Low concentrations of heavy metals such as lead are normal, natural, and necessary in healthy marine and esturine environments. The problem arises when additional amounts of lead are discharged into the surface waters of these environments from industrial and sewage treatment plants.
The map above illustrates lead concentrations in surface sediments taken around Long Island Sound. Each circle on the map represents a single sample and the colors are interpolated across samples. In the map, high lead concentrations (46.7-218 micrograms of lead per gram of sediment) are shown in red, moderate concentrations (23-46.7) are shown in yellow and low concentrations (0-23) are shown in green. The numbers in parentheses represent the number of samples falling into each of the concentration categories. ER-L and ER-M stand for Effects Range-Low and Effects Range-Medium, which are toxicity criteria used to define ranges of sediment lead concentrations that could adversely affect marine life. Lead concentrations above the natural range have the potential of causing toxic effects on marine organisms, and may subsequently affect humans through contamination of seafood.
The concentrations of lead and other metals in bottom sediments are generally higher in the western Sound and lower in the bottom-scoured regions of the eastern Sound. This is primarily due to both the locations of pollutant sources and the westward transport of contaminants associated with fine-grained particles.