Chemical composition of mangrove-generated brines in Bishop Harbor, Florida: Interactions with submarine groundwater discharge

Locations and compositions of submarine ground water discharge (SGD), which constitutes sources to coastal zones of low salinity water, terrestrial contaminants, and solutes with concentrations elevated by diagenetic reactions, may be altered in tropical settings where widespread mangrove forests increase salinity of shallow pore waters through evapotranspiration. The red mangrove (Rhizophora mangle) is ubiquitous in Bishop Harbor on the west coast of Florida, where sampling at four sites shows salinities of pore waters increase to three times the average marine values at depths up to 160 cm below the sediment–water interface in and around the mangroves. The increase in salinity is greatest in open areas within the mangrove forests, while seaward of the forest, the salinity remains near the values of the surface water. Major element concentrations (Cl−, SO42−, Na+, K+, Mg2+, and Ca2+) increase with salinity, but the depth profiles of major element/Cl− ratios vary across the area. The two sites located closest to the mainland have the greatest increase in salinity and are the only sites with major element/Cl− ratios that vary with depth in the sediment, reflecting in situ diagenesis. In contrast, the sites located farthest from the mainland have constant major element/Cl− ratios with depth but with values less than seawater ratios. Low ratios suggest that a non-marine source of water, possibly meteoric, mixes with pore water at these sites before salinity increases through evapotranspiration. If the water is meteoric, the seepage face is located farther from the coast than regions without mangrove forests. The steep salinity gradients indicate that the water bypasses the seepage face by transportation through the mangroves.