1. Occurrence and Significance
Lead (Pb) is the fifth element in Group IVA in the periodic table. It has an atomic number of 82, an atomic weight of 207.19, and valences of 2 and 4. The average abundance of Pb in the earth’s crust is 13 ppm; in soils it ranges from 2.6 to 25 ppm; in streams it is 3 μg/L, and in groundwaters it is generally <0.1 mg/L. Lead is obtained chiefly from galena (PbS). It is used in batteries, ammunition, solder, piping, pigments, insecticides, and alloys. Lead also was used in gasoline for many years as an antiknock agent in the form of tetraethyl lead.
The common aqueous species are Pb2+ and hydroxide and carbonate complexes. Lead in a water supply may come from industrial, mine, and smelter discharges or from the dissolution of plumbing and plumbing fixtures. Tap waters that are inherently noncorrosive or not suitably treated may contain lead resulting from an attack on lead service pipes, lead interior plumbing, brass fixtures and fittings, or solder pipe joints.
Lead is nonessential for plants and animals. It is toxic by ingestion and is a cumulative poison. The Food and Drug Administration regulates lead content in food and house paints. Under the lead-copper rule, the U.S. EPA drinking water 90th percentile action level is 15 μg/L.