Application of Lagrangian-Based Transport Model to Estimate Pollutant Source Locations at Southport Harbor, Connecticut

Abstract

Recreational shellfish beds in the Southport Beach area of Southport Harbor, Connecticut are impacted by elevated levels of fecal coliform bacteria. However, the locations and relative magnitudes of the sources of these bacteria have not yet been identified and quantified. In this study, a computer modeling approach to characterize the sources of the bacteria responsible for closure of recreational shellfish beds in Southport Harbor is demonstrated. Field studies provided data to support the modeling and included physical and chemical measurements, bacterial sampling, and dye studies for two suspected sources was conducted to better understand flow in the area and to support the modeling effort. Circulation in Southport Harbor was found to be primarily driven by the tides and secondarily by winds. The tidal response in the harbor is of a mixed standing and progressive wave. The bacteria surveys indicated highest fecal coliform (FC) concentrations in the Mill River and at the mouth of Sasco Brook, locations on either side of the beach area. An integrated hydrodynamic and pollutant transport modeling framework (WQMAP / OILMAP) was used to identify the location and evaluate the bacteria sources responsible for closure of the recreational shellfish beds. The hydrodynamic model was applied to the greater Southport Harbor estuary. The model, which was calibrated to data from the field program, provided temporally and spatially varying currents for use in the subsequent pollutant transport calculations. A Lagrangian (particle-based) approach was used to model pollutant transport both forward and backward in time. In the forward mode the Lagrangian model predicts the evolution of pollutant plumes from specified source locations, while in the backward mode it estimates the likely source locations that would affect a specified end point or resource area, in this case the Southport Beach shellfish bed. Forward transport modeling indicated that pollutants from a distant source (Mill River) were more likely to impact the Southport Beach area than pollutants from an adjacent source (Sasco Brook). Backward mode results from the Lagrangian transport model confirmed that the Mill River was the likely source based on six representative receptor sites located in the beach area. These results indicated that offshore sources might also be significant, but these can be discounted since there are no known sources there. Modeling results thus suggest that control of the more distant Mill River bacteria sources is more likely to improve water quality at Southport Beach than control of the nearby Sasco Brook sources.