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 solute transport in an intertidal coastal wetland. Two distinct pore-size classes in the sediment were identified. Macropores had effective diameters greater than 100 mu m and a normalized volume of 5%; matrix pores had effective diameters smaller than 100 mu m and were the volumetrically dominant pore-size class (95%). We found that infiltration and evaporation-driven water fluxes were segregated between macropores and matrix pores, respectively, which had the effect of enhancing diffusive 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 pore-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 discharge occurred by an upward flux in matrix pores to satisfy evapotranspiration. Without disturbance by the downward infiltration flux in macropores, upward advection of chloride in matrix pores and evapoconcentration increased chloride concentrations in matrix pores to a level well above the concentration in seawater. The resulting high concentrations of chloride in matrix pores induced a large diffusive efflux of chloride into surface water that was sufficient to balance new input of 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 period. We conclude that segregation of water and solute fluxes in two pore-size classes strongly influences sediment salinity of coastal wetlands, which has direct bearing on primary productivity of dominant vegetation and on exchange of dissolved nutrients and contaminants between intertidal wetlands and open water.