Long-term Heavy Metal Loading to Near-Shore Lake Sediments

Long-term sediment pollution by lead (Pb), copper (Cu) and zinc (Zn) is investigated in Karlskärsviken, a typical bay of Lake Mälaren, about 10 km northwest of Stockholm. We find that metal accumulation rates in the sediments of Karlskärsviken increase considerably more in the outer than in the inner bay section, all the time since medieval times and even more so after than before industrialisation. Resulting metal accumulation rates are dominated by increasing sediment accumulation rate and its spatial–temporal patterns. This indicates significant waterborne metal pollution contributions to the bay sediments, which continue until present times and are still increasing for Zn and Cu. The outer Karlskärsviken bay section is then mainly affected by regional waterborne discharges, while the inner bay section is mainly affected by local discharges, in addition to the atmospheric deposition that affects the whole bay and Lake Mälaren similarly. The present results indicate possible important shortcomings in environmental regulation, management and monitoring systems that consider only sediment concentration levels and neglect pollutant accumulation rates and diffuse metal load contributions from slow subsurface transport into surface water environments.