SPATIAL VARIATION OF SEDIMENT-BOUND ZINC, LEAD, COPPER AND RUBIDIUM IN LAKE ILLAWARRA, A COASTAL LAGOON IN EASTERN AUSTRALIA

Forty sites in Lake Illawarra, a shallow (<3.7 m) almost landlocked, N ew South Wales coastal lagoon, were investigated by sediment coring to provide a comprehensive reappraisal of sediment bound, background (pr e-industrial) and polluted trace metal concentrations together with or ganic matter concentrations and direct (on-vessel) determination of pH , redox potential and temperature. The distribution of Zn, Pb and Cu i n the upper 20 cm of sediment is directly related to the proportion of mud-dominated sediment, organic matter content and input from local i ndustrial 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 f acilitate Zn, Pb and Cu retention via sulfide formation in the sedimen t. Trace metal enrichment factors for sediment-bound Zn, Pb and Cu wer e less than 2.5; an exception was in southern Griffins Bay where enric hment factors were greater than 5.9, 3.5 and 1.8 for Zn, Pb and Cu res pectively. Trace metal contamination can be monitored geochemically by trace element divergence from the Rb concentration versus depth profi les. A major contributor to sediment trace metal contamination in Lake Illawarra consists of distinctive, fugitive industrial particles. Tra ce metal-depth concentration profiles and anthropogenic markers can be used in conjunction with the time of industrial development to determ ine the sedimentation rate which ranges from 3.3-16 mm y(-1)-a five-to ten-fold increase compared with Holocene rates. The low mean concentr ations of Zn (<170 mu g g(-1)), Pb (<60 mu g g(-1)) and Cu (<60 mu g g (-1)) present in the top 20 cm of Lake Illawarra sediments (except for southern Griffins Bay) appear to pose few short term environmental co ncerns. However, management strategies, especially dredging of this sh allow lagoon, should address issues such as transference of sediment f rom an anoxic to oxic environment during foreshore reclamation, with t he resultant development of acid soils from sulfide decomposition.