Geogenic and anthropogenic metal enrichment in aquatic sedimentary environments – What are they and can they be differentiated

Erosion of metal-rich country rock in a catchment may strongly influence the distribution of metals in sediments of a receiving basin. These geogenic deposits often obscure anthropogenic enrichment trends in aquatic environments confounding interpretation and resulting in erroneous decisions for management. High geogenic Cu concentrations up to 89 μg/g in total sediment in Burrill Lake estuary in New South Wales (Australia) hindered attempts to establish anthropogenic Cu enrichment trends using traditional local, regional and global background values and prevented an accurate assessment of impact. A new approach using a spatially variable Cu background (94 μg/g to 28 μg/g) reduced geogenic influence and provided the magnitude and distribution of anthropogenic enrichment (up to 2.7 times). The method also identified subtle impact by Cu near urban development on the Lake. Similar enrichment distributions by other metals (Pb and Zn) not associated with local geology, provided confidence in the new methodology to differentiate geogenic enrichment from anthropogenic enrichment. Although not employed previously, this methodology will be more frequently used considering the number of waterbodies influenced by geogenic enrichment. The method using sedimentary cores is a standard technique employed routinely in aquatic contaminant surveys and does not require any complex methodologies, however the data need to be normalized to reduce the confounding effects of variable sediment size.