PREFERENTIAL FLOW AND SEGREGATION OF POREWATER SOLUTES IN WETLAND SEDIMENT

Sediment macropores (with effective diameters larger than 100 mu m) co mprise 11% of the hulk sediment volume in a tidal freshwater wetland v egetated with Peltandra virginica. In order to determine effects of ma croporous sediment structure on solute transport, we conducted a solut e tracer experiment in the sediment. The effective transport volume (t heta(eff), the volume of sediment through which solute was transported normalized to sediment hulk volume) was 0.15 cm(3) cm(-3), which is c onsiderably smaller than the total pore space that is potentially avai lable for transport (porosity of sediment is 0.63 cm(3) cm(-3)). A mea n transport time of 13 d was required to flush preferential flow paths in Peltandra hummocks; hydrologic turnover of the volumetrically domi nant matrix pores (0.53 cm(3) cm(-3)) was apparently much slower. Base d on porewater sampler design and hydrological principles, we suggest that N-2-purged tension solution samplers and diffusion equilibrators preferentially sample porewater from macropore and matrix domains, res pectively. Dissolved ammonium and orthophosphate concentrations were t hree-fold higher in matrix pores compared to macropores, which is cons istent with our finding that more rapid hydrological flushing occurred in macropores compared to matrix pores, Further evaluation of porewat er sampler designs in macroporous sediment is needed to improve studie s of hydrologic transport and biogeochemical cycling in wetlands.