Natural and anthropogenic factors influencing spatial and temporal patterns of metal accumulation in inland lakes

The natural cycles of trace metals in the environment have been significantly influenced by humans, resulting in toxicological concerns for both humans and wildlife. Although environmental legislation reduced loading of some metals, humans continue to alter the environment in ever increasing ways. In order to reduce the input of metals to the environment, it is necessary to understand both current and historical sources and contaminant pathways. To help determine this, sediments were collected from 17 lakes in Michigan, USA, sediment profiles were examined and interpreted, and anthropogenic accumulation rates were correlated to characteristics of the watershed during two time periods (1970s and 1990s) to understand both natural and anthropogenic factors influencing contaminant accumulation in the sediment. Lakes record the input of contaminants from both the local watershed and from atmospheric deposition over time, and are therefore ideal for identifying sources and evaluating temporal changes.; Elements within each lake were grouped using cluster analysis based on the similarity of depth profiles, with each group representing a unique source and/or process, including terrestrial, diagenetic, anthropogenic and carbonate groups. Representative elements were chosen for each group, and these elements were used to help quantify anthropogenic accumulation rates of cadmium, copper, lead and zinc.; Cadmium, zinc and lead anthropogenic accumulation rates decreased from the 1970s to the 1990s in most lakes, probably in response to environmental legislation. Copper accumulation rates remained the same or increased over that time period for many of the study lakes, indicating that historical sources have not been reduced by environmental legislation, or that new sources have emerged.; Multivariate regression models using a maximum of two watershed characteristics to predict anthropogenic accumulation rates for each metal were determined for both time periods to provide insight into the dominant sources and pathways of these metals in the environment. Cadmium and lead accumulation rates were best predicted by characteristics that seem to represent atmospheric transport and deposition in the 1970s, including sulfate deposition rates and either average slope of the watershed or slope within a 100 m buffer of the lake. The reason for a negative correlation between slope and lead and cadmium accumulation rates is unclear, but the average slope of the watershed has a regional gradient similar to that of the overall population distribution in the state and may represent local atmospheric deposition. Copper and zinc are best predicted by population density in the 1970s. Percentage urban land cover in the watershed is the best predictor of all four metals in the 1990s, indicating that local sources are currently more important that regional atmospheric sources. Future work can now focus on identifying the specific sources of these metals that are represented by urban land cover.