Temporal, spatial, and resolution constraints for in situ sampling devices using diffusion equilibration : dialysis and DET.

Harper, Michael P. and Davison, William and Tych, Wlodek (1997) Temporal, spatial, and resolution constraints for in situ sampling devices using diffusion equilibration : dialysis and DET. Environmental Science and Technology, 31 (11). pp. 3110-3119. ISSN 0013-936X

Abstract

The techniques of dialysis and diffusional equilibration in thin films (DET) are used to measure solute concentrations in sediment porewaters. 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 results. Three alternative types of supply to the samplers were considered: (i) where porewater solute concentrations are well buffered by desorption 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. Using typical sampler designs (DET gels 0.4 mm thick and peeper cells 6 mm deep), the times for 99% equilibration in the buffered case were calculated 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 equilibration times ( hours for DET, up to 2−3 weeks for peepers) lie between the modeled buffered and diffusive case values and are consistent with the modeled partial resupply from solid phase to porewater. The equilibration time is inversely proportional to the solute diffusion coefficient and increases with peeper cell depth or gel layer thickness. To ensure minimum equilibrium times in multiple depolyments, DET devices should be separated by >2 cm and peepers by >16 cm. Porewater concentration maxima and gradients are underestimated by both techniques, resulting in underestimations of vertical fluxes calculated from Fick's first law. DET performs better for a given width of a porewater maximum. As concentration maxima become wider, the fidelity of measured profiles is improved. The fidelity of peeper profiles is limited largely by the cell size and separation, and that of DET by slicing interval and gel/filter thickness. Peeper cell depth does not affect the measured peak concentrations, but increasing DET gel thickness underestimates measured peak concentrations. To minimize back-equilibration times for DET analysis, diffusional path lengths and eluent volumes should be as small as possible.