SUBMARINE GROUNDWATER DISCHARGE IN LAGOONAL SEDIMENTS OF A HOLOCENE TRANSGRESSIVE BARRIER ISLAND SEQUENCE

Submarine ground water discharge (SGD) is derived from two primary sources: fresh water, discharging from terrestrial aquifers, and marine water, circulating through surface sediments. Recent studies in the Indian River Lagoon, Florida have found SGD to be variable both spatially and vertically. Terrestrial aquifer discharge is driven by gravity flow at ~ 0.1 cm/day, and decreases to zero within a few tens of meters from shore. Marine waters circulating through the surface sediments appear to be driven by bioirrigation at ~1 to 10 cm/day to depths as great as 70 cm below seafloor. To test the hypothesis that the observed heterogeneity in SGD measured across the sediment water interface is due to variations in sedimentary properties, primarily hydraulic conductivity, 28 vibracores were collected from five sites across the northern half of the lagoon. Currently, the Indian River Lagoon system is ~250 km long, microtidal, and has restricted connection to the Atlantic with only five inlets. Overall, local (0.04 km²) and lagoon-wide spatial variability in sediment physical properties within the upper three meters is minimal. Median grain sizes range from approximately 1.7 to 2.5 Φ (medium to fine sand) and in general show modest variation (~1 Φ unit) down core. All sites are moderately to poorly sorted, reflecting the broad range and grain sizes from shell to mud. Within each site, distinct beds ~10 - 100 cm in thickness can be followed between cores. Radiocarbon ages obtained from two sites indicate sedimentation rates of 0.5 – 0.7 mm/yr. Roughly equal to relative sea level rise, this suggests that the lagoon has existed in its current morphologic state over the last few millennia. The relative homogeneity of the sediments is likely the result of intense bioturbation and lack of spatial variability in sediment transport mechanisms (e.g. few inlets, no tidal creeks, minimal runoff). Although it is certainly possible that substantial variability may exist elsewhere in the lagoon, modeled and measured hydraulic conductivity values simulated in MODFLOW were unable to reproduce the observed heterogeneity in marine SGD.