Storm-Driven Variability of Particulate Metal Concentrations in Streams of a Subtropical Watershed

Abstract

Extensive urbanization in Hawai’i, and Honolulu in particular, during the 20th century presents an opportunity to examine the effects thereof upon the storm-driven transfer of terrestrial material from the land to the ocean. This contribution focuses on the variability of Pb, Zn, Cu, Ba, Co, As, Ni, V, and Cr concentrations in streams during storm events in the small subtropical Ala Wai Canal Watershed in Honolulu, O’ahu. As expected, a comparison of metal loads for particulate and dissolved phases revealed the dominance of suspended particulate matter as a means of metal transport through the watershed. Particulate Pb, Zn, Cu, Ba, and Co displayed enhanced concentrations and elevated particulate loads during storm flow in the urbanized lower watershed. Enrichment of these metals likely derives from automotive or industrial-related sources. Agricultural fertilizer use in conservation areas, particularly the association of As with phosphate, appears to be responsible for an upper watershed enrichment of particulate As concentrations and loads. Storm-derived concentrations and loads of particulate Ni, V, and Cr exhibited a relative spatial invariance throughout the watershed, suggesting primarily mineralogical controls on their distributions. Principal components analysis (PCA) was applied to the particulate metal concentrations from samples collected in both the upper and lower portions of the watershed. PCA established eigenvalues explaining 77% of the total variance and separated particulate metals into two distinct factors. Factor 1 elements, including particulate Pb, Zn, Cu, Ba, and Co, were interpreted to represent metals exhibiting anthropogenic enrichment in the urban watershed. The association of particulate As, Ni, V, and Cr within Factor 2 likely denotes metals whose concentrations do not display enhancements in urban segments of the watershed. Examination of solid phase metal concentrations during a “Kona” storm (offshore low-pressure system) revealed that the downstream transport of relatively unimpacted upper watershed material during tradewind-derived rains results in an approximately 3-fold dilution in the urban concentrations of Pb, Zn, and Cu.