Nutrient Loading in the Indian River Lagoon from Groundwater at the River Walk Transect

The Indian River Lagoon (IRL) is located on the east coast of Florida and is protected from the Atlantic Ocean by coastal islands known as the “barrier-island chain”. The IRL is an estuary that is approximately 250 km long and stretches from Volusia County in the north to Palm Beach County in the south. Its width varies of 0.8 km to 8 km. The IRL watershed has been modified by urban and agricultural development. The main concern is that the IRL system is becoming more eutrophic, particularly in the northern reaches where major algae blooms of long duration (months) are becoming more frequent. A study was conducted to determine the groundwater seepage and the associated nutrient (dissolved nitrogen and phosphorus) loads transported into the lagoon via groundwater at River Walk, Florida, from June 2014 to September 2015. The width of the lagoon at River Walk is approximately 1.56 km. The nutrient loads were determined using the finite difference model SEAWAT which was calibrated by comparing model predicted hydraulic heads with measured hydraulic heads. During the fifteen-month period from June 2014 to September 2015, the groundwater seepages, per unit length of shoreline, ranged between 0.67 m³/day and 0.85 m³/day. The total nitrogen loads, per linear meter of shoreline, ranged from 562 mg/day to 1866 mg/day or 205.1 kg/year per km of shoreline to 681 kg/year per km of shoreline whereas, the total phosphorus loads ranged from 177 mg/day to 248.3 mg/day per meter of shoreline or 64.6 to 90.6 kg/year per km of shoreline. The assessed TN and TP loads due to groundwater showed a high level of temporal and spatial variation, and can be a significant contributor to the eutrophication within the IRL.