"We knew pH would be an important variable," he says. "You don't want your pH to fall too low, especially for the lead carbonates, which dissolve at lower pH."
"But we didn't realize how important pH was. We studied water samples with a pH of 10, 8.5 and 7.5. You wouldn't think there would be a much of a difference between a pH of 8.5 and 7.5, but there was.
"In some cases, the pH made the difference between a lead concentration that met the drinking water standard and one that didn't."
Phosphate also turned out to be important. "It had a huge impact on the dissolution rates of all the lead corrosion products we studied," says Giammar.
So one way to bring down lead levels is to add phosphate. But phosphate costs money, so the utilities want to add as little of it as they can to produce a good-quality water.
"It's never going to be a one-size-fits all solution because the source-water compositions are different," says Giammar, "but we came up with some pretty strong recommendations."
The final report on the project, "Influence of Water Chemistry on the Dissolution and Transformation Rates of Lead Corrosion Products," was published last year.
Should you let the tap run?
"Another thing we studied in that first project," Giammar says, "was whether we should be more worried about reaction rates or about the equilibrium state of the reactions.
"To put it another way, if you let the water sit in the pipes for six hours, will it be different from water that sat in the pipes for only an hour?"
It turned out that most lead species dissolve relatively quickly, so reaction rates are not particularly important. The water is the same no matter how long the water has been sitting in the pipes.
But this isn't true of the lead +4 oxides, the material that formed the scale inside the Washington, D.C., pipes. "They are never at equilibrium with the water flowing over them," Giam
|Contact: Diana Lutz|
Washington University in St. Louis