Lead isotopic chronologies from inland lakes: Watershed vs. regional scale sources of lead in the Great Lakes region

Environmental regulations have greatly reduced anthropogenic Pb input to the environment from the mid 1970's to the present. However, Pb concentration profiles from inland lake sediment cores in Michigan show that the rates of decrease to natural background concentrations of Pb differ among lakes. This disparity may be due to the dominance of watershed scale sources of Pb over the past three decades following a reduction in the major regional sources of Ph in the 1970's. In the 1970's and 1930's regional sources may have provided the dominant contribution of Pb to inland lakes. For this study, sediment Pb isotopic chronologies from six inland lakes across Michigan were compared to determine temporal and spatial trends. These chronologies were also examined to determine the possible sources of current and historical Pb input to inland lakes. Results show that watershed scale sources of Pb such as deforestation were dominant before 1900. From 1900 to the mid 1970's the regional scale sources of coal combustion, ore smelting, and leaded gasoline combustion were dominant. However, watershed scale sources of Pb also impacted inland lakes during this time period. From the 1970's to 2000 the major sources of Pb were watershed scale sources such as industrial emissions or municipal waste water discharges. This study may have future implications for the control and environmental monitoring of Pb and other metal pollutants.