The elevated lead concentrations found in the District of Columbia’s drinking water distribution system in 2004 gained national attention and refocused the U.S. EPA’s attention on corrosion control, public health implications of trace metals and conflicting drinking water regulations. The progressive approach taken by the DC Water and Sewer Authority (WASA) to study and control the issue has provided a new understanding of corrosion chemistry and monitoring, and a new way to look at simultaneous compliance. HDR worked with the district, EPA and a variety of other consultants to help define an optimal corrosion-control strategy for the district. HDR also undertook a variety of special projects related to lead service line replacement and investigation of the underlying causes of the accelerated lead release.
Arguably, the district’s lead problem was the most intensely covered national drinking water issue in recent years. In 2004-2005 there were:
- More than 230 articles in The Washington Post;
- More than 120 stories on local television and radio stations;
- One new proposed bill in Congress; and
- Numerous district city council and congressional hearings.
Origins of the problem
As is now known, the conversion in 2001 from a free chlorine disinfecting residual to a combined (chloramine) residual caused a substantial change in lead release from the DC WASA lead service lines (LSLs). The disinfectant change was brought about by more stringent federal regulations limiting the acceptable concentrations of disinfection byproducts (DBPs) produced via reactions between free chlorine and naturally occurring organic matter in the raw water. These more stringent DBP regulations essentially require the water treatment provider change treatments or alter its disinfection strategy and utilize a disinfectant with a reduced oxidation potential relative to free chlorine—hence the application of chloramines.
Over the past two years, largely as a result of the DC WASA experience, substantial progress has been made in improving the understanding of lead solubility and the impact of chloramines on lead release. The DC WASA work makes clear that the levels of lead found in consumers’ taps is dependent on the chemistry of the water, the time the water has been in contact with lead-bearing materials, and the age, surface character, metallurgical properties, electrochemical proprieties and water chemistry of the piping. Because lead levels are a complex function of all of these parameters, it is common for elevated lead levels to vary by household, and, over time, even within the same household.
With the help of scientists at the EPA, the University of Washington and other institutions, the DC WASA experience has advanced our understanding of lead chemistry, strongly suggesting that Pb(IV) oxides play a more important role in determining the overall solubility of lead in drinking water distribution systems than previously recognized. This recognition is important because the basis for most guidance on lead corrosion control presumes that the Pb(II) solids, not Pb(IV), control solubility.
This previous “conventional” understanding assumed that manipulation of basic water chemistry (pH, alkalinity) could produce stable mineral forms (cerrusite and hydrocerrusite) that would passivate a corroding lead surface. It was not fully understood that in many cases Pb(II) was not controlling, and that changing redox conditions would alter the preferred mineral forms, destabilizing existing corrosion scales and producing metal release rates that represent both a regulatory and public health concern. The role of naturally occurring organic matter in the source water is also becoming clearer. Moreover, because many large utilities are now considering conversion to chloramination, the scope of this problem is national, and not limited to the DC WASA experience.
The DC WASA pioneered the concept of lead profiling to determine the extent and magnitude of the lead release from various components in service lines and premise piping and plumbing systems.
In this sampling regime, consecutive 1-liter water samples were collected from kitchen taps after the water had been standing motionless in piping for an overnight period. Depending on the length of the piping, two or three dozen consecutive samples may have been collected to gain a fingerprint of the lead contribution from the multiple sources involved.
Selected samples were analyzed for total and soluble lead. The first liter was representative of the faucet and immediate connective piping. The next few samples would have been representative of home plumbing, followed by the service line, and then the utility water main. As indicated by the data, the highest lead levels are generated by the LSL. In addition, it is clear that most of the lead is in soluble, not the particulate form.
Partial LSL replacement
Research on the DC WASA lead issue has gone forward on a variety of fronts; however, one of the most perplexing issues has been the issue of LSL replacement. The DC WASA has long recognized that the preferred approach for resolving the drinking water lead issue is the replacement of as many of the existing 28,000 LSLs as possible.
This, however, is a daunting and expensive task, consuming more than $20 million in resources annually, while causing traffic disruption and inconvenience to both neighbors and neighborhoods. Moreover, legally, the DC WASA can replace only the portion of the LSL that lies within the public right-of-way and cannot force a homeowner to replace the privately owned potion of the LSL. Partly because of this, the DC WASA has invested considerable effort in evaluating the impacts of partial LSL replacements.
Partial LSL replacement raises another corrosion issue, namely the possibility that the coupling (or near coupling) of a partial LSL to the replacement copper line creates a galvanic corrosion cell that may accelerate corrosion on the remaining portion of the LSL. The concern is that coupling of the dissimilar metals may create a localized condition having the potential to evaluate overall lead release rates above pre-replacement levels; hence, not only defeating the intent of the replacement program, but exacerbating the situation.
The DC WASA established a set of complementary and overlapping bench/pilot-scale studies designed to quickly assess lead (and in some cases copper) release. These complementary analyses measured corrosion rates and metal release under a variety of simulated distribution system conditions. Substantial efforts were placed on developing a test cell that could duplicate the effects of LSL galvanic coupling. In the end, this testing demonstrated that galvanic coupling has little relevance to accelerating metal release on the LSL, and is an easily managed process.
The experiences, research and improved understanding of lead corrosion processes have come at substantial cost. Some of the more important lessons learned from the district’s experience are summarized in the following points.
- The chemistry of lead solubility, oxidation and mobility is substantially more complex than had been previously understood. Re-equilibration is always an issue, and substantial changes in redox chemistry of a distribution system will likely have profound effects on lead solubility—possibly on other trace metals as well;
- Concerns about possible acceleration of lead release due to partial LSL replacement, or other potential galvanic-related impacts on lead-bearing materials, have been overstated;
- Pilot-scale pipe loop programs can recreate the lead release process in a distribution system, and can be particularly accurate and useful tools for predicting optimal corrosion control conditions;
- Conventional LCR compliance monitoring may not be adequate in all cases to define the nature of LSL metals release—household plumbing lead profiles are frequently a necessary tool to identify the sources and magnitude of lead release; and
- Phosphates are highly effective at passivation of LSL surfaces in chloraminated systems. In some circumstances, orthophosphates may, in fact, be necessary to achieve substantial passivation. However, not all phosphates are created equal—polyphosphates are likely problematic for lead passivation. Nonetheless, orthophosphates are clearly an effective lead passivation agent, and represent a corrosion-inhibition program that can be implemented with minimal adverse system-wide impact.