If normal lead (Pb) concentrations in freshwater are much lower than the standards set for drinking water, how does lead concentration get high enough in drinking water to become a health issue?
For many of the smaller, newer, and more modern water treatment and distribution systems, Lead exposure is not as likely to be a pertinent issue. However, for many older systems where pipe scales have accumulated over a long period of time, this could be a problem. The first consideration is whether a system has pipe scaling or not, followed by where the scaling is located within the system, and then whether the scales contain lead salts or lead oxides or not. Many systems know very little about the nature of the scales within their distribution system.
Some scaling is unavoidable and is often beneficial up to a point for corrosion protection. Potential lead release from pipe scales became more of an issue when water systems began switching from chlorine to chloramines for secondary disinfection. In some cases, chlorination caused lead levels at the tap to increase significantly, sometimes exceeding the drinking water standard by a hundredfold or more. Water chemists and engineers have found that this increased lead was coming from pipe scales.
One theory of the release mechanism is that lead dioxide scales generated from precipitation of lead under high redox conditions of chlorination over many years is now going back into solution under the lower redox conditions that exist with the switch to chloramination. Since many water systems have switched to chloramines for secondary disinfection with little impact on lead levels in tap water, this does not appear to be a high risk for most people on most public water systems where chloramines are now used for secondary disinfection.
It is a greater risk for customers on those systems that have lead-containing components within the distribution system, scales that contain lead, or lead service lines from the main lines to individual homes. Most systems do not contain lead service lines anymore and most distribution lines containing lead components have been replaced, but some older systems still have both. Your water utility management personnel can answer any questions on the makeup of your distribution system from the treatment plant to your tap.
Many people wonder if lead (Pb) leaching from copper plumbing in their house can also pose a health hazard? The only way to be certain, however, especially if you have copper plumbing and are on a private water system, is to have your water tested under the worst-case conditions--after the water has stood in the plumbing system overnight or for several days. Persons with the greatest risk for lead exposure from drinking water are those on a private well water system where the water has a low pH making it corrosive and capable of leaching lead from copper plumbing and brass fixtures (most of these contain some lead). Even some PVC plastics contain a small amount of lead to reduce their degradation from light, especially UV light. Most PVC plumbing is not exposed to direct sunlight though and will last a long time without significant degradation.
The concern for PVC today is chlorinated organics leaching from the pipe material. Lead exposure risk is greater in homes where lead-based solder was used prior to 1986. Old copper plumbing is less of a threat to human health if you are on a community water system because regulations have required that they increase pH to reduce corrosion potential. Public systems generally inject a variety of salts and orthophosphate to precipitate any lead where that may be an issue. Flushing stagnant water from the lines before filling containers for drinking or cooking will eliminate most of the lead risk in homes with private systems.
For added measure, it is a good idea to neutralize the acidic water for both corrosion control and lead reduction benefits. If you are on a public water system, the water utility is required by law to periodically check tap water from those areas most likely to have lead contamination. This especially applies to housing areas with lead service lines or copper plumbing with lead-based solder that has not been replaced.
In the end, there are two ways lead (Pb) levels in drinking water may exceed Pb concentration in the raw water supply. Number one is when distribution systems contain lead (Pb) within plumbing components such as pipes, pipe joints (solder) or water fixtures (valves, faucets, etc.) and the water has a pH less than 6.5. The more acidic the water is (pH below 6.5), the greater the capacity for the water to leach lead from these plumbing components. This type of problem is associated primarily with private water systems using groundwater that has a pH below 6.5.
The second way that lead can become a problem is when some change in water chemistry occurs, either through mixing water sources or making adjustments in water chemistry to deal with drinking water issues other than Pb. Over time, Pb salts or Pb oxides may have built up as films or in scales within distribution systems. A sudden change in water chemistry may cause Pb to be released from scales back into treated water within the distribution system. This type of problem is more likely to occur in community water systems with old distribution lines containing lead.
