The Effects of Changing Water Chemistry on the Stability of Lead Service Lines

Lead-containing plumbing materials are widely used in drinking water systems across the globe. Being a toxic heavy metal, the release of lead within the drinking water distribution systems poses a major threat to public health. This study examined lead(II) mineral phases present in typical drinking water distribution systems by utilizing chemical equilibrium modeling. The predominant lead(II) minerals were exposed to free chlorine, a residual disinfectant, to study their oxidative transformation. In the absence of phosphate, cerussite PbCO3(s) and hydrocerussite Pb3(CO3)2(OH)2(s) are the predominant phases, dependent on pH. The addition of phosphate induced the precipitation of pyromorphite Pb5(PO4)3Cl(s), lowering the total dissolved lead concentrations. The lead phosphate minerals resisted oxidation by chlorine, but the lead carbonate minerals reacted readily. The resulting solids were analyzed with XAS and revealed the generation of plattnerite -PbO2(s) as the dominant Pb(IV) mineral, with trace amounts of scrutinyite -PbO2(s). Bromine oxidation of these Pb(II) minerals resulted in oxidation rate constants up to an order of magnitude greater than with chlorine. Using the derived rate constants, models of drinking water distribution systems were developed to show in the impact of bromide contamination. Although the lead(IV) phosphate minerals resisted oxidation, the presence of bromide indicated these systems would be oxidized within a century. Next, the mobility of toxic metals in an oxidized system was investigated. Vanadinite Pb5(VO4)3Cl is present in systems with vanadium contamination and has the potential to release both vanadium and lead if oxidation occurs. Like the lead(II) phosphates, vanadinite resists chlorine oxidation but can be oxidized by bromine. This oxidation causes to the release of both lead and vanadium over time. Vanadium is very mobile, as almost 80% that passes through the 0.2 µm filter also passes through the 0.025 µ m filter. Lead is release occurs in the 0.2 µm filter but is effectively removed by the 0.025 µ m filter. This study provides fundamental understanding of lead surface chemistry in drinking water as well as strategies to prevent toxic metal release in drinking water distribution systems