EVALUATING CHANGES IN SOURCES AND PATHWAYS OF TRACE METALS OVER TIME USING LAKE SEDIMENTS

Environmental regulation has reduced the loadings of metals to the environment since the advent of the Clean Air and Water Acts in the 1970’s. Reducing emissions may have changed transport pathways since this legislation targeted major sources of metals which influenced the environment on a regional scale. Watershed-scale processes may now provide the significant contribution to metals loadings. Thus, our working hypothesis is that the reduction of major emission sources has allowed local influences and watershed features to affect metal transport pathways. If true then there should be differences in watershed attributes/influences (e.g., %urban, population density) changed the pathways of metals transport of these metals. In this study we examine Pb, Cu, Zn, and Ni because these metals 1) build from previous studies, 2) exhibit similar environmental behavior but separate pathways, 3) represent various degrees of contaminant enrichment. To test this hypothesis, sediment accumulation rates of these metals at decadal intervals were compared to selected watershed attributes. Sediment cores were collected from the deepest portion of 35 inland lakes representing diverse land uses. Cores were sectioned, microwave digested, and analyzed using mass spectrometry. Sedimentation rates, ages and sediment focusing were determined via ²¹⁰Pb/¹³⁷Cs/stable Pb profile analysis. Similar to the previous work, the data show regional sources of the metals studied to subside upon introduction of environmental legislation in the 1970’s. However, watershed-scale sources provide a more significant portion contaminant loading in nearly all lakes studied, preventing lakes from reacquisition of reference condition values. When compared to landscape attributes, these patterns provide some insights into the causes for the continued contaminant loading. Population density and percent urban provide excellent predictors for recent metals loadings. Lead also showed excellent correlation with slope percent. Unexpectedly all metals showed a change in watershed attributes influencing accumulation rates between the two decades, even though some have not had a significant atmospheric transport pathway or contaminant loading. The reasons for this are subject to further study.