Use of radium isotopes to examine pore-water exchange in an estuary, Limnology and Oceanography 39(8

The measured distributions of four isotopes of Ra along the estuary of the Bega River are used to examine sediment-water column exchange. Ra is created in estuarine sediments by the radioactive decay of insoluble Th parents residing close to or on the surfaces of the sediment grains. Ra desorbed into the pore water is continuously lost to the water column due to the cyclical draining and filling of the sediments by the tides. The distribution of Ra in the estuary is governed by its rate of loss from the sediments, its advection along the estuary resulting from river discharge into the estuary’s head, tidal mixing, and radioactive decay. These processes are all described in a model. Matching of model-predicted Ra con-centrations with measurements allows an estimate of the effective depth in the sediments to which the pore water is exchanged every tidal cycle. This depth is large (15 cm), but it is shown to be reasonable for the Bega estuary. The importance of biochemical reactions occurring in sediments to the chemical state of estuarine waters depends on the rate of transfer of solutes through these sediments an