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- Author or Editor: J. Byegård x
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Abstract
Studies of the stability of various metal EDTA, DTPA and DOTA complexes in order to evaluate their applicability as non-sorbing tracers have been performed. In laboratory tests, the stability generally increases for the individual metal ions in the EDTA<DTPA<DOTA order. For most metal ions, the same trend can be observed for the thermodynamic stability constants. In the in situ experiment, various metal EDTA tracers were used in very low concentrations; YbEDTA−, for example had a breakthrough and recovery which were very similar to the non-sorbing tracers used. According to the extremely low tracers concentrations used, thermodynamic data indicate that all metal EDTA tracers should have been decomplexed as a result of the competition with the naturally occurring cations in the groundwater. This was not found, which indicates that the decomplexation rate and sorption mechanism are important in estimating the applicability of the metal complexes as tracers. The DOTA complexes of elements in the middle of the lanthanide series have indicated high stability in the laboratory tests and therefore appear to be good candidates as non-sorbing tracers. However, in contrary to the metal EDTA, tracers, the DOTA complexes of La3+ and Lu3+ seemed to be slightly delayed in the in situ experiment.
Abstract
Three radially converging in situ migration experiments over a distance of 5 m were performed in a single fracture at a depth of 400 m in the Äspö Hard Rock Laboratory. Injection and breakthrough curves were obtained for uranine,H3HO (HTO), 22Na+, 42K+, 47Ca2+, 58Co(II), 82Br–, 85Sr2+, 86Rb+, 99mTc (no breakthrough), 131I–, 131,133Ba2+ and 134,137Cs+. The in situ experiments lasted for nearly 1.5 years, with single experimental times up to 10,000 hours. The tracer concentrations span over seven orders of magnitude between injection and sampling under practically undisturbed chemical conditions. Dynamic ranges in the breakthrough curves of up to four orders of magnitude were obtained. Thus, the use of radioactive (especially 
-emitting) tracers showed to be a most useful tool for in situ tracer experiments. The relative retardation sequence obtained in the field experiment was Na < Ca 
Sr < K < Ba Rb < Co Cs, which was the same as the relative sequence of the sorption coefficients obtained in the laboratory experiments using crushed rock material. Thus, no scale effect was indicated in the relative retardation sequence between laboratory and field experiments. High recoveries, >90%, were obtained for uranine, HTO, Br, I, Na and Sr and lower recoveries for Ba, Rb, Cs and Co. However, there were indications that there would have been higher recoveries of these elements if it had been possible to continue monitoring over longer experimental times. The low recoveries of Cs and Co indicate either slowly reversible or non-reversible sorption behavior. The laboratory diffusion experiments showed lower diffusivities and porosities and somewhat lower sorptivity of all studied tracers in the site-specific rock samples, dominated by mylonite, than in the diorite host rock. Matrix diffusion and associated sorption within the rock matrix is indicated in the in situ experiments, although this can not be verified without modeling that involves such processes.
