FLUXES OF WATER AND SOLUTE IN A COASTAL WETLAND SEDIMENT .1. THE CONTRIBUTION OF REGIONAL GROUNDWATER DISCHARGE

Upward discharge of fresh groundwater into a mid-Atlantic intertidal wetland contributed 62% of the water needed to replace evapotranspiration losses from the sediment during an 11 day period in September. Infiltration during flooding by tides provided most of the balance; thus there was a net advection of salt into the sediment. The amount of groundwater discharge was estimated from changes in water storage in the sediment, as inferred from measurements of hydraulic head made every 10 min. We argue that this approach is inherently more accurate than calculating the flux as the product of hydraulic conductivity and head gradient. Evapotranspiration was estimated from direct measurements of net radiation. On an annual time-scale, our results suggest that groundwater discharge at this site may exceed the evapotranspiration flux during months of reduced evapotranspiration. Should this occur, groundwater-driven advection would supplement diffusion, during flooding, in removing salt from the sediment.