April 12, 2006
Experiment confirms chloramine’s effect on lead in drinking water
The water chemistry that caused Washington, D.C.’s lead problem isn’t unique.
Chloramines, which have been linked to elevated levels of toxic lead in drinking water in Washington, D.C. [316KB PDF], and Greenville, N.C., have again been implicated in high lead concentrations, but this time the evidence comes from real-time experiments. In research published today on ES&T’s Research ASAP website (DOI: 10.1021/es052411r), chemist Jay Switzer at the University of MissouriRolla describes real-time corrosion studies that show that lead scales on pipes are more likely to dissolve into drinking water when chloramines are used as a disinfectant.
Lead water pipes Michael DeSantis Tetravalent lead scales (inset) have been found inside lead water pipes from several utilities. (a) In a free chlorine solution, tetravalent lead (triangular-faced crystals) are stable, but (b) in the presence of chloramine they dissolve into the water and only a different form of the toxic metal, lead carbonate (flakes), is left behind.
Concerns about lead in drinking water rocketed to prominence in 2004 following the discovery of concentrations of up to 48,000 parts per billion (ppb) in D.C. drinking water. The dangerously high levels came after the district’s water-treatment system switched from chlorine to chloramines for disinfection. Oddly, the switch to chloramine had been prompted by federal limits on another problemdisinfection byproducts (DBPs), potentially harmful compounds formed when chlorine reacts with organic matter in the water. Chloramine, a combination of chlorine and ammonia, is a weaker oxidizer than free chlorine, so it produces lower levels of the unwanted DBPs.
At the time of the D.C. lead crisis, EPA chemist Mike Schock hypothesized that the switch reduced the water’s oxidation potential and caused mineral scales made of relatively insoluble tetravalent lead inside pipes to dissolve. Before that, experts believed that a different form, divalent lead scales, dominated drinking-water systems.
Corrosion expert Marc Edwards at Virginia Polytechnic Institute and State University first identified the severity of the D.C. problem and says that the new research “provides yet another independent confirmation of Michael Schock’s hypothesis as to the importance of tetravalent lead in drinking water systems, and illustrates that the problems experienced in Washington, D.C., are likely to occur in many other systems that switch from free chlorine to chloramine disinfectant.”
Using a quartz-crystal microbalancea device capable of ultrasensitive mass measurementsSwitzer could track reactions between lead and chlorine or chloramine. Chlorine stabilized the lead film but “with the chloramines, the lead film almost completely dissolved,” he says. The experiment took 20 hours with concentrations about 10 to 12 times higher than those in drinking water; a real system would likely take longer, he notes.
“Confirmation by an independent third party using an entirely different experimental setup and analytical technique is gratifying,” says Schock, whose recent work shows that “tetravalent lead scales are not a freak occurrence.” He has found such scales in pipes from roughly a third of the systems he’s examined.
Schock is currently trying to identify a surrogate set of water-quality indicators to predict the occurrence of such scales. Tetravalent lead scales are associated with waters that have persistently high redox potentials. Such water chemistry can occur when engineers use high concentrations of free chlorine to combat bacteria, as was done in D.C.
But these conditions can also characterize pristine, hard, high-alkalinity groundwaters that are low in organic matter as well as various waters treated with other oxidative processes such as greensand filtration or ClO2, he says.
“A system with lead pipes, but lead concentrations that seem anomalously low, is a likely candidate for tetravalent lead scales,” says Schock. Managers must carefully evaluate the pipe-scale chemistry of such systems before they contemplate a switch to chloramines, he adds.
Switzer’s experiment and Schock’s studies focus on situations that involve lead water pipes. Elevated levels of lead were discovered in 2005 in drinking water and in the blood of two children in Greenville. Those levels were also associated with a switch to chloramine disinfection but occurred in a system with no lead service lines. Schock wonders if tetravalent lead scales may be present on solder or brass plumbing devices.
Given the conflict water utilities now face on how to reduce DBPs and keep lead levels low, perhaps the best solution is a new approach, says Schock. “Removing [DBPs] at the plant would be the ideal way to optimize the system,” he notes. This would take “a lot more research,” and he hopes that funding agencies and scientists are up to the challenge. REBECCA RENNER