Modeling bacterial pollution in estuaries after high-intensity rainfall: A watershed-estuary approach in Frenchman Bay, Maine

Forecasted increases in intense rainfall events in northern New England will impact the export quantity of polluted runoff, affecting shellfish harvesting activities in receiving estuaries. Evaluations of how estuaries with varied hydrodynamic characteristics respond to relatively intense rainfall can thereby provide a base of knowledge important for coastal management decision-making with implications to aquaculture and shellfish harvesting. Here we summarize results from runoff simulations implemented to quantify hydrologic responses to relatively high-intensity rainfall events in 10 different watersheds entering four estuaries connected to Frenchman Bay, Maine (USA). Stream outflow hydrographs predicted from 24-h precipitation time series are used to force the streamflow in three-dimensional, idealized hydrodynamic simulations implemented to examine the spatial extent of the polluted freshwater in the four estuaries in the days after the rainfall event. By utilizing a "worst-case scenario" load concentration of fecal coliform bacteria, it is shown that estuary geometry exhibits major control on water quality recovery after periods of heavy rainfall. The simulations predict that the estuaries governed by density-driven dynamics take over one week to recover from runoff generated from 2″ (∼51 mm) of precipitation in 24 h, a quantity that triggers a management response along Maine's coast. Some estuary sub-areas have relatively lower susceptibility to contamination following runoff events due to variations in estuary channel geometry. The outcomes demonstrate a pathway to advance the basis for shellfish management decision-making using place-based hydrologic and hydrodynamic simulations to highlight the implications of local coastal geomorphological conditions for water quality recovery following rainfall runoff events.