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Challenged to solve the mystery of the red-doored vault on Rock Creek Parkway, Vincent M. Destajo—a National Public Radio employee—faxed off a four-page, single-spaced, impeccably researched theory detailed enough to put an All Things Considered opus to shame.

Then he faxed it again.

And again.

And again.

Vinny—relax. You win the goddamn T-shirt.

Destajo began his research by poring over the United States Geological Survey’s “Washington—West Quadrangle” map. He notes that a damlike structure with a small tower spans the creek. “On either side of this structure in the banks of the creek, are manhole covers with the label “Water,’ ” he writes. “If you stand at the far edge of this damlike structure and look directly across Rock Creek Parkway, you will be looking right at the vault. With this evidence, I believe the vault is somehow, either currently or formerly, part of D.C.’s water supply system.”

After a lengthy—and largely accurate—description of how water is pumped through the city’s mains, Destajo adds that “a couple of other clues led me to my theory. For about five months, I observed an Army Corps of Engineers (ACE) truck parked in the grass by the southbound lanes. They would have a manhole cover open and seemed to be taking flow measurements of some sort, based on the equipment that I would see. Out of pure curiosity, I called and asked what they were doing. I was told, “Oh, nothing special. They may rebuild the roadway and are surveying the area.’ I don’t think so. I know what surveying equipment looks like and they had none. I concluded that the Army Corps’ answer was an attempt not to disclose a water main route for fear of tampering.”

Not deterred by the ACE brushoff, Destajo continued to plumb the city’s waterways. “When the huge 36-inch water main burst at 21st and M Streets several years ago, I tried to sneak a peek at a water main route map the Department of Public Works (DPW) had spread across a pickup truck’s hood. The maps were snapped closed by a DPW employee, who followed with a threat to have me arrested for attempting to view “extremely confidential information.’ My brief peek saw several mains crossing Rock Creek.”

Destajo was right about the vault’s function, and as he prophesied, ACE initially feared that disclosing the purpose of the vault would prompt miscreants to tamper with the water supply—or at least use the unlocked building as a crash pad. But Tom Jacobus, the new chief of the Corps’ Washington Aqueduct Division, decided that honesty is the best policy. “The more people know about their water supply, the more confidence they have in it.” In the spirit of municipal perestroika, Jacobus invited me to ACE’s Dalecarlia treatment plant to learn more about the vault from engineer and de facto aqueduct historian, Ray Ferrara.

Ferrara began with a primer on D.C.’s water supply. Our drinking water is culled from the Potomac. Some water is treated at the Dalecarlia plant and pumped to the city and suburbs. The rest flows into the Georgetown Reservoir. Once sediment has settled out, this water is shunted down a 150-foot shaft and into a 4-mile-long, century-old masonry tunnel that runs beneath the city to the McMillan Reservoir and water treatment plant.

Using blueprints and diagrams, Ferrara explained that the vault stands almost directly above the low point in the city tunnel. When ACE needs to repair the tunnel, it shuts off the flow at the Georgetown Reservoir. Gravity propels most of the 13,085,750 gallons running through the tunnel at any one time to McMillan, but about 5 million gallons get stuck at the low point. That water must be pumped out into Rock Creek. To accomplish this, engineers attach a pump to “blow

(Lest waders worry, it’s not every day that the Corps swells the creek with an extra 5 million gallons. Frequently emptying the tunnel and allowing its masonry to dry only increases the need for repairs. ACE last drained the tunnel in the ’70s, when Metro construction required that the conduit be reinforced to protect it from train vibrations.)

So where does the vault fit in? Originally, a building containing a pump sat above the manholes. But when the parkway was widened, the building was torn down. Its replacement—our vault—was constructed in 1941 across the street. Inside lies a large pump, a generator and transformer to run the pump, a giant “A-frame”—a kind of horizontal crane—used to haul the pump across the road, and a tripod to lower the pump down the shaft. (Would-be trespassers take note: After last week’s column, ACE locked the building.)

Alas, modern technology has rendered the vault’s contents obsolete. If the tunnel needed to be drained today, ACE would truck in portable pumps and generators. “We just attach a 4- inch discharge hose, and that’s that,” says Ferrara.

Next Week’s Mystery: Old Ironsides