Modelling the Impact of Rain-Driven Pollutant Plumes on Lake Michigan Beaches

Abstract:

Water quality impairments at urban beaches can be difficult to remediate due to a wide array of possible pollutant sources in urban watersheds. In particular, the Milwaukee Estuary on Lake Michigan receives significant pollutant inputs during large rain events, but the extent to which these pollutants impact beaches is unknown. In this study, we aimed to understand the transport and fate of these contaminants in the Milwaukee nearshore by detecting the existence and duration of contaminant plumes in Lake Michigan. High temporal resolution water samples were collected during rain events and analyzed for total suspended solids (TSS), fecal indicator bacteria (FIB), and a human Bacteroides microbial source tracking marker. These data were incorporated into a previously developed and validated 3D hydrodynamic model to better understand the distribution of contaminants in the nearshore and on local beaches. 13 weather events were sampled in 2018 and 2019 with 11 wet weather events, including 3 combined sewage overflows. Contaminant plumes were considered detectable if predicted fecal coliform concentrations exceeded 100 CFU per 100 mL in the inner harbor. Six events resulted in detectable contaminant plumes, with four of these plumes reaching beaches. The timing of contaminants varied by event, with FIB plumes sometimes preceding or following a TSS plume. When contaminant plumes impacted beaches, human fecal contamination preceded elevated FIB. This study demonstrates that a visible plume may not always correlate with health risk due to the variations in timing of contaminant plumes. While this model accounts for input from the estuary, other sources of runoff may also contribute health-relevant contaminants. Future work will examine other sources by sampling at beaches and stormwater outfalls, increasing our understanding of this EPA designated Area of Concern during high risk weather events.