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

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.