Spatial Variation of Sediment-Bound Zinc, Lead, Copper and Rubidium in Lake Illawarra, a Coastal Lagoon in Eastern Australia

Abstract

Forty sites in Lake Illawarra, a shallow (<3.7 m) almost landlocked, New South Wales coastal lagoon, were investigated by sediment coring to provide a comprehensive reappraisal of sediment-bound, background (pre-industrial) and polluted trace metal concentrations together with organic matter concentrations and direct (on-vessel) determination of pH, redox potential and temperature. The distribution of Zn, Pb and Cu in the upper 20 cm of sediment is directly related to the proportion of mud-dominated sediment, organic matter content and input from local industrial sources. Alkaline (pH >7.6), reducing (redox potential <-50 mV) conditions, 5 cm below the water-sediment interface, high organic matter content (mean 14.0 ± 2.1%  in muddy substrates) and biogenic processes (bacterially-mediated sulfate reduction) in Lake Illawarra facilitate Zn, Pb and Cu retention via sulfide formation in the sediment. Trace metal enrichment factors for sediment-bound Zn, Pb and Cu were less than 2.5; an exception was in southern Griffins Bay where enrichment factors were greater than 5.9, 3.5 and 1.8 for Zn, Pb and Cu respectively. Trace metal contamination can be monitored geochemically by trace element divergence from the Rb concentration versus depth profiles. A major contributor to sediment trace metal contamination in Lake Illawarra consists of distinctive, fugitive industrial particles. Trace metal-depth concentration profiles and anthropogenic markers can be used in conjunction with the time of industrial development to determine the sedimentation rate which ranges from 3.3-16 mm y ^-1 - a five- to tenfold increase compared with Holocene rates.

The low mean concentrations of Zn (< 170 µg g ^-1), Pb (<60 µg g^-1) and Cu (<60 µg g^-1) present in the top 20 cm of Lake Illawarra sediments (except for southern Griffins Bay) appear to pose few short term environmental concerns. However, management strategies, especially dredging of this shallow lagoon, should address issues such as transference of sediment from an anoxic to oxic environment during foreshore reclamation, with the resultant development of acid soils from sulfide decomposition.