Natural levels of lead and cadmium in a remote mountain stream

Abstract

Lead and cadmium concentrations in marine and terrestrial ecosystems, on surfaces of soil particles, and in the atmosphere have been highly elevated on a global scale because of industrial pollution. In order to ascertain the natural (rock-derived) levels of lead and cadmium in streams, a pristine mountain watershed in the Sierra Nevada, California, was studied in respect to its lead and cadmium contents. The transport of lead and cadmium was compared to iron since it shares similar transport patterns with lead and cadmium, and is relatively uninfluenced by pollution. The concentrations of lead and cadmium in a late-summer mountain drainage water are shown to be close to natural levels that are controlled by the weathering of bedrock and soil. This is demonstrated by (1) measurements of lead isotopic composition, and Fe/Pb ratios in stream water, ground water, soil and bedrock, and (2) the rapid removal rate of excess atmospheric lead and cadmium from the water as it flows downstream. Lead is taken up by particles (mostly hydrous iron oxides) in the studied stream during the autumn at a relatively constant rate. On the other hand, cadmium behavior in the stream water is erratic and cannot be explained by the same adsorption mechanism as lead. The Fe/Pb ratio is constant within an order of magnitude in the bedrock, the soil, soil and rock leachates, unfiltered and filtered ground water, and in filtered and unfiltered stream water. Therefore, the Fe/Pb ratio may provide a first-order estimate for the concentration of natural lead in streams draining through granitoid bedrocks.

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