Ra isotopes and Rn in brines and ground waters of the Jordan-Dead Sea RiftValley: Enrichment, retardation, and mixing

Thirty-six springs and wells from the Dead Sea Rift Valley were periodicall y sampled and analyzed. The latter included full chemical analyses, Rn-222 and Ra-226 (by alpha-counting emanometry), as well as Ra-228/Ra-226 and Ra- 224/Ra-228 activity ratios (gamma spectrometry). Sampling stretched over al most 2 yr. Several hundred Rn-Ra analyses and close to a hundred isotopic r atios were measured. Most waters in the Rift Valley have both elevated Ra ( several to 750 dpm/L) and Rn (a few hundred to almost 60,000 dpm/L) content . In practically all samples Rn-222 activity considerably exceeds that of i ts parent Ra-226. Th, Ra content is the result of all Dead Sea Rift Valley waters being mixtures of fresh water with saline brines. Ra is efficiently extracted from surrounding rocks into the brine end member. Ra-228/Ra-226 r atios are exceptionally low -0.07 to 0.9, mostly less than 0.2. This appare ntly reflects the U over Th enrichment in the source rocks that contribute the Ra. Ra enrichment (both Ra-228 and Ra-226) is locally correlated throug hout the Rift Valley with water salinity. This correlation can be used to c onstrain the age of the brine-freshwater mixing process. In one of the hydr ologic subsystems studied (the Fuliya block), the mixing of the shallow gro und water must have occurred in less than 200 to 300 yr ago, probably befor e no more than some 30 yr. High radon activities in surface waters along th e Dead Sea Rift Valley result from the formation of radium-enriched linings on the aquifer rock surface. The mixing of the radium-extracting brines wi th fresh water leads to continual adsorption of radium as water salinity de creases. The unique combination of fast upward how and continual mixing in the Dead Sea Rift Valley accounts for a constant replenishment of radium in the waters. This causes a gradual buildup of a radium lining on the aquife r walls until eventually a steady-state surface activity is established. Co pyright (C) 2000 Elsevier Science Ltd.