Exploring Variations in Properties of Lead Pipe Scales from Different Drinking Water Systems

Abstract

The corrosion products formed in lead water pipes exert strong control over lead concentrations in tap water. Compositions of pipe scales from different drinking water distribution systems vary in appearance, crystalline phases present, and elemental concentrations. This study is based on 76 harvested pipes from 17 different systems across the United States together with data from previously published research. Factors impacting lead pipe scale composition are identified. The characterization data are compared with chemical equilibrium predictions. The specific crystalline lead carbonate solid present depends on the pH and dissolved inorganic carbon (DIC) concentration. Systems with only hydrocerussite [Pb₃(CO₃)₂(OH)₂] tend to have a higher pH (8.5 ± 0.8) and a lower DIC (1.3 ± 0.6 mM) compared to those of systems with only cerussite (PbCO₃) (pH 7.5 ± 0.2 and DIC of 5.5 ± 1.3). While lead(IV) oxide solids are predicted in all free chlorine systems, they were observed in only 43% of them. Lead phosphate solids are more commonly found in systems using orthophosphate at the highest concentrations. Amorphous materials are present as components of many pipe scales, and these amorphous materials are often rich in aluminum. Equilibrium predictions for lead carbonate, lead(IV) oxide solids, and lead phosphate correspond to the observed presence of these solids with accuracies of 95%, 43%, and 73%, respectively.