ESTIMATING ENVIRONMENTAL RECOVERY USING STABLE LEAD CONCENTRATIONS AND LEAD ISOTOPIC RATIOS

Sediment chemical chronologies whether from oceans or lakes provide historical records of environmental change and the effectiveness of environmental legislation. One example is that of lead (Pb), whose sediment concentrations peaked in the mid-1970s and have decreased as a result of implementation of the clean air act. However, sediment concentrations have not reached background levels. Questions about the continuing source for Pb in the environment has resulted in the study of Pb isotopic ratios in sediments, since anthropogenic actives have significantly altered the isotopic signal of lead accumulating in the environment. The Pb isotopic ratios also show that most landscapes are not at pre-disturbance values. As sediment Pb concentrations approach background one can hypothesize that the isotopic ratios should also approach background values and at the same rate of change. Thus information about environmental recovery from these two indicators would be the same. To test this hypothesis, sediment cores were collected from the deepest points of six inland lakes of Michigan. Stable lead (and 26 other elements) and Pb isotopic ratios were analyzed using inductively coupled plasma mass spectrometry with hexapole collision cell technology. Results show that changes in both the stable Pb concentrations and isotopic ratios indicate environmental recovery. However, the rates of change are not necessarily equal. In fact in one watershed the isotopic signal remains nearly 100% anthropogenic while recent Pb concentrations in the sediments have been decreasing. Thus the hypothesis is not supported and these two indicators do reveal different aspects of environmental recovery. Used together, these indicators give insight into the mass of anthropogenic Pb on the landscape, sources (e.g., coal burning) for current Pb loadings, and rates of recovery. However, isotopic ratios might be a more sensitive indicator for true environmental recovery and useful for the interpretation of ecosystem recovery in general.