Isotopic characterisation and dating of groundwater recharge mechanisms in crystalline fractured aquifers: example of the semi-arid Banabuiú watershed (Brazil).

Sustainable groundwater management implies a good knowledge of recharge processes, especially in areas with water deficit, like the semi-arid region of Banabuiú watershed (Ceará State, Northeast of Brazil). In this zone, phreatic aquifers consist of Precambrian crystalline fractured reservoirs characterised by a high spatial anisotropy, both in terms of hydrodynamics and water quality. This study implemented a multi-tracer approach (18O, 2H, 14C, 3H, CFC, SF6), combined with hydrodynamic data (i.e. groundwater levels) to identify the groundwater recharge origin and the recharge mechanisms, and to estimate the groundwater residence time. At the basin scale, hydrodynamic data and local observations indicated the high reactivity of aquifers to precipitation and suggested that infiltration processes occur mostly through preferential infiltration zones. Stable isotope data showed a major contribution of evaporated surface water in the recharge process from many artificial or natural ponds. Groundwater residence time determination highlighted the spatio-temporal heterogeneity of flow path organisation within aquifers, with variable contributions between fast vertical flow (present-day end-member; 15-85 %) and a slower horizontal flow (old end-member <1960), underlining the vulnerability of aquifers to present-day environmental stress or pollution.

Application of isotope techniques to enhance the conceptual hydrogeological model and to assess groundwater sustainability in the Pampean plain in Córdoba, Argentina.

The objective of this work is to enhance the conceptual hydrogeological model in the Río Cuarto River basin by using isotope and hydrochemical techniques. The precipitation pattern, as reflected in the average values of δ 2H and δ 18O in stations located in the plains and in the mountains, showed an isotope depletion from the East to the West, attributed to continental and altitude effects. Groundwater quality is mainly the result of two controlling factors: lithology and flow distances from recharge. The aquifers show fresh calcium/sodium bicarbonate water in the upper and medium basin (coarse fluvial sediments) which evolve to sodium sulphate and chloride waters in the low basin (mainly loess and fine alluvial sediments). The confined aquifer systems in the lower basin (C and D systems) averaged more negative stable isotope values, indicating that groundwater recharged during colder climatic conditions (Pleistocene period). Groundwater dating with 14C confirmed that groundwater ages range from modern to 45,000 years BP showing that as the water flows towards deeper layers and farther from the mountainous recharge area, groundwater age increases. The confined aquifers can potentially be exploited in order to partly cover different water needs but they should be managed in a sustainable way.

High spatial resolution analysis of Pb and U isotopes for geochronology by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

Age determinations using the 235U and 238U radioactive decay series to the daughter isotopes 207Pb and 206Pb, respectively, using the mineral zircon (ZrSiO4), are widely used to decipher geological processes. A new method developed in the last couple of years, the laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), overcomes previous laborious sample preparation, and yields isotopic ratios and age data with a high spatial resolution of ten of microns. The present study describes the analytical set-up and data reduction process as presently applied at the Laboratory for Geochronology of the University of Brasília. It explores the precision and accuracy of the method by cross-analysing three international zircon standards. We arrive at a precision of 1.9 to 3.7 percent (2σ SD) and an accuracy of 0.6 to 3.8 percent (2σ SD) for and U isotopic ratios of the standards. We also apply the method to two natural zircon samples, which have previously been dated by other analytical methods. A comparison of the results show a good conformity of the age data,being whitin the error limits. The data demonstrate the great analytical potential of the method for rapid, precise and accurate U-Pb isotopic analyses on the micron scale.

A determinação de idades através do decaimento dos isótopos radioativos 235U e 238U para os isótopos radiogênicos 207Pb e 206Pb, respectivamente, utilizando o mineral zircão (ZrSiO4), é amplamente aplicada para decifrar processos geológicos. Um novo método tem sido desenvolvido nos últimos anos, a ablação a laser multi-coletor espectrometria de massas com plasma indutivamente acoplado (LA-MC-ICP-MS), superando o laborioso trabalho anteriormente necessário em outros métodos, na preparação de amostras, e permite obtenção de razões isotópicas com alta resolução espacial de micrômetros. O presente estudo descreve os procedimentos analíticos e os métodos usados na redução de dados que estão sendo aplicados no Laboratório de Geocronologia da Universidade de Brasília. Explora-se a precisão e exatidão do método através da análise de três padrões internacionais de zircão. Observa-se uma precisão entre 1,9 a 3,7 por cento (2σ desvio padrão) e uma exatidão de 0,6 a 3,8 por cento (2σ desvio padrão) para as razões isotópicas de Pb e U dos padrões. Também foram obtidas idades pelo método LA-ICP-MS de duas amostras de zircões naturais, que já foram datadas anteriormente por outros métodos analíticos. A comparação dos resultados mostra uma boa conformidade das idades obtidas, dentro dos limites de erro.Os dados demonstram o grande potencial do método analítico para análises isotópicas rápidas, precisas e exatas de U-Pb, numa escala de micrômetros.