Pollution history of Neva Bay bottom sediments (eastern Gulf of Finland, Baltic Sea)
Abstract Neva Bay is the shallowest and easternmost part of the Gulf of Finland (Baltic Sea). St. Petersburg, Russia’s second largest city, occupies the coastal area where the Neva River debouches into Neva Bay. St. Petersburg has a protracted history of industrial, transportation and urban related activity that have affected Neva Bay. By the sealing off the bay from the eastern Gulf of Finland, the St. Petersburg Flood Protective Facility, which was constructed from the 1970‘s to 2011, transformed Neva Bay into a “technogenic” lagoon. Neva Bay sediments record a unique history of pollution near the metropolis. Heavy metal concentrations of most elements studied varied consistently throughout sediment cores. Temporal trends indicate that metals started to accumulate abruptly in the first half of the 20th century. Zinc, lead and copper were the first metals to reach contaminant thresholds implicating the regional base metal industry as a source. Significant increase in cadmium levels a decade or two later suggests pollution from the regional chemical industry. Comparison of geochemical data collected from sediment cores and recent annual sediment surveys highlighted the temporal history and potential sources of pollution in Neva Bay. Intensive dredging in 2007–2008 resuspended and redistributed contaminated sediment around Neva Bay causing a dramatic increase in benthic sediment heavy metal concentrations. Concentrations of all measured metals subsequently declined from 2009–2014 relative to the elevated values observed for 2007–2008. Pollution history of Neva Bay bottom sediments is closely linked with changing of sedimentation conditions. Analyses of sedimentological data collected by 20th and 21st century scientific surveys reveal dramatic shifts in Neva Bay sedimentation processes over the last three centuries. The western part of Neva Bay has transitioned from a sand-dominated system to one of mud accumulation with the aerial extent of mud deposition expanding significantly during the 20th century. This inventory coupled with an understanding of primary natural and anthropogenic processes can help inform decision makers to support the overall ecological health of the bay.