COLLOID FORMATION AND REDOX SENSITIVE METAL TRANSPORT IN THE SUBTERRANEAN ESTUARY

In surface estuaries, the transport of organic matter, iron, and trace metals to the sea is modified by the flocculation and removal of colloidal phase (particles from 1 nm to 1 µM). Like surface estuaries, subterranean estuaries (STEs) exert considerable control on the composition of water reaching the coastal ocean; however, little is known about the importance of the colloidal phase and the effect on trace metal transport in these systems. We investigated how the surface charge, abundance, and composition of colloidal particles in an STE changed along salinity and redox gradients. The range of colloidal surface charges was similar to that of surface estuaries, but the relationship between surface charge and salinity was less robust in the STE. The abundance of colloidal particles, as determined by the concentrations of DOC, Fe, and Mn in 3-30 kD and >30 kD size fractions, generally followed the distribution of the total dissolved concentrations. Internal redox processes (largely absent from surface estuaries) and flow through a porous medium led to an increase in the proportion of high molecular weight DOC and Fe with increasing salinity, in contrast to observations from surface estuaries. The importance of colloids for redox sensitive metal (RSM) (Mo, U, V, and Cr) transport in the STE differed from surface estuaries due to strong redox gradients affecting metal speciation in the STE. Specifically, the presence of colloids in a reducing environment facilitates the transport of particle reactive reduced species, rather than driving removal (as is typical in surface estuaries). This work represents some of the first results characterizing the concentration of colloids and their effect of RSM transport across both salinity and redox gradients characteristic of the STE.