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961.
Arsenic concentrations exceeding the U.S. EPA's 10 μg/L standard are common in glacial aquifers in the midwestern United States. Previous studies have indicated that arsenic occurs naturally in these aquifers in association with metal-(hydr)oxides and is released to groundwater under reducing conditions generated by microbial oxidation of organic matter. Despite this delineation of the arsenic source and mechanism of arsenic mobilization, identification of arsenic-impacted aquifers is hindered by the heterogeneous and discontinuous nature of glacial sediments. In much of the Midwest, the hydrostratigraphy of glacial deposits is not sufficiently characterized to predict where elevated arsenic concentrations are likely to occur. This case study from southeast Wisconsin presents a detailed characterization of local stratigraphy, hydrostratigraphy, and geochemistry of the Pleistocene glacial deposits and underlying Silurian dolomite. Analyses of a single core, water chemistry data, and well construction reports enabled identification of two aquifers separated by an organic-rich aquitard. The upper, unconfined aquifer provides potable water, whereas arsenic generally exceeds 10 μg/L in the deeper aquifer. Although coring and detailed hydrostratigraphic characterization are often considered impractical, our results demonstrate that a single core improved interpretation of the complex lithology and hydrostratigraphy. This detailed characterization of hydrostratigraphy facilitated development of well construction guidelines and lays the ground work for further studies of the complex interactions among aquifer sediments, hydrogeology, water chemistry, and microbiology that lead to elevated arsenic in groundwater. 相似文献
962.
Ghylaine Quitt Eric Robin Sylvain Levasseur Franoise Capmas Robert Rocchia Jean‐Louis Birck Claude Jean AllGre 《Meteoritics & planetary science》2007,42(9):1567-1580
Abstract— It is now established that a large extraterrestrial object hit the Earth at the end of the Cretaceous period, about 65 Ma ago. We have investigated Re‐Os, Hf‐W, and Mn‐Cr isotope systems in sediments from the Cretaceous and the Paleogene in order to characterize the type of impactor. Within the Cretaceous‐Tertiary (K‐T) boundary layer, extraterrestrial material is mixed with terrestrial material, causing a dilution of the extraterrestrial isotope signature that is difficult to quantify. A phase essentially composed of Ni‐rich spinel, formed in the atmosphere mainly from melted projectile material, is likely to contain the extraterrestrial isotopic signature of the impactor. We show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor, and that W and Cr isotopes confirm that the projectile was a carbonaceous chondrite. 相似文献
963.
Abstract Current understanding of the possible nature of climatic change at the regional scale is limited by the spatial resolution of General Circulation Models (GCM). The use of GCM outputs without correction linked to the spatial variability of the variables can bring significant errors in their utilization at the regional scale. The potential of the Canadian GCM for regional applications in Quebec has been analysed by comparison to the climatic normals of temperature and precipitation, measured over the Quebec climatological network, on an annual and seasonal basis. This analysis has been undertaken with the support of a geographical information system (GIS) (PAMAP). In summary, a difference between the climatic normal and the GCM output has been estimated at 20% for temperature and 30% for precipitation. We present an analysis of a corrected regionalized scenario for the province of Quebec of the possible climatic change simulated by the Canadian GCM under the hypothesis of a doubling of atmospheric CO2. Results show an increase of the annual average temperature of 4° C for summer and 6°C for winter, associated with an average increase of 80 mm (10%) in annual precipitation, reaching 25% in some regions. 相似文献
964.
Poroelastic two‐phase material modeling: theoretical formulation and embedded finite element method implementation 下载免费PDF全文
Nathan Benkemoun Rachel Gelet Emmanuel Roubin Jean‐Baptiste Colliat 《国际地质力学数值与分析法杂志》2015,39(12):1255-1275
This paper presents the formulation of FEMs for the numerical modeling of a poroelastic two‐phase (aggregates/mixture phase) solid. The displacement and pressure fields are decomposed, following the Enhanced Assumed Strain (EAS) method, into a regular part and an enhanced part. This leads to discontinuous strain and pressure gradient fields allowing to capture the jump in mechanical and hydrical properties passing through the interface between the aggregates and the mixture phase. All these enhanced fields are treated in the context of the embedded FEM through a local enhancement of the finite element interpolations as these jumps appear. The local character of these interpolations leads after a static condensation of the enhanced fields to a problem exhibiting the same structure as common poroelastic finite element models but incorporating now the mechanical and hydrical properties of a two‐phase solid. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
965.
We challenge the former interpretation of the ‘sedimentary mélange’ of the Makran accretionary complex as a tectonic mélange diapirically emplaced from below and provide evidence for its sedimentary gravitational emplacement from the north during Tortonian–Messinian times (between 11.8 and 5.8 Ma). It is an olistostrome that includes blocks of ophiolites and oceanic sediments derived from the ‘coloured mélange’ to the north, and reworked chunks of the turbidites on which it rests with an erosional truncation. The chaotic scattering of blocks of any size and lithology and the weak, soft‐sediment deformation of the matrix argue against a tectonic emplacement of the chaotic formation. Its size and internal structure, together with the size of the individual blocks, make the olistostrome a fossil equivalent of the large, gravitationally emplaced debris flows observed along present‐day continental margins and unstable volcanic edifices. 相似文献
966.
An accurate simulation of snowmelt runoff is of much importance in arid alpine regions. Data availability is usually an obstacle to use energy‐based snowmelt models for the snowmelt runoff simulation, and temperature‐based snowmelt models are more appealing in these regions. The snow runoff model is very popular nowadays, especially in the data sparse regions, because only temperature, precipitation and snow cover data are required for inputs to the model. However, this model uses average temperature as index, which cannot reflect the snowmelt simulation in the high altitude band. In this study, the snow runoff model is modified on the basis of accumulated active temperature. Snow cover calculation algorithm is added and is no longer needed as input but output. This makes the model able to simulate long‐time runoff and long‐time snow cover variation in every band. An examination of the improved model in the Manas River basin showed that the model is effective. It can reproduce the behaviour of the hydrology and can reflect the actual snow cover fluctuation. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
967.
Catherine Kuzucuoğlu Aurélien Christol Damase Mouralis Ali‐Fuat Doğu Ebru Akköprü Monique Fort Daniel Brunstein Halil Zorer Michel Fontugne Mustafa Karabiyikoğlu Stéphane Scaillet Jean‐Louis Reyss Hervé Guillou 《第四纪科学杂志》2010,25(7):1124-1137
Sedimentological and geomorphological studies of terraces around Lake Van (1647 m) provided a preliminary framework for lake‐level variations. The elevations of terraces and past lake level were measured with a differential global positioning system. A chronology is developed using 234U/230Th dating of travertines, 39Ar/40Ar dating of pyroclastites and 14C dating of organic matter. Facies and stratigraphic correlations identify four transgressions (C1′, C1″, C2′ and C2″), each followed by a regression which ended with low lake levels that caused river incision and terrace formation. Evidence of the oldest transgression (C1′) is found in the uppermost reaches of valleys up to 1755 m, an altitude higher than the present lake threshold (1736 m). This C1′ transgression may be related to pyroclastic flows which dammed an outlet located in the western part of the lake basin and which is dated to before 105 ka. After 100 ka, a second transgression (C1″) reached 1730/1735 m, possibly related to a younger ignimbrite flow, in association with high water inflow (warm and/or wetter conditions). The two younger transgressions reached 1700–1705 m. The first one (C2′) is dated to 26–24.5 cal. ka BP and the second one (C2″) to 21–20 cal. ka BP. Available data suggest that the long‐term lake‐level changes responded mainly to climate oscillations. Additional events such as river captures caused by volcanic falls filling valleys, tectonism, erosion and karstic diversion may have impacted these long‐term lake‐level changes. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
968.
Behavior of Niobium, Tantalum and other high field strength elements in adakites and related lavas from The Philippines 总被引:3,自引:0,他引:3
Gaëlle Prouteau René C. Maury Fernando G. Sajona Joseph Cotten and Jean Cotten 《Island Arc》2000,9(4):487-498
Abstract Pliocene–Quaternary adakites and spatially and temporally associated niobium-enriched basalts (the latter thought to be derived by melting of slab melt-metasomatized mantle) from the Philippine island arcs have been selected for analysis of high field strength elements (HFSE). All these samples have nearly constant and chondritic Zr/Hf ratio (36.5) and slightly infrachondritic Nb/Ta ratio (14.7). We interpret adakitic magmas to be derived from the partial melting at approximately 900°C of subducted mid-ocean ridge basalts (MORB) crust, with rutile and/or ilmenite as residual minerals. Melting calculations show that, under these conditions, slab melts should have suprachondritic Nb/Ta ratios. The obvious discrepancy with our data is attributed to insufficient knowledge of rutile-melt partition coefficients for HFSE. Consequently, abnormal Nb/Ta or Zr/Hf ratios cannot be considered as potential markers of slab melting processes in island arcs. 相似文献
969.
On aggregation in spatial econometric modelling 总被引:1,自引:1,他引:0
Jean H.P. Paelinck 《Journal of Geographical Systems》2000,2(2):157-165
The spatial aggregation problem – also termed the modifiable areal unit problem – has attracted regular attention in spatial
statistics and econometrics. In this study econometric aggregation analysis is used to investigate the formal composition
of meso-areal parameters given micro-areal underlying relations with spatial dependence. Impact on stochastic terms (possible
meso-areal spatial autocorrelation) is also studied. Finally consequences for meso-areal estimation are derived, the general
finding having been that spatial aggregation leads to meso-region specific parameter values, with the estimation problems
this implies. 相似文献
970.
四川盆地南缘早三叠世古地磁研究表明,川东和川东南地区一系列北北东、北东向褶皱的形成,并非同造山期形成的弧形弯曲构造,它是受深部断裂控制形成的简单褶皱。这一结果再次表明,华北与扬子地块的拼合晚于早三叠世,可能于早/中侏罗世完成拼合过程。 相似文献