Tidally regulated chemical fluxes across the sediment-water interface in elkhorn slough, california: Evidence from a coupled geochemical and hydrodynamic approach

Measurements of four Ra isotopes, Ba, Mn, U, and dissolved nutrients were used in concert with hydrodynamic modeling to reveal a large, tidally regulated sediment-surface-water exchange within Elkhorn Slough, an estuary in Monterey Bay, California. Hydrologic and oceanographic data from in situ sensors and geophysical surveys of estuarine bottom sediments were integrated with the chemical measurements to develop Ra flux boundary conditions for a three-dimensional hydrodynamic transport model of the estuary. Sediment and surface-water exchange is the dominant source of Ra isotopes within this system. This process occurs without significant fresh terrestrial groundwater discharge. A number of known solute transport and pore-water flushing mechanisms can explain the 224Ra input, including diffusion, evaporative transport, bioirrigation, and inundation and drainage of borrows and macropores. The 226Ra results suggest that some of these mechanisms, or others, are capable of mobilizing Ra from deeper sediments. The sediments supplying this flux may cover an area as large as the entire intertidal zone, primarily salt marsh, to a depth of 1-2 cm. In addition, the Ra delivery to the estuary appears to be temporally regulated by the tidal inundation of the salt marsh and subsequent drainage through tidal creek channels. These results illustrate the importance of the intertidal zone as a flowpath in the estuarine water cycle and the tidal phase lag that results when solute from this zone is transported into the estuary. This process is essential to understanding estuarine biogeochemical budgets of elements with significant sedimentary sources.