Water quality–pipe deposit relationships in Midwestern lead pipes

Abstract

The conventional wisdom of lead‐scale solubility has been built over the years by geochemical solubility models, experimental studies, and field sampling using multiple protocols. Rarely have the mineral phases from scales formed in real‐world drinking water lead service lines (LSLs) been compared with theoretical predictions. In this study, model predictions are compared with LSL scales from 22 drinking water distribution systems. The results show that only 9 of the 22 systems had LSL scales that followed model predictions. The remaining systems had unpredictable scales, some with unknown lead release characteristics, demonstrating that predicting scale formation and lead release solely by models cannot be relied on in all cases to protect human health. Therefore, for many systems with LSLs, pilot studies with existing LSL scales will be necessary to evaluate and optimize corrosion control, and correspondingly, appropriate residential water sampling will be needed to demonstrate consistent and optimal system corrosion control.

Lead pipe andlead–tinsolder scale formation and structure: A conceptual model

“…Analyses of lead scale formed during uniform corrosion have been reported by others (Tully et al, 2019;Walsh et al, 2017;Wasserstrom et al, 2017). Walsh et al (2017) presented a schematic of the "typical" structure and composition of lead scale they observed in a Denver Water system pipe loop study, which is reproduced as Figure 4.…”

Worth a Closer Look: Raman Spectra of Lead-Pipe Scale

“…Many water utilities add orthophosphate in an effort to form these low-solubility solids, but very few of them have actually analyzed scale samples to determine if such solids are forming and, if so, which ones and of what specific chemical composition. In those cases in which analyses have been made, the results can be unexpected [11].…”

“…To understand what controls the concentrations of dissolved and particulate lead in drinking water requires knowledge of what phase is precipitating on the walls of the LSLs. The precipitating phase is not always that which has been calculated to be in equilibrium with the composition, pH, and Eh of the water [3,4,11]. RS can help determine if this apparent discrepancy between theory and observation is due to the development of an unpredicted phase or simply the ability of an earlier-precipitated phase to adjust its composition via chemical substitution, i.e., to exhibit solid solution.…”

“…The fact that type (b) material directly overlies type (a) suggests that the spectra have captured plattnerite in the process of maturation from recent precipitate (b) into a more ordered, more stable crystalline solid (a) as it grows outward from the pipe wall over time. There is repeated recognition in the literature [3,11,63] of a large component of retrieved scale material that is called amorphous, which typically means undefinable by XRD. Both the chemical composition (in many cases rich in Al, Si, and P, in addition to Pb, which may be minor) and the degree/lack of crystallinity of such material are important to its physical and chemical stability and, therefore, important to define.…”

“…In principle, one can calculate which phase will be stable under specific compositional and pH conditions in the drinking/synthetically produced water. However, bulk XRD analyses of scale material in some cases show deviations from the predicted scale minerals [2,4,11,12], which means that the predicted lead concentrations in the water will be in error [25]. These deviations should be investigated.…”