The Soil Mineralogy of Lead at Horace's Villa

Abstract

Lead pipe buried by calcareous alluvium at Horace's Villa near Licenza, Italy, for more than a millennium were excavated, along with soil from around and within the lead pipe, to identify the stable corrosion products and Pb-bearing soil minerals. The corrosion crust of the pipe, soil from inside the pipe, and soil from around the pipe were characterized by chemical means and by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). The chemical and thermal conditions necessary to produce the identified Pb mineral assemblage was investigated through the development of activity and predominance diagrams. Analysis of total soil Pb concentrations indicates that the lateral movement of Pb from the pipe in the pH >8 soil has exceeded 9 cm. However, background soil Pb concentrations are observed at a lateral distance of 50 cm. The corrosion crust of the lead pipe is composed of litharge [α–PbO(s)], cerussite [PbCO3(s)], and hydrocerussite [Pb3(CO3)2(OH)2(s)]. Similarly, soil inside the pipe and in the 0- to 1-cm zone around the pipe contains cerussite and hydrocerussite. A detailed thermochemical analysis indicates that the formation of hydrocerussite is favored, relative to cerussite, when the CO2 partial pressure is approximately atmospheric (10−3.5 atm or 10−4.5 MPa) or less, or when the soil temperature is approximately 25°C or greater. The formation of stable lead phosphates has not occurred in these alkaline environments, even though apatite is present and thermochemical analysis predicts the formation of chloropyromorphite [Pb5(PO4)3Cl(s)]. If chloropyromorphite is removed from consideration, hydroxypyromorphite [Pb5(PO4)3OH(s)] is not predicted to form in the presence of apatite.