Trace metals and Copper Isotope Dynamics in the Ala Wai watershed, O‘ahu

Abstract

Transfer of material from land to sea is important to our understanding of global geochemical cycles. High rates of weathering and erosion in Hawai’i create an ideal laboratory to study natural weathering processes. However, increasing urbanization has led to concerns about heavy metal contamination in both urban streams and coastal waters. Additionally, storm runoff is often enriched in contaminants compared to baseflow, and storms are expected to increase in intensity due to anthropogenic climate change. The transport of the transition metal copper (Cu), both a bio-essential nutrient and toxic pollutant, is investigated in the small, tropical Ala Wai watershed. Distinguishing the natural versus anthropogenic sources of Cu and determining the key processes that influence its distribution and transport are critical to developing a base for the evaluation of Cu contamination and toxicity in ecosystems. Concentrations of various elements, as well as isotopic ratios of Cu (δ65Cu) and Fe in suspended particles and the dissolved fraction are analyzed. Sampling sites extend from pristine mountain streams to an urban canal, and cover baseflow and storm conditions, providing a transition from natural to anthropogenic dominance in Cu supply and cycling. Dissolved δ65Cu during baseflow is heavier than both bedrock and particulate fractions, and relatively consistent throughout the watershed, indicating control by complexation with organic ligands, while particulate δ65Cu is more variable. Urban stations and storm runoff are both enriched in Cu compared to upstream, baseflow sites. Heavier δ65Cu of estuarine particles likely stems from road-derived materials, and δ65Cu of storm runoff indicates both light, low concentration- and heavy, high concentration sources of Cu, potentially from soils, fungicides, and vehicle-related sources.