HEAVY-METAL AND ORGANIC-MATTER CONCENTRATIONS AND DISTRIBUTIONS IN DATED SEDIMENTS OF A SMALL ESTUARY ADJACENT TO A SMALL URBAN AREA

The concentrations and distributions of the heavy metals Cr, Mn, Fe, Ni, Cu, Zn and Pb and organic matter were investigated in dated sediment cores of a small micro-tidal estuary adjacent to a small urban area. Metal concentrations are greatest in sediments deposited since European settlement, when the sedimentation rates have been slowest. The concentrations are lowest in rapidly deposited sediments and in sediments formed in pre-European times. Significant heavy metal pollution has entered the estuary since 1850. The concentration distribution of the metals and organic matter, however, has been modified by the sedimentation rate which in tum was determined by changes in drainage and the use of the adjacent land area. For example, rapid sedimentation occurred when the contributing rivers were swept of excess sediment derived from stormwater drains and nearby horticulture areas. Despite maximum anthropogenic metal fluxes to the estuary the extra 'clean' sediment diluted the heavy metal concentrations in the sediments. The concentration profiles of the metals can be explained by the historical events occurring around the estuary and rivers, and the introduction of storm water drains and a sewage plant.