Stream sediment geochemistry of four small drainages on the north shore of Kauai west of Hanalei

Abstract

Geochemical compositions of fine-grained stream sediment from four drainages on the north shore of the island of Kauai, Hawaii, west of Hanalei and two back-beach sites were explored to increase understanding about land-based runoff and ecological risk from runoff to nearshore coral communities. Stream and beach sediment were collected between July 30 and August 2, 2016, and major, minor, and trace elements in the less than 63 micrometer-diameter fraction were analyzed by inductively coupled plasma optical emission spectroscopy and mass spectroscopy. The potentially toxic metals Cr, Cu, Ni, and Zn exceeded levels at which adverse biological effects could be observed; however, these metals seemed to be largely mineral-bound and thus were unlikely to harm organisms. Cd and Pb were below levels of ecological concern. Only a small amount of fine-grained sediment was retained on beaches west of Hanalei sampled in summer 2016 (mean=8.8 percent, median=0.4 percent, range=0–92.8 percent, n=41). Although the scarcity of fine-grained sediment precluded land-based runoff sourcing to the nearshore region, it did indicate that fine-grained sediment and associated contaminants did not accumulate over the long term in the sampled intertidal, subtidal, and reef-flat environments, which would reduce sediment-related pressures on coral communities there.