Search

The transport of Ni2+ions in a column, filled with porous media, was observed in three dimensions and time by magnetic resonance imaging (MRI) in a clinical scanner. For porous media we used glass beads or quartz sand in a saturated continuous flow mode. The magnetic moment of Ni2+decreased the T1relaxation time of 1H in aqueous solution. This concentration‐dependent effect was used by a fast low angle shot (FLASH) MRI sequence for imaging the concentration of the dissolved ions. Since Ni2+behaves as a conservative tracer under the chosen conditions, the tracer motion was representative for the water flow in the porous medium. Currently, we can achieve an isotropic spatial resolution of 1.5 mm and a temporal resolution of 170 s. The transport observation gives direct access to hydraulic flow properties of the porous media. The fluid flow velocity field was calculated by a fronttracking method and the statistical properties of the velocities were investigated. We also compared the experimental data with the three‐dimensional particle tracking model PARTRACE, which uses the experimental flow field as input.

In Germany, geochemical modelling takes a strong position in two aspects of broader public interest. The first aspect is the safety assessment of a nuclear waste repository, the second is remediation of uranium mining areas. In both aspects, the application of geochemical modelling is stipulated by authorities. This situation results from the possibility to model highly complex situations by computers. The increase in computing power experienced in recent times now offers techniques to assess the sensitivity of modelling results to uncertain input data both in the thermodynamic data base and the site-specific field data. Both aspects are investigated by using Monte Carlo methods in combination with non-parametric statistics. A probabilistic geochemical modelling of uranium mill tailings leaching is demonstrated by application of TReaC modelling code using a simplified site model.

Abstract:  Knowledge of the transport behavior of radionuclides in groundwater is needed for both groundwater protection and remediation of abandoned uranium mines and milling sites. Dispersion, diffusion, mixing, recharge to the aquifer, and chemical interactions, as well as radioactive decay, should be taken into account to obtain reliable predictions on transport of primordial nuclides in groundwater. This paper demonstrates the need for carrying out rehabilitation strategies before closure of the Königstein in-situ leaching uranium mine near Dresden, Germany. Column experiments on drilling cores with uranium-enriched tap water provided data about the exchange behavior of uranium. Uranium breakthrough was observed after more than 20 pore volumes. This strong retardation is due to the exchange of positively charged uranium ions. The code TReAC is a 1-D, 2-D, and 3-D reactive transport code that was modified to take into account the radioactive decay of uranium and the most important daughter nuclides, and to include double-porosity flow. TReAC satisfactorily simulated the breakthrough curves of the column experiments and provided a first approximation of exchange parameters. Groundwater flow in the region of the Königstein mine was simulated using the FLOWPATH code. Reactive transport behavior was simulated with TReAC in one dimension along a 6000-m path line. Results show that uranium migration is relatively slow, but that due to decay of uranium, the concentration of radium along the flow path increases. Results are highly sensitive to the influence of double-porosity flow.

Abstract: Mining and milling has taken place in Saxony and Thuringia, in eastern Germany, for over 800 years. Radioactive waste rock was dumped without regard for the environment for centuries. The area is now rather densely populated, and some of the smaller dumps and tailings have been used as municipal or industrial landfills after the uranium mine and mill closed. Ground water contamination has occurred down-gradient of an industrial disposal plant in Dresden (Saxony). To determine the source of the contamination, a multi-parameter interpretation was done using234U,238U,226Ra,222Rn,2H,18O,34S, and13C, as well as conventional hydrochemical parameters. Ground water contamination in the natural aquifer beyond the disposal site was determined to be from the tailings due to its oxidizing, acidic character and high concentrations of sulfate, uranium and heavy metals. In this case, the contamination was not caused by recent seepage water from the dump. Water currently leaching from the tailings has been modified by water draining from the superimposed waste, which has had a positive influence on the tailings. Uranium and heavy metals have been immobilized in place due to the changes in the chemical environment.

Groundwaters in the area of a mixed landfill (domestic waste above uranium mill tailings) in the Erzgebirge (Saxony, Germany) were investigated for their isotope signatures to distinguish between different groundwater types. The seepage water from the landfill shows higher δ34S values as the inflow whereas the parameters tritium and δ18O do not significantly vary. Uranium values are lower than at other comparable sites, The maximum of 0.7 mg/l Unat was measured in the geological underground of the landfill. Significant radium concentrations occur in the geological background and are due to former mining activities. The landfill does not produce increased radium values in the water.