Estimation of Submarine Groundwater Discharge into the Indian River Lagoon

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A calibrated and validated finite difference numerical model was used to estimate the spatial and temporal distribution of the Meteoric Groundwater Discharge (MGWD) and Submarine Groundwater Discharge (SGD) into a coasted estuary known as the Indian River Lagoon (IRL) at two transects. Specifically, this paper describes the methodology used to determine: a) the quantity of MGWD originating from the mainland and the barrier island, b) the spatial distribution of the SGD into the IRL, c) the groundwater salinity and hydraulic head distribution below the IRL, and d) the regional flow directions of the MGWD and the Oceanic Groundwater Discharge (OGWD) in a vertical plane below the transects. It was found that a brackish transition zone, in which groundwater salinity varies from freshwater salinity to lagoon water salinity, exists in the surficial aquifer, below the IRL, at both the Palm Bay and Titusville transects. The daily SGD flow into the IRL for each month ranged from 1.77 to 2.10 m³/d, and from 0.37 to 0.42 m³/d, per meter of lagoon shoreline, at the Palm Bay and Titusville transects, respectively. These numbers are close to the 0.45 m³/d per meter of lagoon shoreline MGWD, through a 22 m outflow face, estimated by at the Eau Gallie transect. The mainland produced 98% of the MGWD at the Palm Bay transect and 86% of the MGWD at the Titusville transect. The estimated annual MGWD were 9.0 % and 1.6 % of the annual rainfall at the Palm Bay and Titusville transects. These numbers are reasonable given the impervious character of the watersheds that discharge into the IRL at these transects. The MGWD can occur at distances of several kilometers from the groundwater divide and up to a kilometer away from the IRL shoreline, and can affect the brackish water salt concentration below the IRL. Also, it is possible that, below the lagoon, zones of meteoric ground water may occur below the brackish water at depths of 20 to 30 m as in the case of the Palm Bay transect. The MGWD appears to be the primary source of SGD into the IRL at the study transects, as no ocean water enters the lagoon, and there is virtually no tidal influence at any of the transects which implies that the reversed estuarine water discharge (REWD) is also negligible.