SOURCES OF GROUND WATER DISCHARGE AND NUTRIENT LOADING TO THE INDIAN RIVER LAGOON, FL

Nutrient loading to estuaries may be enhanced by ground water discharge depending on nutrient concentrations and flow rates. Both of these variables can be influenced by the sources of the discharging water. Flow rates of several meters per year have been measured using seep meters and radioisotopes in the Indian River Lagoon system, located along Florida's Atlantic coast. These rates are several orders of magnitude greater than those calculated using numerical ground water flow models. Two sources of water discharging to the Indian River Lagoon can be identified in pore water profiles, which were recently measured to a maximum depth of 230 cm below the lagoon floor (cmblf) at seven locations throughout the lagoon. Profiles of Cl concentrations reveal minima of up to 30% less than lagoon water concentrations at depths of around 70 cmblf. At depths greater than ~70 cmblf, the Cl concentration profiles constrain an advection-diffusion model that indicates an upward flow of ~5 cm/yr. This rate is similar to those calculated from numerical ground water models and may represent flow driven by hydrostatic head in regional aquifers such as the Floridan or Surficial aquifers. At sediment depths above ~70 cm, Cl concentrations adjust over a period of months to changes in concentrations of the overlying water column. These changes reflect rapid mixing between pore water and lagoon water which is superimposed on the slow upward flow from the aquifer. The mechanisms driving the shallow mixing are unknown, but could include wave, tidal, or biological pumping. Water pumped through the shallow sediments would be measured as high discharge rates using the seep meter or radioisotope techniques, but would not be included in numerical ground water models, thereby accounting for the discrepancy in flow rates between the techniques. Total nitrogen concentrations increase with depth in the sediment to ~70 cmblf, and then decrease or remain constant at depths below 70 cmblf. Elevated nutrient concentrations may result from pumping of oxygenated surface water into the shallow sediments, thereby enhancing organic matter remineralization. These data suggest that fluxes of nutrient from ground water discharge, including the shallow mixed water, are similar to fluxes from surface water runoff.