SEDIMENT FACIES AND PERMEABILITY OF A LATE QUATERNARY BARRIER ISLAND SEQUENCE, INDIAN RIVER LAGOON, FLORIDA

Sediment permeability can fundamentally influence the magnitude of fluid flow through sandy coastal sediments. Submarine groundwater discharge (SGD) is recognized as a potentially important pathway for dissolved fluxes across the sediment-water interface in coastal and estuarine settings. However, difficulties remain in determining the spatial heterogeneity in SGD in sandy shallow coastal settings because there are significant changes over short distances in sediment facies and corresponding permeability. The Indian River lagoon (IRL) Florida is a transgressive barrier island system and an ideal location to examine lateral and vertical facies changes on permeability and SGD. Existing facies models for transgressive barrier environments predict a complex spatial distribution of sediment textures that range from highly permeable, mature sands to nearly impermeable muds. Four sites in the northern ~45 km of IRL, previously identified as representing a wide range of groundwater discharge rates and bottom sediment textures, were each selected for a 200 m wide spatial vibracoring program of the upper three meters of lagoonal sediments. Four major depositional environments were identified in the upper 3 m of Indian River Lagoon sediments: marine, brackish, lacustrine, and lagoonal representing the past 125 ka. However, the limited tidal prism of the northern Indian River Lagoon has prevented any significant redistribution of the Holocene brackish and lagoonal deposits as is more common in mesotidal barrier island systems. The result is a stratigraphic sequence that is not predicted by facies models for transgressive barrier island systems. Overall hydraulic conductivity values for the upper 3 m of sediments in IRL range from 10 ^-2- 10^-8cm/sec. However, within the upper ~70 cmbsf where fluid flow is rapid, values only varied by two orders of magnitude (10^-2- 10^-4 cm/sec). Therefore, at the sites tested, observed spatial variability in SGD is not related to changes in permeability resulting from vertical facies changes.