The fate of uranium in the three lakes of the Jordan Rift Valley

Analyses of uranium were carried out by delayed neutron activation on the waters and sediments of the Jordan River and of the lakes of the Rift Valley that this river connects: Lake Huleh, Lake Kinneret, and the Dead Sea. Between the headwaters of the Upper Jordan River and the northernmost part of the Lower Jordan, the soluble uranium concentration is relatively low, about 1.5 μg liter^-1. The concentration more than doubles before it reaches the Dead Sea. In the upper oxic brines of the Dead Sea, 2.5 μg uranium liter^-1 is preserved in solution and about 0.16 μg liter^-1 yr^-1 is being removed. In the earlier, peaty stage of Lake Huleh the sediments accumulated uranium from the overlying waters. The peaty sediments contain up to 11.8 ppm uranium. During the later lacustrine phase the concentrations of U in the sediment column are much lower, 0.6 to 2.8 ppm. They reflect varying mixing ratios between two end members: the endogenic carbonate and the U-containing (about 3.5 ppm) detrital material. Similarly, in the sediments of Lake Kinneret, the endogenic carbonate is effectively diluting the U-containing detrital phases. In the Kinneret, however, due to the narrower spread of CaCO₃ concentrations, the variations in uranium content are smaller too, between 1.3 and 2.6 ppm. In contrast to Lake Huleh, where we did not find clear evidence for removal of U from the water column, it seems that in Lake Kinneret about 2% of the inflowing U is lost to the sediments. The proposed removal mechanisms are coprecipitation with calcite and/or through reduction to insoluble tetravalent species. Only in the sediments of the Dead Sea are U concentrations found to be similar to, or slightly higher than, in the allogenic material. Here, non-efficient dilution by the relatively low endogenic carbonate content, is coupled to addition of U by removal from the oxic Upper Water Mass. This uranium removal appears to be restricted to an area close to the Jordan inflow rather than uniformly distributed over the entire Dead Sea floor. From the Lower Water Mass of the Dead Sea uranium is being removed only at the initiation of a meromictic phase, during the onset of anoxia. The horizon of sediment coeval with the onset of anoxia in the Lower Water Mass, which took place about 300 years ago, has been found to contain, as anticipated, a higher uranium concentration (7.2 ppm U). It is proposed that similar horizons of U enrichment can be looked for in paleolimnological studies for defining the onset of past meromictic cycles.