The effects of a known exposure source on Pb isotopes in bones from Pb-dosed cows

Abstract

Pb accumulates in bone over long periods of time, and the isotopic effects of this accumulation are poorly studied. The use of Pb isotopes in bones as a metric of exposure requires additional constraints on the relationship between the isotopic composition of the exposure source and resulting isotopic composition of the bone. Here, we present a Pb isotope study of two cows that were associated with the blood Pb proficiency testing program at the University of Wisconsin-Madison State Laboratory of Hygiene (WSLH). These two cows were born and raised under the same conditions on the same farm, but were subjected to different levels of Pb exposure during that time period. Pb isotopes were measured in femur bones subsampled from each cow and the Pb isotopic composition of the exposure source (Pb nitrate) used for dosing the animals was also measured. Pb isotopic compositions of nearby (<0.5 km) wild grasses, silage, well water, and other potential environmental sources were measured to obtain the background Pb signal. Our data show that the Pb isotopic compositions of the cow bones reflect a mixture between background Pb sources mostly in the diet and the Pb isotopic composition of the Pb nitrate exposure source. The bones approach but do not attain the Pb isotopic composition of the exposure source at the levels of exposure in this study. Using estimates for daily food and water consumption, we are able to evaluate the relative contributions of different sources of Pb to the isotopic composition of the bone. The data show that the Pb isotopic composition of bone provides a weighted average of bioaccumulated Pb sources. Additional bone major and trace element composition data show that Pb competition with certain other trace elements in Ca substitution sites depends on the degree of Pb exposure. This result has implications for the use of bone in exposure studies, as well as the use of bone to understand background Pb signals in the environment.