During the 19th and early and mid-20th centuries, industries discharged large amounts of heavy metals into the Sound and its tributaries as byproducts of manufacturing processes. While large-scale industrial release of heavy metals has been reduced, other modern sources still exist. Atmospheric deposition into the Sound directly and into the larger watershed can be a source of heavy metals, as well as particles discharged from wastewater treatment plants.
Data from sediment cores is an indicator of trends in heavy metal contamination of Long Island Sound over the last thousand years.
The chart above uses data collected from a core sample taken off Norwalk Harbor. It represents a historical record of heavy metal concentrations in a formerly industrial area of Long Island Sound. Concentrations of mercury, copper and zinc began to increase during the Industrial Revolution. Although concentrations have begun to decrease in the most recent decades, overall concentrations are still far above the pre-industrial baseline.