Concentrations of polycyclic aromatic hydrocarbons (PAHs) were measured in sediment cores collected from 11 inland lakes in Michigan for examination of spatial and temporal trends in accumulation and sources. Concentrations of PAHs varied widely among these lakes; the highest concentrations were found in sediment collected from Cass Lake, located in an urbanized watershed. Concentrations of PAHs were correlated with watershed population density on a spatial scale. PAH concentrations have decreased over time since the 1980s in nine of the 11 lakes studied, most of which are located in suburban and rural areas. The rate of decline in PAH concentration was faster in Gratiot Lake, a remote lake with no known local sources of pollution in recent years. Concentrations of PAHs in surface sediments from Cass and Cadillac Lakes were greater than the threshold for adverse effects. Despite the wide variations in total PAH concentrations, fluoranthene, pyrene, chrysene, benzo[b]fluoranthene, and benzo[k]fluoranthene were the major PAHs in all of the lakes studied. The proportions of individual PAHs within the total PAH concentrations have changed both temporally and spatially, suggesting a shift in the sources of PAHs in the environment. A general increase in the proportion of high-molecular-weight PAHs suggests an increase in combustion-related sources recently.

Several studies of chronologies of mercury (Hg) in inland lake sediments have demonstrated that Hg accumulation decreased in recent decades. However, episodic mercury accumulation events were recorded in some of these lakes, but not investigated in detail. Recent decreases had been attributed to the reduction of regional Hg consumption and secondary removal during process waste treatment. In addition to regional sources, local sources, including watershed disturbance, might significantly contribute to Hg loading. Here, mercury chronologies of Hg loadings based on dated sediment cores are presented for 26 inland Michigan lakes. Although spatial trends of anthropogenic inventories suggest a regional pattern dominated by human activities, sub-regional to local scale sources are also found to be significant. Temporal trends show episodic Hg accumulation events superimposed on a more general, long-term trend. Episodic increases common to lakes suggest a common source or processes common to lakes. Episodic increases unique to a lake indicate a more local scale source. Similar Hg profiles from lakes that are geographically proximal provide evidence for sub-regional to regional scale sources. Local sources and pathways for mercury to inland lakes need to be more fully understood to effectively reduce Hg loading to the environment.