Trace metals and copper isotope dynamics in the Ala Wai watershed and estuary, Hawaiʻi: Comparing the 1990s to today, and storm runoff to baseflow.

Transfer of material from land to sea is important to our understanding of geochemical cycles. High erosion rates in Hawaiʻi create an ideal laboratory to study natural weathering processes. However, increasing urbanization leads to concerns about heavy metal contamination in 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 climate change. The transport of copper (Cu), a bio-essential nutrient and toxic pollutant, is investigated in a small, tropical 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) 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 solid 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 samples. 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.