Anthropogenic effects in a coastal lagoon: geochemical characterization of Burrill Lake, NSW, Australia
Burrill Lake, a small coastal lagoon on the south coast of New South Wales, developed as an impounded drowned river valley following the post-glacial marine transgression. Marine sand fills the entrance to the lagoon whereas the back-barrier basin has accumulated organic-rich mud and sandy bayhead deltas. The bilobate form of the estuary coincides with two different catchment lithologies and land use patterns. The northern lobe drains an agricultural catchment and has a much larger bayhead delta than the southern lobs that drains natural and state forest. Sedimentation rates within the muddy lagoonal deposits are about 1.7 mm/yr. The distributions of sediment-bound trace elements within the bottomsediments in Burrill Lake show strong positive correlations with the amountof finegrained muddy sediment and organic matter preserved in the stronglyreduced, deeper portions of the lagoon. Most of the trace elements reflectthe composition of their source lithologies rather than pollution. A clearsouthward decrease in abundance is apparent for trace elements (e.g. Ce, La)that have raised values in the Milton Monzomte compared to the Snapper Point Formation. The main anthropogenic influence is the increased rate of sedimentation in the bayhead delta and the increased quantities of monzonite-derived trace elements that reflect perferential land clearing on the monzonite for agriculture in the last 150 years. Anthropogenically introduced heavy metals (Zn, Pb and possibly Cu) are minor, widely dispersed and concentrated in the reduced mud-dominated sediment.