TEMPORAL, SPATIAL, AND RESOLUTION CONSTRAINTS FOR IN-SITU SAMPLING DEVICES USING DIFFUSIONAL EQUILIBRATION - DIALYSIS AND DET

The techniques of dialysis and diffusional equilibration in thin films (DET) are used to measure solute concentrations in sediment porewater s. Their performance was assessed using two-dimensional modeling with a view to establishing their limitations and providing guidelines for their application in the field and the subsequent interpretation of re sults. Three alternative types of supply to the samplers were consider ed: (i) where porewater solute concentrations are well buffered by des orption from or dissolution of the solid phase; (ii) where there is no resupply to the porewater apart from diffusion; (iii) where there is a partial resupply to the porewater from the sediment solid phase. Usi ng typical sampler designs (DET gels 0.4 mm thick and peeper cells 6 m m deep), the times for 99% equilibration in the buffered case were cal culated as 18 min and 36 h for DET and dialysis peepers, respectively. For the purely diffusive case, 99% equilibration times are 78 h (DET) and 1380 days (dialysis peepers). Experimentally observed equilibrati on times (similar to hours for DET, up to 2-3 weeks for peepers) lie b etween the modeled buffered and diffusive case values and are consiste nt with the modeled partial resupply from solid phase to porewater. Th e equilibration time is inversely proportional to the solute diffusion coefficient and increases with peeper cell depth or gel layer thickne ss. To ensure minimum equilibrium times in multiple depolyments, DET d evices should be separated by >2 cm and peepers by >16 cm. Porewater c oncentration maxima and gradients are underestimated by both technique s, resulting in underestimations of vertical fluxes calculated from Fi ck's first law. DET performs better for a given width of a porewater m aximum. As concentration maxima become wider,the fidelity of measured profiles is improved. The fidelity of peeper profiles is limited large ly by the cell size and separation, and that of DET by slicing interva l and gel/filter thickness. Peeper cell depth does not affect the meas ured peak concentrations, but increasing DET gel thickness underestima tes measured peak concentrations. To minimize back-equilibration times for DET analysis, diffusional path lengths and eluent volumes should be as small as possible.