TIDAL WETLAND FLUXES OF DISSOLVED ORGANIC CARBON AND SEDIMENT AT BROWNS ISLAND, CALIFORNIA: INITIAL EVALUATION
Ganju*, N.K.
B.A. Bergamaschi, D.H. Schoellhamer
U.S. Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA, 95819-6129
Carbon and sediment fluxes from tidal wetlands are of increasing concern in
the Sacramento-San Joaquin River Delta (Delta), because of drinking water issues
and habitat restoration efforts. Certain forms of dissolved organic carbon (DOC)
react with disinfecting chemicals used to treat drinking water, to form disinfection
byproducts (DBPs), some of which are potential carcinogens. The contribution
of DBP precursors by tidal wetlands is unknown. Sediment transport to and from
tidal wetlands determines the potential for marsh accretion, thereby affecting
habitat formation.
Water, carbon, and sediment flux were measured in the main channel of Browns
Island, a tidal wetland located at the confluence of Suisun Bay and the Delta.
In-situ instrumentation were deployed between May 3 and May 21, 2002. Water
flux was measured using acoustic Doppler current profilers and the index-velocity
method. DOC concentrations were measured using calibrated ultraviolet absorbance
and fluorescence instruments. Suspended-sediment concentrations were measured
using a calibrated nephelometric turbidity sensor.
Tidally averaged water flux through the channel was dependent on water surface
elevations in Suisun Bay. Strong westerly winds resulted in higher water surface
elevations in the area east of Browns Island, causing seaward flow, while subsiding
winds reversed this effect. Peak ebb flow transported 36% more water than peak
flood flow, indicating an ebb-dominant system. DOC concentrations were affected
strongly by porewater drainage from the banks of the channel. Peak DOC concentrations
were observed during slack after ebb, when the most porewater drained into the
channel. Suspended-sediment concentrations were controlled by tidal currents
that mobilized sediment from the channel bed, and stronger tides mobilized more
sediment than the weaker tides. Sediment was transported mainly to the island
during the 2-week monitoring period, though short periods of export occurred
during the spring tide. Future deployments will characterize the seasonal variability
of these fluxes.