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 flat in the Manukau Harbour, New Zealand. Measurements of water table position, and sediment saturation based on buried lysimeter cups and electrical conductivity, accompanied by an analysis of sediment characteristics were used. The level of the water table drops following exposure on the ebb tide up to 41cm in the vicinity of drainage channels but much less over most of the sand flat. Despite the drop in water level, the surface sediments remain close to saturation throughout the period of exposure, with the extent of water table lowering being primarily influenced by topographic change close to surface drainage channels and the duration of exposure. It was determined that at all stages of exposure, the capillary fringe is well developed, intersecting with the surface, meaning the addition of a very small amount of water brings the water table to the surface. As a consequence tidal pumping and the total flux of water and gaseous exchange through the upper layers of sediment are likely to be small.