Relative size distributions of lead, aluminum, and iron were determined for a subset of the profile samples described above (see "Point-of-use corrosion control monitoring"), using size-exclusion chromatography with multielement detection (SEC-ICP-MS). The full method is detailed in a previous publication. Briefly, we separated samples on a stationary phase composed of cross-linked agarose and dextran (Superdex 200, 10 × 300 mm, 13 μm particle size, GE Healthcare) with 50 mM tris-HCl (pH 7.3) as the mobile phase. The flow rate was 0.5 mL min -1 and the injection volume was 212 µL. We monitored Al, Fe, and 208 Pb in the column effluent as a function of time by ICP-MS (see Point-of-use corrosion control monitoring above). The retention volume of thyroglobulin (669 kDa, Stoke's radius 8.5 nm), indicated in chromatograms as a qualitative point of reference, was monitored as 127 I. Chromatograms were summarized as the sum of two skewed or exponentially modified Gaussians using the R package fffprocessr, as described elsewhere. R code to reproduce the analysis is included as Supplementary Text S1, the individual chromatograms are shown in Figure , and the data are available at doi.org/10.5281/zenodo.5139734.