Changes in Lead Levels during Annual Switch to Free Chlorine
WASA did some lead sampling during the annual switch to free chlorine in April.
How did this affect the lead levels in tap water?
As part of its efforts to better understand the occurrence of high lead levels in many D.C. homes, the D.C. Water and Sewer Authority (WASA) has been conducting lead profile monitoring in homes to evaluate differences between lead levels in homes served by lead and non-lead service lines, the effects of lead service line replacement, and techniques for cutting lead service lines.
The annual changeover of secondary disinfection chemicals from chloramine to free chlorine occurred during the period from April 2 to May 7, 2004. This presented an excellent opportunity to assess any differences in lead corrosion rates during chlorine and chloramine treatment on a more detailed basis.
Details of testing
Seven household locations served by full or partial lead service lines were used by WASA to conduct detailed profiles of lead levels and other metals over the course of time. Roughly 20-30 one-liter samples were taken at each location before and during free chlorine addition to determine metal concentrations in different sections of a home's plumbing after a period of stagnation. The lead data discussed here are a subset of a larger amount of information collected by WASA for home plumbing/service line/water main profiling purposes.
The preliminary data show a substantial difference in lead levels from before and during free chlorine application in homes served by lead service lines. Data from two sampling sites illustrate this difference: during the free chlorine treatment period, lead levels were up to 10-fold lower than chloramine treatment periods at the same households. During chlorination, the lead levels were generally below the action level of 15 ppb in water that sat in lead service lines. Other samples identified by WASA to have been taken during the period of free chlorine feed confirm this finding: lead levels in tap samples taken in April (free chlorine addition) were generally lower than those from March (chloramine addition), though not to the same extent as in the profiled locations.
What does the data mean?
While one could conclude that chloramines are the cause of the elevated lead levels in DC, experts thus far suspect that free chlorine actually served as a form of corrosion control treatment. The hypothesis is that it is not chloramine addition per se, but instead the absence of the free chlorine that is contributing to lead leaching. Elemental chlorine combines to form an oxide with lead to create a protective layer inside distribution system pipes/plumbing, slowing the rate of lead corrosion. The use of free chlorine may have slowed a process already underway.
How will this data affect treatment decisions?
Firm conclusions should not be drawn from this limited data set. This data provides significant new information to the various agencies working to resolve this matter. The study data are being provided to the Technical Expert Working Group and their conclusions, requested by June 30, 2004, will be reviewed by the Independent Peer Review Panel.
If the switch to chloramines appears to be the cause of elevated lead levels, why wasn't the Aqueduct required to go back to free chlorine immediately?
Why the Aqueduct switched to chloramines
In November 2000, the Washington Aqueduct changed their treatment process from the use of free chlorine to reduce the levels of disinfection by-products (DBPs) and the public health risks associated with them. The change resulted in an estimated 47% reduction in levels of DBPs, on average. In January 2002, a running annual average maximum contaminant level (MCL) of 80 parts per billion (ppb) was set to help prevent risks of possible cancer-causing agents and suspected risks of miscarriage at acute levels.
Why switching back to free chlorine isn't the answer
Simply going back to chlorine may trade one public health problem for another: though lower lead levels may result, reverting to chlorine may result in higher cancer risks for the entire service area of the Washington Aqueduct. EPA is concerned about potential effects related to miscarriage and birth defects, as some studies have suggested a weak link between elevated levels of trihalomethanes (THMs) and reproductive and developmental effects.
As we enter the warmest months of the year which typically create the highest levels of disinfection by-product formation, it is prudent to avoid a quick decision to turn off chloramine treatment in favor of free chlorine at this stage.
What about zinc orthophosphate?
The proposed treatment using zinc orthophosphate will continue as planned. Further analysis of the new data may provide suggestions on adjusting that treatment. We know from other systems' experiences that zinc orthophosphate can be effective in controlling corrosion while chloramination is retained as the disinfectant. The desire is that lead levels will eventually be reduced and District of Columbia and Northern Virginia residents will not be exposed to higher cancer risks and other potential effects.
Continuing to protect public health
Effective public education programs should be instrumental in protecting the target population from lead exposure through the flushing guidance and the distribution of water filters. Other options will be evaluated by EPA and partner agencies to determine if changes in treatment are feasible without unacceptable risks from lead levels, disinfection by-products, or other regulated parameters. WASA will offer to resample any residence or location tested during the free chlorine period to ensure that the most accurate picture of lead levels is provided during chloramine treatment. WASA is also conducting additional assessments to study this phenomenon in detail.