Normalisation as a tool for environmental impact studies The Gulf of Gdansk as a case study

Geochemical maps of the Gulf of Gdansk, based on data obtained using partial extraction techniques on the <200 ?m fraction of sea bed sediments, show patterns of metal distribution that are closely related to sediment grain size, with low concentrations in the sandy coastal zone and high concentrations in the muddy central part of the Gulf. This reflects the commonly observed relationship between clay mineral content and trace element concentration. Normalisation to Al, as a proxy for clay mineral content, lead to the recognition of five geochemical anomalies with suspected anthropogenic origin. Analysis of new samples from the anomalous sites, using total concentration techniques on the <2 mm (whole sediment) fraction, together with data on contaminant sources, hydrodynamics, salinity profiles and water stratification (obtained from local authorities) verified and helped explain four of the anomalies: Cd, Cr and P near the mouth of the Vistula River; Cd, P, Ni, Pb and Zn east of the Hel Peninsula; P, Pb and Zn south-east of the Hel Peninsula; and Cd, Cr, P, Pb and Zn in Puck bay. Thus geochemical data not fully 'fit-for-purpose' can be used in environmental studies to establish potential anomalies that can be verified by follow-up sampling. This reduces research and monitoring costs by minimising the number of samples that need to be processed by the more expensive 'total' concentration methods. Using data on deposition rates and sedimentological characteristics at the anomalous sites, local background concentrations and enrichment factors (EF) were estimated for the sea bed sediments of three different sedimentation areas in the Gulf. After correction for variations in grain size, each sedimentation area was found to have different EFs.