Fig. 3.

(A) Plot of ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁴Pb/²⁰⁶Pb for leachates and residues from the ancient Neapolis harbor deposits. Neapolitan Yellow Tuff (open circles), travertine (filled squares), and fistulae (filled triangles) (21, 23) are also shown. The residues define a mixing line between a volcanic component best represented by the Neapolitan Yellow Tuff and a natural fluvial (soluble) component represented by the star symbols. The leachates define two well-separated fields, which both can be accounted for by a mixture between a fluvial component and the imported (anthropogenic) component β. (B) Similar plot using the geochemically informed parameters κ (²³²Th/²³⁸U) and tectonic model age T_mod of the lead sources. This plot shows that the imported Pb component β is of Variscan (Hercynian, ∼300 Ma) age: such values of T_mod are unknown in peninsular Italy, demonstrating that this component reflects massive contamination of the harbor by lead from the water distribution system. The two groups of κ values are distinct, which indicates that a new network of Pb fistulae was installed in the wake of the Somma-Vesuvius AD 79 eruption.