the Washington Aqueduct, which serves D.C., Arlington, and Falls Church, only has the Potomac River. Arlington, which is the home of the Pentagon and D.C., which is the home of the federal government, are the most vulnerable cities in the United States to a cutoff of their drinking water supply.