Geochemical Distribution of Heavy Metal Elements and Potential Ecological Risk Assessment of Matsushima Bay Sediments During 2012-2016.

Heavy metal pollution of marine sediments has attracted a great deal of attention because of its persistence, bioaccumulation, and toxicity. To evaluate the effects of mega-tsunami, anthropogenic activities, and redox conditions on heavy metal accumulation in coastal areas, sediments from Matsushima Bay, Miyagi Prefecture, Japan, were sampled to test variations in heavy metal spatial distribution on the bay floor during 4 years following the 2011 Tohoku Earthquake tsunami. Cluster analysis and principal component analysis were performed to assess the influencing factors and potential sources of heavy metal enrichment in the sediments of the bay. Additionally, the sediment enrichment levels of heavy metals were assessed on the basis of the enrichment factor (EF). The results of multivariate statistical analyses showed that the Ti, Fe, V, Pb, and Zn contents in Matsushima Bay sediments, which were transported mainly from Sendai Bay, depended on the mud content. The value of EF < 2 for Fe, V, Pb, and Zn indicated that these elements were not enriched. The value of EF > 7 for Cu suggested that the contamination levels in western Matsushima Bay were moderate to severe in every sampling year from 2012 to 2016 by anthropogenic activities. From the values of EF > 5 for U and Mo during 2012 and 2014, the severe enrichment of both elements in these periods may be explained by contamination with 2011 tsunami deposits; the improvement in 2015-2016 suggests that there was recovery of the tsunami-affected sediment composition to its original state. The values of EF > 3 for Mn and As indicated moderate to severe contamination with these heavy metals in the bay mouth area during 2015. This was likely explained by more oxic bottom conditions in the mouth of Matsushima Bay during that year.