We present a new approach to studying the behavior of radium isotopes in a coastal aquifer. In order to simulate radium isotope distributions in the dynamic flow field of the Dead Sea aquifer, a multi-species density dependent flow model (SUTRA-MS) was used. Field data show that the activity of 226Ra decreases from 140 to 60dpm/L upon entering the aquifer from the Dead Sea, and then further decreases linearly due to mixing with Ra-poor fresh water. On the other hand, an increase is observed in the activity of the shorter-lived isotopes (up to 52dpm/L 224Ra and 31dpm/L 223Ra), which are relatively low in Dead Sea water (up to 2.5dpm/L 224Ra and 0.5dpm/L 223Ra). The activities of the short lived radium isotopes also decrease with decreasing salinity, which is due to the effect of salinity on the adsorption of radium. The relationship between 224Ra and salinity suggests that the adsorption partition coefficient (K) is linearly related to salinity. Simulations of the steady-state conditions, show that the distance where equilibrium activity is attained for each radium isotope is affected by the isotope half-life, K and the groundwater velocity, resulting in a longer distance for the long-lived radium isotopes. K affects the radium distribution in transient conditions, especially that of the long-lived radium isotopes. The transient conditions in the Dead Sea system, with a 1m/yr lake level drop, together with the radium field data, constrains K to be relatively low (<10). Thus, the sharp decrease in 226Ra cannot be explained by adsorption, and it is better explained by removal via coprecipitation, probably with barite or celestine.
|Number of pages||18|
|Journal||Geochimica et Cosmochimica Acta|
|State||Published - 1 Jul 2012|
Bibliographical noteFunding Information:
We thank W. Sanford, A. Provost and J. Hughes for their help with SUTRA-MS. We thank H. Michael for fruitful discussions. We thank Y. Rosenberg for his code and help with calculating the activities of radium, calculations of saturation degrees, and for fruitful discussions. We thank M. Feldman and Y. Shalem for their help with lab work ( 226 Ra, 224 Ra and 223 Ra measurements). We thank B. Burnett for the 228 Ra analysis. We thank O. Yoffe and D. Stiber from the Geological Survey of Israel lab for the major elements and Ba analysis. We thank the reviewers and the associated editor, C. Zhu, for their many constructive comments that improved this paper significantly. We thank Y. Erel, B. Lazar and Y. Kolodny for their advice. We thank the BSF Rahamimoff Travel Grant for Young Scientists which funded the travel expenses of the first author (YK) to the USGS, the Hoffman Leadership and Responsibility Fellowship of the Hebrew University and the Rieger Foundation for the support given while working on this study. We thank the Ministry of National Infrastructure for supporting this research.