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41.
42.
The concentration of heavy metals and metalloids (As, Cd, Cr, Cu, Hg, Ni, Pb, Sb and Zn) in sediments from the River Llobregat and its tributaries (Anoia and Cardener) was studied. Samples collected at 17 locations during four different periods were analysed by ICP-MS. The heavy metal enrichment at some sites along the rivers reflects the effects of agricultural activities, sewage treatment plant effluents, collectors' discharges and industrial activities. Principal component analysis (PCA) was used to describe trends in contamination and to find groupings among the investigated areas. The Llobregat and Cardener sediments appeared to have features of an unpolluted river, even though significant amounts of domestic and industrial wastewater are discharged into these rivers. On the other hand, the sediments from the River Anoia showed high Cr and Hg levels originating from industrial activities. The concentrations of Cr and Hg ranged from 91–540 and 0.28–2.29 µg/g respectively. 相似文献
43.
Sean Y. Sol Lyndal L. Johnson Beth H. Horness Tracy K. Collier 《Marine pollution bulletin》2000,40(12):1139-1147
Following the Exxon Valdez oil spill (EVOS) in 1989, the effect of oil exposure on reproductive parameters was investigated in wild populations of female dolly varden, yellowfin sole, and pollock. Exposure to oil was the highest in the first year of sampling and decreased in the subsequent years of sampling. Both positive and negative correlations between reproductive parameters and indices of exposure were detected in varying degrees for all three species. Reproductive parameters were not measured in the first year of sampling. For those parameters found to be significantly correlated to oil exposure, an exponential regression function was used to project reproductive parameters of fish sampled in the first year of the spill. A higher proportion of dolly varden sampled in 1989 were projected to have depressed plasma estradiol-17β compared to the fish sampled in 1990. 相似文献
44.
Beatriz Modenutti Esteban Balseiro Marcela Bastidas Navarro Cecilia Laspoumaderes María Sol Souza Florencia Cuassolo 《Aquatic Sciences - Research Across Boundaries》2013,75(3):361-371
The North-Patagonian Andean lakes of Argentina are high light, low nutrient environments that exhibit development of deep chlorophyll maxima (DCM) at the metalimnetic layer during summer stratification, at approximately 1 % of surface PAR irradiance. We examined whether the position of DCM changes as a consequence of long-time (global warming: glacial clay input) and short-time (eruption: volcanic ashes) events. We performed different field studies: (1) an interlacustrine analysis of six lakes from different basins, including data of the 2011 volcanic eruption, which caused an unexpected variation in water transparency; and (2) an intralacustrine analysis in which we compared different stations along a transparency gradient in Lake Mascardi caused by glacial clay input at one end of the gradient. In these analyses, we documented changes in DCM depth and its relationship with different parameters. DCM development was not related with thermocline depth or nutrient distribution. In all cases, the only significant variables were Kd 320 nm and Kd PAR. Our study showed that suspended particles (glacial clay and volcanic ashes) can play a crucial role in transparent lakes, affecting lake features such as the phototrophic biomass distribution along the water column. Suspended solid inputs from either glacial clay or volcanic ashes produce a comparable effect, provoking a decrease in light and, consequently, an upper location of the DCM. Thus, the DCM position is highly sensitive to global changes, such as increased temperatures causing glacier recession or to regional changes caused by volcanic eruptions. 相似文献
45.
This study explores the variations in household participation rates in recycling projects implemented in urban and suburban communities. A causal model is developed to compare the demographic attributes of residents, their awareness of the recycling programs, their environmental attitudes, and their overall willingness to recycle. The model is assessed by Linear Structural Equations Methodology (LISREL). The results suggest that household recycling behavior is not consistent across communities due primarily to variations in demographic characteristics of residents and their evaluation of program policies. 相似文献
46.
Solute transport in heterogeneous media: The impact of anisotropy and non-ergodicity in risk assessment 总被引:1,自引:1,他引:0
X. Sánchez-Vila J. Solís-Delfín 《Stochastic Environmental Research and Risk Assessment (SERRA)》1999,13(5):365-379
Conceptual model selection is a key issue in risk assessment studies. We analyze the effect of a number of conceptual aspects
related to solute transport in two-dimensional heterogeneous media. The main issues addressed are non-ergodicity, anisotropy
in the correlation structure of the transmissivity field, and dispersion at the local scale. In particular, we study the development
of a solute plume when mean flow is oriented at an angle with respect to the principal directions of anisotropy. The study
is carried out in a Lagrangian framework using Monte Carlo analysis.
Of special interest is the evolution of individual plumes. A number of aspects are analyzed, namely the location of the center
of mass for each plume and the different ways to compute the angles that the main axes of the plume develop with respect to
the direction of the mean flow. Stochastic theories based upon ergodicity conclude that the plume gets oriented in the mean
flow direction. In our non-ergodic simulations, the mean of the offset angles, for each individual plume in each particular
realization, is offset from the mean flow direction towards the direction of maximum anisotropy. If, instead, the analysis
is performed on the ensemble plume (superposition of all different simulations), it is then found oriented closer to the direction
of the mean flow than the average offset angle for the different plumes considered separately. This last result adds an extra
word of caution to the use of ensemble averaged values in solute transport studies.
Serious implications for risk assessment follow from the conceptual model adopted. First, in any single realization there
will a large uncertainty in locating the plume at any given time; second, real dilution would be less than what would be expected
if the macrodispersion values obtained for ergodic conditions were applied; third, the volume that is affected by a non-zero
concentration is smaller than that predicted from macrodispersion concepts; fourth, the orientation of the plume does not
correspond to that of the mean flow; and fifth, accounting for local dispersion helps reducing uncertainty. 相似文献
47.
Recent results of the gamma-ray Cherenkov astronomy definitely prove the existence of fast variability in the very high energy
(V.H.E.) gamma-ray flux of some active galactic nuclei. The BL Lac PKS 2155-304 for instance showed variations down to a few
minutes time scale. From standard light travel time argument, these variations put extremely strong constraints on the size
of the TeV emitting zone, which has to be of the order of a few Schwarzschild radius, even for high values of the relativistic
Doppler factor of the emitting jets. Such discovery is a challenge for particle acceleration scenarios, which have to imagine
efficient acceleration processes at work in a very compact zone. Eventually, the immediate vicinity of the central black hole
appears as the most conservative choice for the location of the TeV emission region of active galactic nuclei. In this paper,
we propose a two-step mechanism for charged particle acceleration in the magnetosphere of a massive black hole surrounded
by an accretion disk. Particles first gain energy by a stochastic process during the accretion phase. It is shown that effective
proton acceleration up to energies 1017–1019 eV is possible in a low-luminosity magnetized accretion disk with 2D turbulent motion. The distribution function of energetic
protons over energies is a power law function with typical index ≃−1. Here electrons are not very efficiently accelerated
because of their drastic losses by synchrotron radiation. In a second time, part of the fast particles escape from the disk
and are then entrained by the magnetic structure above the disk, in the rotating black hole magnetosphere. They thus gain
additional energy by direct centrifugal mechanism, up to about 1020 eV for the protons and to 10–100 TeV for the electrons when they cross the light cylinder surface. Such energetic particles
can further radiate in the TeV spectral range observed by Cherenkov experiments as HESS, MAGIC and VERITAS. Energetic protons
can produce γ-radiation in the energy band 1 GeV–100 TeV and above mainly by nuclei collisions with the disk matter, clouds, or ambient
low energy photons. Energetic electrons can also reach the required spectral range by inverse Compton emission. However their
acceleration is less efficient due to heavy radiation losses, and only gained by centrifugal process during the second phase
of the whole mechanism we describe. Our present analysis would therefore favor hadronic scenarios for TeV emission of active
galactic nuclei. It is tempting to relate long term variability over years of TeV active galactic nuclei to the first stochastic
acceleration phase, which also provides the needed power law particle distributions, while short term variability over minutes
is more likely due to perturbations of the second fast direct acceleration phase. 相似文献
48.
49.
Jesús Solé Michael Cosca Zachary Sharp Pere Enrique 《International Journal of Earth Sciences》2002,91(5):865-881
The cooling history of Hercynian calc-alkalic, post-kinematic plutonic intrusions of the Montnegre massif (NE Spain) has been determined by 40Ar/39Ar analysis of two hornblendes, four biotites and eight K-feldspars (Kfs). The hornblendes have 40Ar/39Ar total fusion ages of 291Dž Ma and define the magmatic cooling of the basic and oldest structural intrusions. The biotites from the acid and intermediate rocks have 40Ar/39Ar plateau ages of 285Dž Ma, which date cooling of the intrusions through argon closure in biotite. The 40Ar/39Ar ages of the K-feldspars vary widely, ranging from 276-191 Ma. A correlation between K-feldspar 40Ar/39Ar total fusion age and several other features such as structural state, microstructures indicated by obliquity and, to some degree, optically visible perthites, is consistent with post-crystallisation partial argon loss in the K-feldspars. The 'D values of the biotites also correlate with age and chlorite contents, but this is not so for the '18O values of either feldspar or quartz. We infer that most microtextural changes occurred during cooling of the batholith, but a possibly Mesozoic, late disturbing hydrothermal event of weak intensity and with only minor fluid circulation must have occurred. This event provoked significant argon loss in the most structurally complex K-feldspars and is recorded in the hydrogen, but not oxygen, isotope data. 相似文献
50.
Fabian Gutierrez-Aguilar Peter Schaaf Gabriela Solís-Pichardo Gerardo F.Arrieta-García Teodoro Hernandez-Trevino Carlos Linares-Lopez 《地学前缘(英文版)》2021,12(1):293-312
The Yelapa-Chimo Metamorphic Complex forms part of the Jalisco Block in western Mexico and exposes a wide range of Early Cretaceous metamorphic rocks;such as paragneiss,orthogneiss,amphibolites,and migmatites.However,the pressure-temperature(P-T)conditions of metamorphism and partial melting remain poorly studied in the region.To elucidate metamorphic P-T conditions,phase equilibrium modelling was applied to two sillimanite-garnet paragneisses,one amphibole-orthogneiss,and one amphibolite.Sillimanite-garnet paragneisses exhibit a lepidoblastic texture with a biotite+sillimanite+kyanite+garnet+quartz+plagioclase+K-feldspar mineral assemblage.Amphibole-orthogneiss and amphibolite display a nematoblastic texture with an amphibole+(1)plagioclase+quartz+(1)titanite assemblage and an amphibole+(2)plagioclase+(2)titanite+ilmenite retrograde mineral assemblage.Pseudosections calculated for the two sillimanite-garnet paragneiss samples show P-T peak conditions at~6-7.5 kbar and~725-740℃.The results for amphibole-orthogneiss and the amphibolite yield P-T peak conditions at~8.5-10 kbar and~690-710℃.The mode models imply that metasedimentary and metaigneous units can produce up to~20 vol%and~10 vol%of melt,respectively.Modelling within a closed system during isobaric heating suggests that melt compositions of metasedimentary and metaigneous units are likely to have direct implications for the petrogenesis of the Puerto Vallarta Batholith.Our new data indicate that the Yelapa-Chimo Metamorphic Complex evolved through a metamorphic gradient between~23-33℃km^-1and the metamorphic rocks formed at depths between~22 km and~30 km with a burial rate of~2.0 km Ma^-1.Finally,the P-T data for both metasedimentary and metaigneous rocks provide new constraints on an accretionary framework,which is responsible for generating metamorphism and partial melting in the YelapaChimo Metamorphic Complex during the Early Cretaceous. 相似文献