TEMPORAL VARIABILITY IN SUBMARINE GROUNDWATER DISCHARGE: WHAT TIME SCALES ARE IMPORTANT?

It is clear from 15 years of investigations that submarine groundwater discharge (SGD) (1) occurs around the globe, (2) exports nutrients, metals, carbon, and biological agents to the ocean, and (3) is caused by several forcing factors that vary over different spatial and temporal scales. It is not clear, however, how fluxes associated with different driving forces compare, or which scales of temporal variations are most important. We installed monitoring wells in a barrier island in the US Southeast to quantify SGD and to assess the importance of variations in SGD, particularly those caused by large storms. Field observations indicate groundwater flow patterns that are broadly consistent with prior conceptual models for groundwater flow in small islands and salt marshes, including the presence of a freshwater lens below the upland and tidally-influenced groundwater flow in the beach and in the salt marsh on the landward side of the island. Variations in freshwater discharge are well-studied in terrestrial systems and predictably occur in coastal systems, so this talk will focus on variations in saline flow associated with variations in the tidal signal, including daily tidal fluctuations, spring-neap tidal cycles, storm surge, seasonal changes in mean sea level (MSL), and interannual variations in MSL. Variations in the tidal signal also affect the ecological zonation of the island, particularly the hypersaline Salicornia zone, which forms adjacent to the freshwater upland. In this zone, saline water infiltrates and flows downward during inundating spring tides. The Salicornia zone is exposed during neap tides, and high salinities form by evapo-concentration of upward-flowing groundwater. Upward flow is caused by a combination of evapotranspiration and elevated heads at depth, which are boosted by a strong hydraulic connection with the nearby upland. Future field and modeling studies of SGD must consider the variability of the real tidal signals rather than simple "high tide/low tide" descriptions.