Lead can leach into drinking water through the corrosion of indoor plumbing fixtures and old outdoor service lines containing the toxic element. A better understanding of how this leaching occurs could help scientists find new ways to mitigate lead contamination in tap water. Now, for the first time, researchers have directly observed that a lining of PbO2 scales, formed naturally in lead water service lines, can do a better job of keeping lead out of drinking water than a well-established treatment strategy (Environ. Sci. Technol. 2015, DOI: 10.1021/es505886h).
When ingested over time, lead accumulates in the body and can cause long-term damage to health, especially in children. The Centers for Disease Control & Prevention recommends that children with a blood lead level of 5 μg per deciliter or higher be tested for health problems. So to help reduce the amount of lead that leaches into drinking water, water utilities add a variety of chemicals to reduce pipe corrosion and to maintain the scale deposits that act as a protective barrier. In North America and Europe, many utilities use a well-established treatment that adjusts pH and alkalinity levels to maintain the most common type of lead scale, compounds of Pb(II), or that doses the water with inorganic phosphates to create orthophosphate, which forms lead-phosphate compounds making up the pipe coating.
But less soluble lead scales composed of PbO2—formed from Pb(IV)—can also act as a barrier to lead leaching into the water. Michael Schock and colleagues at the Environmental Protection Agency’s National Risk Management Research Laboratory, in Cincinnati, wanted to measure lead levels in water from service lines that have stable, uniform PbO2 films. They also wanted to see if major differences in the pH and alkalinity of the water affect lead release from pipes with PbO2 scales.
The researchers analyzed water samples collected from taps in six homes built between 1884 and 1928 in Newport, R.I.; Cincinnati; and Oakwood, Ohio. They quantified the total lead in the unfiltered water using inductively coupled plasma/mass spectrometry or graphite furnace atomic absorption.
Relatively low levels of total lead, ranging from 1 to 18 µg/L, had been released from the PbO2-coated pipes, even after the water had been standing in the pipes for three days. The total lead levels in these pipes were 50 to 80% lower than the lead levels found in service lines coated with Pb(II) scale, as measured in comparable published studies, Schock notes.
Chlorine, a powerful oxidant widely used by utilities to disinfect water, changes the water chemistry to favor the formation of PbO2 scales, the team hypothesizes, which points to a potential treatment strategy. However, chlorine also boosts the creation of harmful disinfection by-products, Schock says. So if this treatment approach were attempted, it would require sophisticated water treatment and monitoring that not every utility could achieve.
The study fills in important data gaps for utilities, says Jeff Swertfeger, superintendent of water quality management at Greater Cincinnati Water Works, who provided some of the tested pipe specimens but was not involved in the analysis. “We are really trying to understand this scale, and not many people are doing research to find out how it forms and how we can help keep it stable.”