Unusually high sedimentary metal concentrations in intermittently closed and open lakes and lagoons (ICOLLs) in the northern lagoons of Sydney (Australia)

Highlights: • Highest sedimentary metal concentrations in Australian central east coast estuaries • Extensive low-lying areas of catchments reclaimed by infilling of wetlands. • Sedimentary metals mix in urban- and groundwater-influenced lagoons statistically distinct. • Elevated metal levels possibly related to groundwater influx. • Metal enrichment is high and ecological risk moderate and metal levels maybe declining. The environmental status of three Intermittently Closed and Open Lakes and Lagoons (ICOLLs) (Manly, Curl Curl and Dee Why Lagoons) in northern Sydney (Australia) were assessed using sedimentary metals. Results identified the highest sedimentary metal enrichment in the Australian central east coast estuaries, with only a moderate ecological risk. A comparison with surficial sediment concentrations from 1996 to 2008 indicated declining metal concentrations, which is largely supported by subsurface metals data. Extensive low-lying areas of the catchments were historically reclaimed by infilling wetlands, which may have resulted in contaminated groundwater influx to the lagoons. An analysis of the sedimentary metals mix in lagoons surrounded by extensive infilling and estuaries with minor reclamation were statistically distinct. ICOLLs are environmentally sensitive due to large catchment/estuary ratios and restricted access to the ocean. A full set of baseline data is provided to support future monitoring of anthropogenic impacts on the system.