COMPOSITION OF SUSPENDED SEDIMENT AND WATER IN STREAMS OF URBANIZED SUBTROPICAL WATERSHEDS IN HAWAII

Streams in Hawaii often differ from those in continental settings and present unique challenges to investigators attempting to characterize them. Hawaiian streams are short, watersheds are small and steep, and rain events are usually short in duration but intense. Hence, streams in Hawaii are flashy and carry large suspended particulate matter (SPM) loads during storms. Time scales for hydrographs in Hawaii are on the order of hours rather than days and automated sampling is a necessity. We report here on the trace element (TE) composition of water and SPM collected at paired stations in the Ala Wai and South Kaneohe Bay watersheds during the period 1999-2002. Samples were analyzed for TE to elucidate the factors that influence spatial and temporal changes in composition during downstream transport from conservation areas through the urban landscape and ultimate to the coastal ocean. Our results show that the TE composition of SPM derives from natural source rocks and is overprinted by variable anthropogenic contributions that reflect land use. SPM in urban areas is enriched in Ba, Cd, Co, Cu, Pb, and Zn, whereas As enrichments are attributed to agricultural activity. Enrichments of TE between upper, forested portions of the watersheds and the urban landscape vary by a factor of less than two to more than one order of magnitude. Some elements (Cr, Ni, and V) appear to be mostly controlled by natural processes and show relatively little variation through time or space. Statistical relationships obtained from principal component analysis (PCA), which yields factors that account for over 80% of the data variance, are utilized to elucidate sources of element. The three main factors are interpreted to represent 1) urban anthropogenic input (Ba, Cu, Co, Pb, Zn), 2) natural control (Cr, Ni, V) and 3) agricultural input (As). Land use changes can account for a large portion of the spatial trends in distribution and abundance of selected TE, whereas temporal trends reflect natural processes, more so than land use. Comparison of data obtained during storms that develop from the orographic lifting during tradewind conditions with those derived from a large offshore low pressure systems allows calculation of urban contributions to the TE content of SPM. Dissolved trace elements also display spatial and temporal trends that are consistent with observed changes in the composition of SPM.