Using Multiple Geochemical Indicators to Estimate the Environmental Recovery of Lakes from Lead Contamination

Inland lake sediment chronologies provide records of environmental change, and have been used to test the effectiveness of environmental legislation. However, in most lakes, sediment lead (Pb) concentrations have not reached background values. Information about the continuing source for lead to the environment may be revealed by supplementing traditional sediment concentration data with Pb isotopic ratios and loading rates. As sediment Pb concentrations approach background, one can hypothesize that the isotopic ratios and loading rates should also approach background values. The environmental recovery rate of isotopes, loading rates and sediment concentrations should be identical and predict the same time to recovery. To test this hypothesis, sediment cores were collected from six inland lakes in Michigan, dated with 210Pb, and analyzed for stable Pb and Pb isotopic ratios using ICP-MS. Results show that changes in stable Pb concentrations and isotopic ratios indicate environmental recovery in each lake. However, loading rates for two lakes were increasing and time to recovery using this indicator could not be calculated. In all cases isotopic ratios indicate a much longer time to recovery than that of sediment concentration data and loading rates. Relationships of time to recovery to simple landscape variables (e.g. watershed area or watershed/ lake area ratio) remain unclear. Thus, the hypothesis is not supported, but these three indicators do reveal different aspects of environmental recovery. This study demonstrates that isotopic ratios and loading rate data, in conjunction with traditional concentration data, may be more sensitive indicators of recovery than any single indicator by itself. Even though concentration data shows “complete” recovery in some lakes, isotopes do not, revealing gaps in our knowledge of watershed processes. This study reinforces the complexity of ecosystem functions and shows that recovery assessments will likely need to rely on a suite of indicators.