Answer Man Taps Into His High-Pressure Sources

Since I have lived in Tenleytown, I have wondered about that large, flat-topped mound on the corner of Van Ness and 44th streets NW. It has a high fence with barbed wire around it and signs identifying it as U.S. government property. My guesses as to its use have ranged from a subterranean reservoir to a secret weapons cache, but so far I just can't figure it out.

-- Mark Borak, Washington

It's a secret weapons cache. Or it was, until you spilled the beans. Nice move, Borak.

Kidding! Your first guess was closer to the mark. The feature in question -- imagine a grass-covered butte -- is what officials at the Washington Aqueduct call the "second high water reservoir." It's basically a big concrete box full of 14.5 million gallons of "finished water."

The District's current water-supply infrastructure got its start in 1851, after a fire at the U.S. Capitol spurred Congress to mandate a ready water supply for the city. The job of making that a reality fell to Montgomery C. Meigs, a distinguished West Point grad who would go on to design, among other things, the building that now houses the National Building Museum.

The H2O that pours from Washington faucets (as well as faucets in Arlington County and Falls Church) starts out as "raw water" sluiced from the Potomac at Great Falls.

From Great Falls, the water travels downhill through a marvel of 19th-century engineering: a 10-mile underground pipe running from the Potomac to the Dalecarlia Water Treatment Plant near Sibley Memorial Hospital.

Meigs's conduit -- nine feet in diameter -- was completed in 1860. (A second conduit, parallel to the first, was completed in 1927.) Built atop the pipe to provide access for water workers is a winding road: Conduit Road -- at least that's what it was called until 1942, when the name was changed to MacArthur Boulevard to make it a little less obvious to saboteurs what was underneath.

The conduit travels over the delightfully arched Cabin John Bridge and arrives at Dalecarlia, where it's made ready for drinking. Alum is mixed with the water in a process known as flocculation. The alum bonds to yucky water-borne particles and sinks to the bottom of huge sedimentation tanks.

From there the water is filtered through 18 inches of anthracite coal and 12 inches of sand, then disinfected with chlorine and dosed with fluoride. At that point, you can stick a fork in it -- it's done. Customers could head over to Dalecarlia to pick it up -- or to the McMillan Reservoir on North Capitol Street, where about 40 percent of the water is produced. But it's much more convenient if it comes to your house.

"Washington is a fairly flat town, compared to the San Francisco Bay area," said Tom Jacobus, general manager of the Washington Aqueduct. But there are some differences in elevation, resulting in five distinct pressure zones. Water can go to customers directly from the treatment plants or from reservoirs that hold water for high-demand times. Low-lying areas are served by a reservoir in Brentwood. Those a little higher, by one near the German Embassy. The aforementioned reservoir at 44th and Van Ness serves customers at elevations between 335 and 250 feet. There are underground tanks at Fort Reno for the next-highest neighborhoods, and the highest neighborhoods of all are served by Fort Reno's castlelike water tower.

None of the water stays in these tanks very long -- three days, max. It's just a place for it to sit before gravity takes over and brings it out, cool and refreshing, at the turn of a tap.

Send your questions about the Washington area toanswerman@washpost.com.