Meteoric Groundwater Discharge (MGWD), Nutrient and Salt Transport Between a Coastal Aquifer and the Indian River Lagoon

The meteoric groundwater discharge (MGWD) into estuaries can be important for at least two reasons: a) it can be a primary source of bringing land-based nutrients to the estuaries, and b) it is a continuous fresh water source, and perhaps an important source, which may significantly affect the salinity of the estuarine waters. In estuaries, especially tidal estuaries, where many groundwater sources and components are present, estimating the MGWD, a component of the submarine groundwater discharge (SGD), and nutrients loading due to the MGWD, are far more challenging. The main objective is to determine the quantity and spatial distribution of the MGWD, to obtain nutrients loadings due to the MGWD into the estuary and determine saltwater intrusion from the estuary into the coastal aquifer.;Piezometric head, groundwater and the lagoon salinities were measured at several locations of a tidal saltwater estuary known as the Indian River Lagoon (IRL) to develop equi-potential lines. Water table and surface water elevations were measured in the surrounding watershed to establish boundary conditions for the model. Sediments samples, collected from below the lagoon bed, were analyzed for estimation of horizontal hydraulic conductivity. The finite difference (MODFLOW) numerical model was calibrated using the spatial distribution of the vertical hydraulic conductivity (Kv) below the IRL, estimated by comparing model-predicted and measured freshwater equi-potential lines below the surface water body, and not just by visual comparisons of nodal parametric values. Model validation confirmed that calibration by visual comparison was very accurate. Sensitivity analyses of model parameters showed that Kv was the most important factor governing spatial distribution of the MGWD into the IRL. The calibration and validation process showed that use of a single anisotropy ratio value to represent the entire domain could lead to significant errors. The estimated annual MGWD values were approximately 1.6 and 9% of the annual rainfall. The MGWD was not a near-shore phenomenon and occurred at distances of 5000 ft (1.5 km) from the shoreline. Three dimensional steady state model also supported these results.;The Total Nitrogen (TN) loads ranged from 308 to 591 mg/day, 620 to 703 mg/day, and 8 to 28 mg/day per foot of lagoon shoreline at the Palm Bay, Titusville and Vero Beach transects, respectively. The higher Total Phosphorus (TP) loads at the three transects were 591 mg/day, 50 mg/day, and 19 mg/day per foot of the lagoon shore line. The TN and TP loads were found to be highly dependent on the landuse within the watershed and also showed significant variation from week to week primarily due to changes in concentrations. This study presents also the groundwater salinity distributions measured below the IRL at the transect, and demonstrates how the coupled flow and salt transport (SEAWAT) model was calibrated for the one of transects using the measured salinity distributions.