Isotope measurements and ground water flow modeling using MODFLOW for understanding environmental changes caused by a well field in semiarid Brazil     

Luiz Alberto Ribeiro Mendonça  Horst Frischkorn  Marlúcia Freitas Santiago  Josué Mendes Filho 《Environmental Geology》2005,47(8):1045-1053

Measurements of the environmental isotopes carbon-14 and oxygen-18 and of the electrical conductivity in ground water, together with MODFLOW-88 (DOS) simulations of pumping tests, were used to achieve a detailed understanding of the functioning of an aquifer in the area of a well field. The methodology was applied in the sedimentary basin of the Cariri region, in the south of Ceará State in Brazil. Localized hydraulic connections between adjacent aquifer units could be detected, and mixing ratios for the contributions from the units involved were determined. Cone of depression simulations for a three-year drought period revealed well interference and a drastic lowering of the piezometric level, thereby explaining the hydrogeologic changes and transformations in vegetation that were observed in the area of the well field. The combined use of the modeling tools and the geochemical field observations is shown to provide more detailed insight into the conceptual model of the groundwater flow system.  相似文献   

The densest meteorite collection area in hot deserts: The San Juan meteorite field (Atacama Desert,Chile)     

Jérôme GATTACCECA  Millarca VALENZUELA  Minoru UEHARA  A. J. Timothy JULL  Marlène GISCARD  Pierre ROCHETTE  Régis BRAUCHER  Clement SUAVET  Matthieu GOUNELLE  Diego MORATA  Pablo MUNAYCO  Michèle BOUROT‐DENISE  Didier BOURLES  François DEMORY 《Meteoritics & planetary science》2011,46(9):1276-1287

Abstract– We describe the geological, morphological, and climatic setting of the San Juan meteorite collection area in the Central Depression of the Atacama Desert (Chile). Our recovery activities yielded 48 meteorites corresponding to a minimum of 36 different falls within a 3.88 km² area. The recovery density is in the range 9–12 falls km^?2 depending on pairing, making it the densest among meteorite collection areas in hot deserts. This high meteorite concentration is linked to the long‐standing hyperaridity of the area, the stability of the surface pebbles (> Ma), and very low erosion rates of surface pebbles (approximately 30 cm Ma^?1 maximum). The San Juan meteorite population is characterized by old terrestrial ages that range from zero to beyond 40 ka, and limited weathering compared with other dense collection areas in hot desert. Chemical weathering in San Juan is slow and mainly controlled by the initial porosity of meteorites. As in the Antarctic and other hot deserts, there is an overabundance of H chondrites and a shortage of LL chondrites compared with the modern falls population, suggesting a recent (< few ka) change in the composition of the meteorite flux to Earth.  相似文献   

Relationship between hydrogeological parameters for data-scarce regions: the case of the Araripe sedimentary basin, Brazil   总被引：1，自引：0，他引：1  

Sávio de Brito Fontenele  Luiz Alberto Ribeiro Mendonça  José Carlos de Araújo  Maria Marlúcia Freitas Santiago  José Yarley de Brito Gonçalves 《Environmental Earth Sciences》2014,71(2):885-894

This paper applies and validates a method for generating spatially distributed hydraulic conductivity (k) based on the specific capacity (Q _s) for data-scarce regions. This method has been applied to the Araripe sedimentary basin, Brazil, and consists of four steps: (1) selection of (32) wells for which both k and Q _s data are available; (2) estimation of k as a function of Q _s for the (128) wells for which only specific capacity data are available; (3) spatial distribution of k using the kriging geostatistical tool; (4) validation of the method, using (17) representative wells with k measured data. The equation relating k and Q _s showed a statistically significant linear relationship (R = 0.93), from which a database has been generated using kriging with the spherical model. The results showed a calibration coefficient of Nash and Sutcliffe (NS) of 0.54 and moderate spatial dependence ratio of 69 %. The validation process provided only a moderate efficiency (NS = 0.22), possibly due to the geological complexity of the focus system. Despite its limitations, the method indicates the possibility of application of ordinary kriging to generate reliable data from auxiliary variables, especially for the water management of data-scarce areas.  相似文献   

Probing the relationship between surface waters and aquifers by <Superscript>18</Superscript>O measurements on the top of the Araripe Plateau/NE Brazil     

Luiz?Alberto?Ribeiro?Mendon?a  Horst?FrischkornEmail author  Maria?Marlúcia?Freitas?Santiago  Josué?Mendes Filho 《Environmental Geology》2004,46(2):295-302

The Araripe Plateau in northeastern Brazil has an area of about 8,000 km², confined by 39°05E and 40°55E, and 7°10S and 7°50S. Due to high permeability of soils, a surface drainage system is practically inexistent. Water is stored in excavations with clayey soil, the barreiros. Monthly samples were taken for ¹⁸O measurements, from September 1999 to August 2000, from four barreiros, three dug wells and five drilled wells. Results show that (1) groundwaters in the eastern part of the plateau are derived from present-day rainfall (¹⁸O–3.2), whereas groundwaters in the western portion are isotopically different (¹⁸O–5.0); (2) barreiros are strongly marked seasonally by elevated ¹⁸O during the dry period due to elevated evaporation; (3) a dug well at a distance of 30 m from a barreiro exhibits ¹⁸O similar to that of the reservoir, indicating a strong interaction between groundwater and surface water; and (4) a tubular well of 242-m depth, located in a fault, exhibits strong seasonal changes in ¹⁸O and electrical conductivity, revealing downward leakage between aquifers.  相似文献   

Hydrogeochemical and Flow Modeling of Aquitard Percolation in the Cariri Valley-Northeast Brazil     

Carlos José Freire Machado  Maria Marlúcia Freitas Santiago  Luiz Alberto Ribeiro Mendonça  Horst Frischkorn  Josué Mendes Filho 《Aquatic Geochemistry》2007,13(2):187-196

Flow modeling and hydrogeochemical inversion calculations were performed on the aquifer systems of the Crato-Juazeiro Graben in the Araripe Sedimentary Basin of the Cariri Valley, Northeast Brazil, in order to assess aquifer interconnections through an aquitard of the Araripe Plateau. Steady state hydraulic head numeric calculations with the finite difference method used Modflow software with hydrologically based boundary conditions and a piezometric surface as initial condition. Hydrogeochemical inversion with PHREEQC was performed for two water samples representing typical initial and final waters along the flow path. Flow simulation shows that approximately 20% of all recharge water from the superior aquifer system passes through the Santana aquitard reaching the middle aquifer system. Quantification of mineral dissolution/precipitation processes, ion exchange and microbiological redox reactions taking place in the aquifer systems also indicates hydraulic interconnection of the superior and middle aquifer systems through the Santana aquitard.  相似文献   

Diversity and functions of microscopic fungi: a missing component in pelagic food webs     

Marlène Jobard  Serena Rasconi  Télesphore Sime-Ngando 《Aquatic Sciences - Research Across Boundaries》2010,72(3):255-268

Fungi are a highly complex group of organisms of the kingdom Eumycota (i.e. the true-fungi) and other fungus-like organisms traditionally studied by mycologists, such as slime molds (Myxomycota) and oomycota (Straminopiles or Heterokonts). They constitute a significant proportion of the as yet undiscovered biota that is crucial in ecological processes and human well-being, through at least three main trophic modes: saprophytism, parasitism, or symbiosis. In addition to direct benefit (sources of antibiotics) or adverse effects (agents of disease), fungi can impact many environmental processes, particularly those associated with the decomposition of organic matter. They are present in almost all regions and climates, even under extreme conditions. However, studies have focussed mostly on economically interesting species, and knowledge of their diversity and functions is mainly restricted to soil, rhizosphere, mangrove, and lotic ecosystems. In this study, we review the diversity and potential functions of microscopic fungi in aquatic ecosystems, with focus on the pelagic environments where they often are regarded as allochthonous material, of low ecological significance for food-web processes. Recent environmental 18S rDNA surveys of microbial eukaryotes have (1) unveiled a large reservoir of unexpected fungal diversity in pelagic systems, (2) emphasized their ecological potentials for ecosystem functioning, and (3) opened new perspectives in the context of food-web dynamics. In spite of persisting methodological difficulties, we conclude that a better documentation of the diversity and quantitative and functional importance of fungi will improve our understanding of pelagic processes and biogeochemical cycling.  相似文献