TERRESTRIAL BRACKISH GROUNDWATER DISCHARGE OF PHOSPHORUS TO COASTLINES

Despite the recent recognition of submarine groundwater discharge (SGD) as a potentially significant source of both water and chemicals to coastal areas, there has been much controversy as to the actual magnitude of SGD on both local and global scales. This controversy is fueled by differences in the magnitude of this flux in different locations, in the lack of concordance of modeling vs field investigations, and the difficulties in quantifying the freshwater and seawater components of SGD. Here we define a new phenomenon we call Terrestrial Brackish Groundwater Discharge (TBGD), which is related to SGD, and demonstrate that the flux of TBGD is dominated by seawater driven into the coastal aquifer by seawater intrusion. Furthermore we demonstrate that TBGD is a significant addition of phosphorus (the limiting nutrient) to the coastal wetlands of the southern Everglades. Brackish groundwaters of the southern Everglades contain 1 to 2.3 µM concentrations of total phosphorus. These concentrations exceed the expected values predicted by conservative mixing of local fresh groundwater and surface seawater, which both have phosphorus of less than 1 µM. The additional source of phosphorus may be from seawater sediments or result from the desorption from the aquifer matrix due to water-rock interactions induced by mixing fresh groundwater with intruding seawater.