EVALUATING THE ROLE OF BIOIRRIGATION IN SUBMARINE GROUND WATER DISCHARGE

A strong interest has arisen in delineating the pathways and magnitudes of fluid flow in shallow permeable coastal sediments. It has long been recognized that benthic infauna can rapidly exchange porewater with overlying water through bioirrigation, but recent data have also demonstrated possible additional forcing by wave- and tidal-induced physical mixing. The relative balance of biologically to physically driven flow was examined in the shallow back-barrier Indian River Lagoon on the east coast of Florida. Visual observations and shallow coring were used to quantify sediment lithology, physical properties, and bioturbation intensity. At and between the four shallow (<1.5 m water depth) locations examined, there is very little variation in the shelly sandy lithology over the upper ~50 cmbsf, which is the apparent zone of rapid (hours to days) fluid exchange between the water column in the sediments. At lower-energy sites, the mass percent mud in the uppermost sediments is potentially high enough (>2%) to clog pore spaces at the grain-size-modeled hydraulic conductivity (~ 0.5–2.5 x 10 ^-2 cm/sec), thus reducing advective transport of fluid across the sediment-water interface by pressure-induced circulation. A variety of burrowing organisms are endemic to the Indian River Lagoon and using visual observations of the burrow and tube morphologies at the surface and in x-radiographs, we were able to identify three to four dominant types of these infaunal tracemakers. The organisms include two species of worms (Arenicola cristata, Diopatra cuprea) and possibly two types of shrimp (Callianassa sp., Upogebia sp.). Burrow/tube densities of ~ 3-10 m^-2 were determined by counting the features on the sediment surface and published bioirrigation rates for each organism were used to calculated the volume of water exchanged through the burrows. Summing the rates of the individual species across the area influenced by the burrows yields an average value of ~5 cm/day over the observation area. This value falls within the range of those measured using seepage meters and chemical tracers of ~2 to 12 cm/day. Because there are small tidal ranges (~10 cm) and wave heights (< 40 cm), these data suggest that bioirrigation dominates exchange in these shallow sandy sediments.