Isotopic identification of natural vs. anthropogenic sources of Pb in Laramie basin groundwaters, Wyoming, USA

Abstract.

Water well samples, precipitation, and leachates of aquifer rock samples from the Laramie basin, Wyoming, were analyzed to test the suitability of Pb isotopes for tracing hydrologic processes in a basin where Sr isotopes had proven effective. Leachable Pb from host rocks to aquifers in this basin have isotopically distinct compositions and isotopic tracing would be effective in differentiating natural sources of Pb. However, in almost all cases, this natural signal is apparently swamped by anthropogenic Pb sources in drilled water wells. The isotopic compositions of these samples cannot be produced by any combination of Pb in precipitation and leached Pb from the aquifers alone, but are consistent with mixtures of anthropogenic Pb. Thorogenic (²⁰⁸Pb) vs. uranogenic (²⁰⁶Pb, ²⁰⁷Pb) Pb was especially useful for discriminating natural from anthropogenic sources for these samples. It is strongly recommended that all four Pb isotopes be reported in hydrologic and environmental studies that employ this approach.