Examination of the watershed-wide distribution of Escherichia coli along southern Lake Michigan: An integrated approach

Abstract

Recent research has highlighted the occurrence of Escherichia coli in natural habitats not directly influenced by sewage inputs. Most studies on E. coli in recreational water typically focus on discernible sources (e.g., effluent discharge and runoff) and fall short of integrating riparian, nearshore, onshore, and outfall sources. An integrated "beachshed" approach that links E. coli inputs and interactions would be helpful to understand the difference between background loading and sewage pollution; to develop more accurate predictive models; and to understand the differences between potential, net, and apparent cnlturable E. coli. The objective of this study was to examine the interrelatedness of E. coli occurrence from various coastal watershed components along southern Lake Michigan. The study shows that once established in forest soil, E. coli can persist throughout the year, potentially acting as a continuous non-point source of E. coli to nearby streams. Year-round background stream loading of E. coli can influence beach water quality. E. coli is present in highly variable counts in beach sand to depths just below the water table and to distances at least 5 m inland from the shore, providing a large potential area of input to beach water. In summary, E. coli in the fluvial-lacustrine system may be stored in forest soils, sediments surrounding springs, bank seeps, stream margins and pools, foreshore sand, and surface groundwater. While rainfall events may increase E. coli counts in the foreshore sand and lake water, concentrations quickly decline to prerain concentrations. Onshore winds cause an increase in E. coli in shallow nearshore water, likely resulting from resuspension of E. coli-laden beach sand. When examining indicator bacteria source, flux, and context, the entire "beachshed" as a dynamic interacting system should be considered.