Heavy metals and stormwater : an investigation of Hobart, Tasmania, Australia

Stormwater quality in the City of Hobart has been examined by past local studies. However, heavy metals were not critically examined in those studies. This research was undertaken to determine the levels of heavy metals discharged by three major rivulets namely, Sandy Bay Rivulet, Hobart Rivulet and New Town Rivulet into the Derwent Estuary. Nine stormwater samples were collected through grab sampling during the first-flush period at three rivulet sites, including street runoff and stormwater in the suburbs. The sampling period took place from October 2001 till February 2002. Levels of pH and total suspended solids (TSS) concentrations were measured. Levels of heavy metals such as Al, Cd, Cr, Co, Cu, Fe, Mn, Ni, Pn and Zn were determined using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP) in the laboratory. Total metals concentrations, sediment and dissolved heavy metals were calculated and compared against international and national standards. In addition, studies from past stormwater studies, including local studies were used as a benchmark in this study. Results show metals like Cu, Ni and Zn were found mainly in dissolved form while Pb, Al and Fe were mainly in particulate matter. High levels of dissolved Cu and Zn are found in Hobart and New Town Rivulets. Distribution and levels of heavy metals are found to be highly dependent on landuse across the three rivulets' catchments. High levels of Al and Fe were found to their abundance in the Earth's crust. High levels of Mn in Sandy Bay Rivulet were due to geologic makeup of Sandy Bay region. Other metals, excluding Cu, Pb and Zn are found to be within guidelines. Non-point sources such as residential areas and point sources have been identified as potential sources of metals. Zn concentrations are significantly higher than Pb and Cu. Heavy metals in street runoff sample reveal higher levels of Cu, Ni and Zn than Atlanta streets. Comparison with local studies reveal an increase in TSS in Sandy Bay Rivulet, increass in Zn levels in Hobart Rivulet and an overall higher level of heavy metals in stormwater sediment as compared to sediments in the Derwent Estuary. It is recommended reduction measures like efficient sediment traps should be installed at three rivulets, as well as source reduction programs to be adopted by the state government or local councils to curb the level of sedimentation, as well as the amount of heavy metals contaminated sediment from entering the Derwent Estuary.