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Isotopic Clues to Sources of Natural and Anthropogenic Lead in Sediments and Soils from O'ahu, Hawai'i

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Title:Isotopic Clues to Sources of Natural and Anthropogenic Lead in Sediments and Soils from O'ahu, Hawai'i
Authors:Spencer, Khalil J.
De Carlo, Eric H.
McMurtry, Gary M.
Date Issued:Oct 1995
Publisher:University of Hawaii Press
Citation:Spencer KJ, De Carlo EH, McMurtry GM. 1995. Isotopic clues to sources of natural and anthropogenic lead in sediments and soils from O'ahu, Hawai'i. Pac Sci 49(4): 492-510.
Abstract:Stable Pb isotopes, Pb elemental concentrations, and, for some
samples, Nd and Sr isotopes and concentrations have been analyzed on soils
and on stream and estuarine sediments to evaluate the provenance of major
inputs of Pb to the O'ahu, Hawai'i, environment. Core samples from the Ala
Wai Canal, a major estuary draining urban Honolulu, preserve a historical
record of anthropogenic lead input that peaked during the 1970s, the period of
heaviest leaded-gas usage in Hawai'i. The timing of the Pb concentration peak
and the simultaneous rise in Zn and Cd concentrations, two elements used in
tire vulcanization, strongly suggest that the source of this Pb was tetraethyl Pb
used in leaded gasoline. The changing Pb isotopic composition in these sediments
reflects changing sources of ore from which tetraethyllead was produced.
These isotopic signatures can be used to fingerprint anthropogenic Pb elsewhere
on O'ahu. Although leaded gasoline has been phased out of production in the
United States and in many other countries, elevated amounts of lead continue
to deposit from the Ala Wai Canal's watershed. Sediment samples from Manoa
Stream, a principal tributary, suggest that relatively uncontaminated sediments
are eroded from its headwaters while a source (or sources) of lead continues to
discharge into the stream as it nears the south end of Manoa Valley. The
isotopic composition of this lead is similar to that measured in recently deposited
sediments cored from the Ala Wai Canal. An atmospheric dust-enriched
soil collected on the island of Hawai'i contains elevated Pb concentrations
(55 ppm) and a Pb isotopic composition similar to North Pacific pelagic
sediment. In addition, this sample contains unradiogenic Nd (E = -6) and radiogenic
Sr (87Sr/86Sr = 0.722527) confirming an old, continentally derived
provenance. Soils collected in Ha'ikii Valley, a windward O'ahu valley subject
to high rainfall, contain variable Pb concentrations and Sr, Nd, and Pb isotopes
trending toward the isotopic composition of the dust-enriched sample. This
confirms that the Ha'ikii Valley soils contain an aerosol component. Soils
enriched in this component could have natural lead concentrations higher than
soils made up solely of weathered Hawaiian rocks. Hawai'i's soils and sediments
have naturally derived variations in Pb concentration that are caused by
differences in provenance and degree of weathering. Superimposed on this
natural concentration variation is a variable anthropogenic signal. These variations
should be factored into environmental monitoring programs.
Appears in Collections: Pacific Science Volume 49, Number 4, 1995

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