HOW DOES SEA LEVEL RISE AFFECT SALT MARSH HYDROGEOLOGY AND SUBMARINE GROUNDWATER DISCHARGE (SGD)?

Groundwater flow through salt marsh sediments discharges nutrients to adjacent estuaries and plays an important role in salt marsh ecology, but few studies have considered the impact of sea level rise on groundwater flow through salt marshes. We analyzed hydraulic data from three salt marshes in the Southeastern United States to examine hydrogeologic controls on ecological zonation and nutrient export from salt marshes. One marsh was adjacent to a large upland, one was adjacent to a small (<100 m) upland, and one was part of a marsh island that lacks an upland. We found that interactions between fresh and saline groundwater flow systems at the upland/marsh boundary control ecological zonation patterns, particularly the presence of a hypersaline zone. These interactions have the potential to be altered by sea level rise when fresh groundwater discharging from uplands falls below a specific threshold. We also found that groundwater discharge to tidal creeks was inversely proportional to sea level. For example, sea level fell ~50 cm from October 2008 to March 2009 at Cabretta Island, Georgia; this caused groundwater discharge per tidal cycle to rise by 50% over the same period. Spring-neap variations in tidal amplitude caused additional fluctuations in groundwater discharge from the marsh, so that the volume of groundwater discharging per tidal cycle varied by an order of magnitude over the course of a 1-year observation period. Overall our results suggest significant changes in the hydrogeology of salt marshes during rapid sea level rise. Potential thresholds exist for the presence of hypersaline zones, and sea level rise will likely cause a decrease in the export of nutrients from salt marshes to estuaries.