Comparative distributions of size fractionated metals in pore waters sampled by in situ dialysis and whole-core sediment squeezing: Implications for diffusive flux calculations

Trace elements (Ag, Cd, Co, Cr, Cu, Fe, Mn, Pb, V, Zn) and ammonia were measured in pore waters extracted from sediments collected in Green Sands Beach (Mare Island, San Francisco, CA) using peepers and whole-core squeezers. Samples were size-fractionated, either by using 0.45 and 0.1 μm polycarbonate filters (whole-core squeezers), or by passive filtration using peepers with 0.45 μm and 3000 MWCO (3 kDa) membranes. Results indicate that the distribution profiles of pore water constituents in the traditionally defined total dissolved fraction (CTD = <0.45 μm) reflect different diagenetic processes, as well as anthropogenic inputs from the former Mare Island Naval Shipyard. Metal concentrations and profiles in the truly dissolved phase (CD = <3 kDa) for Mn, Co, Cr, V and NH4+ were similar to the ones obtained for CTD. Profiles for the small-sized dissolved fraction (CSSD = <0.1 μm) and in the CTD pool were almost identical for all of the analysed metals. Absolute values of the ratios between the diffusive fluxes of the different size fractions relative to the total dissolved fluxes ranged from 0.012 to 41. Of these, 37% were lower than 0.50 and 12% showed values in excess of 2.0. However, in 22% of the cases, the fluxes were in directions opposite to the metal fluxes calculated using CTD gradients. Hence, gradients of total dissolved trace metals, which are the only ones normally measured in pore waters, may not necessarily reflect the mass flow of dissolved constituents across the sediment column.