Paloma Morales Gómez

The geochemical aspects of toxic waters retained in the Entremuros area (Spain)

Abstract Two walls were built, in order to retain approximately 2 Hm 3 of toxic water from the Aznalcollar mine-tailing dam. This water accumulated in a channel (approx. 10 km long and 1 km wide) in the lower part of the Guadiamar River (Entremuros) and was sampled during the first week of June 1998. The results indicate that the chemistry of the samples located at the northern part of the channel is controlled by a mixture of two processes — dilution of polluted water by the Guadiamar River and precipitation; consequently, medium or low concentrations of toxic elements were measured. The solid phases that determine the chemical equilibrium are calcite, smithsonite and gypsum. On the other hand, the most polluted zone is located to the south of the area studied. These waters are more acidic, and hence, have a higher content of dissolved heavy metals. For example, the Zn concentration is three orders of magnitude greater than in unpolluted samples. Prior to their flushing to the Guadalquivir River, the Entremuros waters were chemically neutralised with Na 2 CO 3 and NaOH. This neutralisation process was modelled and the results were compared with the analytical data obtained during the sampling campaign. The natural removal of heavy metals, which was observed in the waters, was produced by an increase of pH due to a strong evaporation in the zone. This removal produced the precipitation of hydroxides and carbonates of heavy metals, a process, which was also clearly shown in the modelling.

Factors that control the geochemical evolution of hydrothermal systems of alkaline water in granites in Central Pyrenees (Spain)

La evolucion geoquimica de las soluciones de los sistemas hidrotermales de Caldas de Boi, Arties, Tredos y Les, situados en el Pirineo Central (Spain), muestra diferencias reflejadas en la concentracion de solidos disueltos y en la presencia de una situacion de equilibrio o desequilibrio en profundidad de las soluciones respecto a la albita. La datacion (14C) de las aguas pertenecientes a los distintos sistemas hidrotermales ha puesto de manifiesto que el tiempo de residencia no es el factor de control determinante de las diferencias existentes en el grado de evolucion geoquimica alcanzado por las mismas, teniendo que contemplarse otro tipo de factores relacionados con la cinetica de disolucion de los minerales, como responsables de estas diferencias. El analisis de la influencia de los principales factores que controlan la tasa de disolucion de los minerales y, mas en concreto, de los feldespatos, muestra como las diferencias de temperatura de las soluciones en profundidad son capaces de justificar el distinto grado de evolucion geoquimica, incluso en soluciones con tiempos de residencia muy similares.

Modelling of the migration of trace elements along groundwater flowpaths by using a steady state approach application to the site at El Berrocal (Spain)

Abstract The applications of geochemical modelling to natural water systems mostly rely upon the equilibrium assumption. This is in principle justified for deep groundwater systems with long water residence times. In addition, trace element behaviour in these natural systems has been normally modelled by taking into account the precipitation and dissolution of individual trace element phases. Recent geochemical modelling work related to natural analogue systems would indicate that: (1) the equilibrium condition is restricted to a limited number of components in long water residence times and (2) the behaviour of trace metals is very much connected to the major component cycling in the system. There is a need to develop our geochemical modelling capabilities to take these two facts into account in order to be able to predict the behaviour of trace components (radionuclides) in a geological repository. In this work we report on the successful application of steady-state kinetics in conjunction with co-dissolution/co-precipitation approaches to model trace element geochemistry in the natural analogue system at El Berrocal. The evolution of major components of the system (Ca (II), Al (III), CO 3 2− and Si) as well the trace elements investigated (U, Ba and Mn) is well reproduced by using the coupling between steady-state kinetics and co-dissolution/co-precipitation approaches.

Mineralogical Control of the REE Distribution in the Fracture Fillings of an Uranium-Ore (Caceres-Spain)

The behavior of actinides in a deeply buried radioactive waste cannot be predicted from evidence of their movement in geological environments in the geological past because these elements do not occur naturally in appreciable concentration. A useful means of reducing the uncertainty is to observe the behavior, both past and present, of chemical analogues in geological environment. In this sense, the rare earth elements (REE) have been used to predict the behaviour of actinide series elements in water-rock systems on account of the similar valence and ionic radii and high similarity in its electronic structure. This paper describes the factors that control REE signature in the fracture fillings of an ancient uranium mine (dug granite) located in the Central-Iberian Zone of the Hesperian Massif (Caceres, Spain). The study of REE distribution in the fracture fillings provides interesting information about the REE distribution in the hydrothermal fluids that flow through the system. In order to obtain this information, it is necessary to evaluate the control of different minerals in the total REE content of the fracture fillings. The fracture fillings display a large mineralogical variety dominated by K-feldspars, quartz, plagioclase, micas, monazites, xenotimes, apatites, uraninites, inherited of the granites, and carbonates, quartz, apatites, phyllosilicates, sulphides and Fe-oxihydroxydes, neoformed in successive hydrothermal stages. In general, the fracture fillings present different kind of REE patterns, just as positive or negative Eu anomalies. The REE final distribution in the fracture fillings is controlled by the presence of REE-minerals, both inherited and neoformed, and of carbonates, with great capacity to fractionate HREE in the system. On the other hand, the chemical composition of the carbonates and accessory minerals is responsible, as a last resort, for the Eu anomalie (positive or negative) observed in the fracture fillings.

Rare Earth Elements: A Tool for Understanding the Behaviour of Trivalent Actinides in the Geosphere

Rare earth element (REE) concentrations have been determined in groundwaters, granite and fracture fillings in a restored uranium mine. The granitoids normalized REE patterns of groundwaters show heavy rare earth elements (HREE)-enrichment and positive Eu anomalies. This suggests that the REE are fractionated during leaching from the source rocks by groundwaters. Preferential leaching of HREE would be consistent with the greater stability of their aqueous complexes compared to those of the light rare earth elements (LREE), together with the dissolution of certain fracture filling minerals, dissolution/alteration of phyllosilicates and colloidal transport. (authors)