The effect of salinity on the concentrations of radium and thorium in sediments

Abstract

The effect of salinity on the sorption behaviour of radium and thorium has been studied with the aim of assessing the suitability of the 226Ra/232Th activity ratio (AR) as a tracer in saline waters. Radium has been reported in the literature as being highly mobile in saline water, and easily desorbed from sediments, whereas thorium is believed to remain tightly bound. Results of analyses of bed sediments from an inland river system with a record of past regimes of high dissolved salt concentrations were consistent with this behaviour. Radium-226 activity in the bed sediments was shown to be deficient with respect to its parent, 23°Th, indicating a loss of radium from the sediments. A more detailed field study examined the extent of radium loss as a function of salinity in an estuary. Desorption of ²²⁶Ra from freshwater suspended sediment increased to a maximum value (58% ±7 of the total sediment concentration) as the salinity of the estuary rose. Maximum radium desorption was reached in the 6-10 ppt salinity range. Net desorption then remained unchanged despite further rises in salinity. Laboratory experiments indicated that the kinetics of radium desorption is rapid, with maximum desorption from river sediments reached within 1 h of contact with saline water. The net radium loss from freshwater sediment occurs as a result of the competition effects from other dissolved cations for ion-exchange sites on the sediment. Cation exchange is probably the dominant mechanism of radium release, although alpha recoil appears to be at least partly responsible for the higher losses of ²²⁴Ra and ²²³Ra by increasing their accessibility for ion-exchange reactions. The effects of solid/liquid ratio and particle size of the sediment on radium sorption in saline water were also investigated. The results indicate that, when exposed to saline water, the greatest radium loss from freshwater sediments will occur from clay sized particles when they are present in the water column at low concentrations. Proportionally less radium is desorbed from coarse-grained bed sediments. There was no evidence of any net loss of thorium from sediment as salinity is increased. In fact, laboratory experiments indicate that the transfer of dissolved or colloidal thorium onto sediment particles is enhanced in saline water. From the differing sorption behaviour of radium and thorium it is concluded that the ²²⁶R a / ²³²Th AR is not a suitable sediment tracer in saline water. However, the 23° T h / ²³²Th AR can provide a stable tracer of sediment movement in estuaries and is the preferred sediment tracer in saline rivers. In some circumstances the ²²⁸R a/²²⁶Ra AR may also be useful. The dissolved concentrations of all radium isotopes show strongly non-conservative increases in the Bega estuary. The shorter-1 ived isotopes, ²²⁴Ra, ²²³Ra and ²²⁸Ra are particularly highly enriched. The most likely source of these isotopes is the pore water of bottom sediments. This work has shown that radium isotopes may give information on the rates and extent of surface-pore water mixing in an estuary.