The capillary fringe and the water table in an intertidal estuarine sand flat

A study of pore water exchange was undertaken within an intertidal sand fla t in the Manukau Harbour, New Zealand. Measurements of water table position , and sediment saturation based on buried lysimeter cups and electrical con ductivity, accompanied by an analysis of sediment characteristics were used . The level of the water table drops following exposure on the ebb tide up to 41 cm in the vicinity of drainage channels but much less over most of th e sand flat. Despite the drop in water level, the surface sediments remain close to saturation throughout the period of exposure, with the extent of w ater table lowering being primarily influenced by topographic change close to surface drainage channels and the duration of exposure. It was determine d that at all stages of exposure, the capillary fringe is well developed, i ntersecting with the surface, meaning the addition of a very small amount o f water brings the water table to the surface. As a consequence tidal pumpi ng and the total flux of water and gaseous exchange through the upper layer s of sediment are likely to be small. (C) 1999 Academic Press.