Sources of land-derived runoff to a coral reef-fringed embayment identified using geochemical tracers in nearshore sediment traps Renee K. Takesue a, * , Michael H. Bothner b , Richard L. Reynolds c a US Geological Survey, Coastal and Marine Geology, 400 Natural Bridges Drive, Santa Cruz, CA 95060, USA b US Geological Survey, Woods Hole Coastal and Marine Science Center, 384 Woods Hole Road, Woods Hole, MA 02543, USA c US Geological Survey, Denver Federal Center, MS 980, Box 25046, Denver, CO 80225, USA article info Article history: Received 26 January 2009 Accepted 18 September 2009 Available online 24 September 2009 Keywords: sediment traps provenance trace elements radioisotopes magnetic properties coral reefs abstract Geochemical tracers, including Ba, Co, Th, 7 Be, 137 Cs and 210 Pb, and magnetic properties were used to characterize terrestrial runoff collected in nearshore time-series sediment traps in Hanalei Bay, Kauai, during flood and dry conditions in summer 2006, and to fingerprint possible runoff sources in the lower watershed. In combination, the tracers indicate that runoff during a flood in August could have come from cultivated taro fields bordering the lower reach of the river. Land-based runoff associated with summer floods may have a greater impact on coral reef communities in Hanalei Bay than in winter because sediment persists for several months. During dry periods, sediment carried by the Hanalei River appears to have been mobilized primarily by undercutting of low 7 Be, low 137 Cs riverbanks composed of soil weathered from tholeiitic basalt with low Ba and Co concentrations. Following a moderate rainfall event in September, high 7 Be sediment carried by the Hanalei River was probably mobilized by overland flow in the upper watershed. Ba-desorption in low-salinity coastal water limited its use to a qualitative runoff tracer in nearshore sediment. 210 Pb had limited usefulness as a terrestrial tracer in the nearshore due to a large dissolved oceanic source and scavenging onto resuspended bottom sediment. 210 Pb- scavenging does, however, illustrate the role resuspension could play in the accumulation of particle- reactive contaminants in nearshore sediment. Co and 137 Cs were not affected by desorption or geochemical scavenging and showed the greatest potential as quantitative sediment provenance indi- cators in material collected in nearshore sediment traps. Published by Elsevier Ltd. 1. Introduction Runoff of nutrients, sediment, and contaminants from land is the largest source of pollution to marine waters and has harmful impacts on nearshore ecosystems (Fabricius, 2005; UNEP/GPA, 2006). Shallow water communities such as coral reefs are partic- ularly vulnerable to the effects of terrestrial runoff because they are close to land and are sensitive to changes in water quality (Jackson et al., 2001; Bellwood et al., 2004; Fabricius, 2005). In coastal regions with coral reefs, population growth, coastal development, and reef exploitation are contributing factors in the observed decline in coral reef ecosystems worldwide (Gardner et al., 2003; McCulloch et al., 2003; Bellwood et al., 2004; Wilkinson, 2004; Pandolfi et al., 2005). The Hawaiian Islands provide many examples of coral decline in regions with large population and impaired watersheds (Jokiel et al., 2004). To reduce threats to its coral reefs, the state of Hawaii is working to reduce land-derived sediment and pollutant runoff. This study explores whether geochemical and magnetic prop- erties in nearshore trapped sediment can be used to identify source regions of eroded terrestrial sediment in the Hanalei watershed, and to trace its dispersal in coral reef-fringed Hanalei Bay, Kauai. The watershed is characterized by heavy precipitation, steep hill slopes, areas denuded of vegetation, cultivated farmland and a flood-prone river. An understanding of sediment erosion patterns will allow mitigation strategies to target regions or land-use prac- tices that contribute to nearshore sedimentation. We used select trace elements, atmospheric-fallout radioiso- topes and magnetic properties to characterize, or fingerprint, nearshore and terrestrial sediment. Sediment trace element chemistry reflects the composition of the parent rock, chemical weathering, sorting during transport, and post-depositional diagenesis (McLennan et al., 1993). Elements such as aluminum (Al), cobalt (Co) and thorium (Th) that are relatively immobile in rocks and sediment are the most useful provenance indicators * Corresponding author. E-mail addresses: rtakesue@usgs.gov (R.K. Takesue), mbothner@usgs.gov (M.H. Bothner), rreynolds@usgs.gov (R.L. Reynolds). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.ecss.2009.09.014 Estuarine, Coastal and Shelf Science 85 (2009) 459–471