The study investigates the impact from point source loadings into the Lake Maurepas near New Orleans, LA, which feeds into the Lake Pontchartrain, a large estuarine lake along the city of New Orleans connecting with the Gulf of Mexico. The Gulf of Mexico along the coast of Louisiana had been experiencing numerous dead zone water quality episodes triggered by heavy nutrient loadings, contributed by both point source (PS) and nonpoint source (NPS).
Lake Maurepas receives water and nutrients from three rivers (Blind, Amite, and Tickfaw) with occasional tidal interaction from Lake Pontchartrain through a narrow canal, the Manchac Pass. Nutrients from industrial, residential, and agricultural sources in the three river basins have lead to an increase in nutrient loadings into Lake Maurepas. This study used historical flow and nutrient loading records from the US Geological Survey (USGS) and the Louisiana Department of Environmental Quality (LDEQ) and developed a finite segment water quality model (FSM) with four equal volume segments. FSM was based on the 4 by 6 steady-state response matrix (SSRM) transport scheme to characterize PS nitrogen (TKN) loading, transport, and flux in the lake.
Dispersive flux among finite segments is the primary mechanism of substance transport and fate in an impounded such as the Lake Maurepas. Salinity gradient method is used to determine dispersion coefficients and estimate resultant nitrogen flux. Salinity from each segment boundaries was estimated, which were used to estimate the bulk dispersion coefficients, E’, at the interfacing boundaries of segments. Subsequently, with TKN loadings from each point source with USGS and LDEQ data sources, annual TKN concentrations in each segment of the lake were estimated.
Nitrogen loading to Lake Maurepas at Segment 1 is dominated by the Blind River PS, fed by an adjacent swamp as well as man-made canals with elevated houses. The magnitude of dispersion coefficient for Segment 1 is over 10 times higher than the dispersion coefficients in the other 3 segments due to its large mass inflow, pronounced bathymetric slope and direction of internal substance flux. On the other hand, the magnitude of TKN loading into segment 3 from the Tickfaw River, which has a larger number of contributing residential and agricultural PS sources than segment 1 by the Blind River, was found lower in TKN loading magnitude due to mass flow and pre-dilution compared to segment 1.