Radium fluxes from a salt marsh

Abstract

Dissolved 226Ra (half life 1,600 yr), 228Ra (half life 5.7 yr) and 224Ra (half life 3.64 days) activities in estuarine and coastal waters higher than values reported for open ocean or river waters have been explained by desorption and diffusion of radium from coastal and estuarine sediments1–4. Fluxes of radium isotopes from sediments in Long Island Sound, the Hudson River estuary, Chesapeake Bay and Winyah Bay, South Carolina, have been estimated from mass balances, diffusive bottom fluxes and desorption from suspended sediments4–7. These estimates may not be correct if salt marshes which are intimately associated with estuaries and the coastal ocean are an additional source or sink of radium4. In this study Bly Creek, a small marsh-tidal creek system near Georgetown, South Carolina, was chosen to determine the flux of 228Ra and its great granddaughter, 224Ra, from the sediments to the tidal creek water. We find that the highest activities of 224Ra and 228Ra in the tidal creek waters occur during low tide, but 226Ra activity remains relatively constant throughout the tidal cycle. Fluxes of 224Ra and 228Ra calculated from mass balances required to support the observed activities in the creek waters and measured directly using a flux chamber are consistent. These measurements reveal a large input of radium from the marsh sediments to the overlying waters. To produce the necessary fluxes, irrigation and bioturbation must replace interstitial water in the top 5 cm of the marsh sediments every 2–3 days.