FLUXES OF WATER AND SOLUTE IN A COASTAL WETLAND SEDIMENT .2. EFFECT OF MACROPORES ON SOLUTE EXCHANGE WITH SURFACE-WATER

We investigated interactions between sediment physical structure and s olute transport in an intertidal coastal wetland. Two distinct pore-si ze classes in the sediment were identified. Macropores had effective d iameters greater than 100 mu m and a normalized volume of 5%; matrix p ores had effective diameters smaller than 100 mu m and were the volume trically dominant pore-size class (95%). We found that infiltration an d evaporation-driven water fluxes were segregated between macropores a nd matrix pores, respectively, which had the effect of enhancing diffu sive effluxes of chloride from the sediment to surface water. Chloride was highly concentrated relative to seawater in matrix porewater but was comparatively dilute in macropores. Concentration differences in p ore-size classes declined with depth until indistinguishable below 10 cm. The segregated chloride distribution can be explained if recharge to the sediment occurred by downward infiltration in macropores and di scharge occurred by an upward flux in matrix pores to satisfy evapotra nspiration. Without disturbance by the downward infiltration flux in m acropores, upward advection of chloride in matrix pores and evapoconce ntration increased chloride concentrations in matrix pores to a level well above the concentration in seawater. The resulting high concentra tions of chloride in matrix pores induced a large diffusive efflux of chloride into surface water that was sufficient to balance new input o f chloride by infiltration of seawater in macropores (0.085 mmol Cl cm (-2) day(-1)). Transport models that were constrained by water balance measurements at the field site explained both the exponential form of the vertical distribution of chloride in matrix pores and the rate of change in storage of chloride in sediment porewater over a 1 month pe riod. We conclude that segregation of water and solute fluxes in two p ore-size classes strongly influences sediment salinity of coastal wetl ands, which has direct bearing on primary productivity of dominant veg etation and on exchange of dissolved nutrients and contaminants betwee n intertidal wetlands and open water.