Retrospective Analysis of Anthropogenic Inputs of Lead and Other Heavy Metals to the Hawaiian Sedimentary Environment

Abstract

In this paper we present and discuss heavy-metal concentrations in sediment cores from the Ala Wai Canal, a small urban drainage estuary in Honolulu, Hawaii (Oahu Island), and from selected soils and sediments collected in less impacted areas of Hawaii. We focus on lead and its isotope ratios to document the introduction, proliferation and subsequent phasing-out of alkyl-lead fuel additives in Honolulu over the past 60 years. Sedimentary lead increases from <10 ppm in the oldest (deepest) unimpacted portions of the cores to ˜750 ppm in sediments from the mid-1970s; lead contents then decrease to a range of 100–300 ppm in the most recent deposits. Lead concentrations in the canal sediments closely track the consumption of alkyl-lead fuel additives used on Oahu in the middle of this century. More than 90% of the lead in certain core intervals is of anthropogenic origin. Stable lead isotope ratios reflect changes in the estuary's sources of lead and show that this lead has an isotopic signature distinct from natural samples. Undisturbed layers of sediments in the Ala Wai canal track the average composition of anthropogenic lead used in its Honolulu watershed through time. The distinctive pattern of lead isotopes measured in Ala Wai Canal sediments can be used to distinguish anthropogenic from natural lead in Hawaiian rural locations. Soils located away from point-sources of contamination have isotopic compositions defined by mixtures of locally erupted volcanic rock, atmospheric dust and components similar to Ala Wai Canal anthropogenic lead. Natural lead concentrations in Hawaii are in the range of a few to a few tens of parts per million, with the high-concentration end-member occurring in a few selected intramountain basins where soils contain a significant component of continental dust fines scavenged by rainfall and concentrated by local geological conditions. Rural watershed material exhibits small but discernible elevations in lead concentrations due to anthropogenic effects. Other metals (cadmium, copper and zinc) in the Ala Wai Canal sediments also display patterns consistent with increased fluxes of metals attributable to anthropogenic activity. Unlike lead, no clear maxima are observed in sediments deposited during the mid 1970s. Rather, a continued input of these metals is attributed to expanding anthropogenic activity in the Ala Wai Canal watershed. © 1997 by John Wiley & Sons, Ltd.