UNDERSTANDING THE RESPONSE OF WATERSHEDS TO URBANIZATION USING SEDIMENT CHEMICAL CHRONOLOGIES

Urbanization has been shown to disrupt the natural biological, chemical, and physical processes influencing the flow of mass within a watershed. Sediment chemical chronologies can help to understand some aspects of these interactions including responses and changes in exposure as a result of this disruption, effectiveness of environmental policies, relationships to production and consumption of chemicals and recovery of the system from disturbance. The hypothesis for this research is that the flow of mass will not return to a pre-disturbance state because of both permanent anthropogenically-sourced changes to the landscape chemical interactions and changes in regional climactic regimes. The focus here will be on the former. The chronologies are explored from a lake impacted by clear cutting and urbanization (Thompson Lake, Michigan, USA) using chemical proxies for terrestrial inputs (e.g., Al, Mg); for contaminants (PAH’s, PCB’s, DDT, heavy metals) and for productivity (N, P, Ca, LOI). Results show that concentrations of terrestrial proxies increase after clear cutting, decrease slightly and then fluctuate again to elevated concentrations. Trajectories in productivity proxies show that Ca patterns are inversely related to those of N and P. P and N concentrations decrease after clear cutting and rapidly increase near the present. All suspected contaminants show elevated concentrations with watershed urbanization. Some have peak concentrations and onset times related to their production/consumption (e.g. Pb, DDT), some (e.g., PCBs) do not, while others have peaks related to terrestrial runoff and not the use. These results clearly show the impacts of clear cutting, recovery from clear cutting and then impacts of urbanization on mass flows. None of the proxies measured have returned to pre-disturbance states and recent trends give no indication that they will. These results also provide insight into the complexities in understanding urban geochemistry and the need for the integration of physical, biological, and chemical processes along with human influences and historical legacies.