Lead Service Lines
Also known as service branches, service lines connect the water main in the street to your house. Normally the service line is copper, but in older cities in the eastern US and Canada it is commonly lead. Galvanized iron is also used but is less durable.
Also in the system are various brass components – the curb stop valve, the meter, and the shutoff valve in the house. Thus a Pb supply line will have a number of brass connections along its length. These bimetallic connections have the potential for accelerated galvanic corrosion, as illustrated on the right
The Lead and Copper RuleIn the U.S., the metal content of drinking water is regulated at the tap for lead and copper, using the lead and copper rule of the USEPA. Briefly, a utility must demonstrate that fewer than 10 percent of homes have Pb levels greater than 15 micrograms/liter (ug/L) and Cu levels less than 1300 ug/L. Clearly the Pb is of greater concern, and it mostly comes from the service line. To meet this standard utilities employ water treatment changes or service line replacement or both.
Water treatment changes that lower Pb levels at the tap include raising the pH of the water and raising the phosphate content. Raising the pH lowers the solubility of Pb carbonate minerals [see Overview of Lead Scale Formation and Solubility]. Addition of phosphate replaces more soluble carbonates and oxides with insoluble Pb phosphates.
Most utilities routinely replace their side of the service line (from the main to the curbstop) when doing water main repairs or replacements, but it is left to the homeowner to do the remainder. Such partial replacements produce some reduction in Pb at the tap, once the disturbance from cutting the pipe has settled down, but it is only a full replacement that significantly lowers Pb levels [see Partial v Full Service Line Replacement].
The inner surfaces of service lines are coated with minerals that are less soluble than the bare metal and so confer a measure of resistance to dissolution. For lead, common mineral components of these corrosion scales are the oxides litharge and plattnerite and the carbonates cerussite and hydrocerussite. Systems that use a phosphate corrosion inhibitor may in addition have pyromorphite, and non-lead minerals such as calcite are common. Systems with stable plattnerite or pyromorphite scales tend to have lower lead in their tap water.
Changes in water chemistry can lead to destabilization of these scales, however. A classic observation is that, especially for systems on surface water supplies, summer levels of lead measured at the tap are considerably higher than winter levels. Many systems have attempted to reduce lead levels by raising the pH of their water, but this approach is limited by the precipitation of calcium carbonate minerals at higher pHs. Profound changes have been observed by some utilities that switched from chlorine to chloramine disinfection. The apparent cause is the lowered oxidation potential in chloramine that makes plattnerite soluble.