THE INFLUENCE OF MANGROVE-DERIVED BRINES ON WATER CHEMISTRY AND SUBMARINE GROUNDWATER DISCHARGE IN BISHOP HARBOR, FLORIDA

Terrestrial groundwater discharged to coastal zones, known as submarine groundwater discharge (SGD), is typically fresher than marine water. Salinity of shallow pore waters can increase through evapotranspiration by certain species of mangroves, which exclude of 90 to 99% of salts, creating residual brines in the soils. We are studying the influence of these brines on discharge locations of SGD, water chemistry, and diagenetic reactions in Bishop Harbor, near the mouth of Tampa Bay, Florida. At the site, a significant portion of the coastal area is covered by mangrove forests. The study involves measurements of salinity, major element chemistry and ²²²Rn activity in the surface and pore waters. The salinity of pore water at four sites (labeled Sites B, C, D, and F) in and around the mangroves increases to three times average marine value at depths up to 160 cm below the sediment-water interface. The increase in salinity is greatest within the mangrove forests, while bayward of the forest, the salinity remains near the value of the surface water. Radon is a good tracer of SGD because evasion causes low activity in surface water, decay of ²²⁶Ra in detrital sediments elevates its activity in ground water. Continuous measurements of ²²²Rn activity were collected over six hours of flood tide at Site F, located the closest to the mainland. Radon activity increased with salinity, suggesting the source of ²²²Rn (i.e. SGD) is bayward of mangrove forests. At Site F, the major element/Cl^- ratios are similar to seawater values. At Sites B and D, which are bayward from the other sites, the major element/Cl^- ratios are less than those of average seawater, reflecting mixing with a source of water depleted of major elements relative to seawater. The spatial distribution of salinity and major element/Cl^- ratios and the elevated ²²²Rn activities in the saline water suggest that the mangrove-derived brines divert SGD to locations offshore from the mangrove forests. The diversion may be controlled by an ~5% increase in the density of the brines over normal seawater. This influence on the location of SGD may be important for assessing magnitudes and locations of discharge, associated mass fluxes, and for the ecology of mangrove forests.