One thing the Obama family may not like about Washington, D.C., is the water, which has trace amounts of weed killer, gasoline additives, and industrial chemicals. All these pollutants are found in the Potomac River, the source of most drinking water in D.C.

But the problem extends far beyond the Potomac. Some 200 million Americans have measurable levels of agricultural or industrial chemicals in their water supplies. Water companies use advanced filters, UV light, and other treatments to kill harmful bacteria and reduce contaminants. But to truly clean up our water would require curbing runoff from agricultural waste and banning industries from pumping pollutants into rivers.

"We could save a tremendous amount of money and effort if we kept pollutants out of our drinking water in the first place," says Lynn Thorp of Clean Water Action, an environmental advocacy group. "With some pollutants in the water today, we don't even know which treatments will work, making it even more important to keep our water clean from the beginning." Public water systems have to list their source waters in their annual report.