Historical analysis (2000-2005) of the coastal water quality in San Andrés Island, SeaFlower Biosphere Reserve, Caribbean Colombia     

Brigitte Gavio  Shelly Palmer-Cantillo  J. Ernesto Mancera 《Marine pollution bulletin》2010,60(7):1018-1030

To understand the coastal water quality of San Andrès Island, and provide tools for the management of its marine resources, we present the historical analysis of the island monitoring, which includes ammonia, nitrites, nitrate, phosphates, fecal and total coliforms. The anthropogenic pressure on the coastal system is heavy, with water nutrification, posing at risk seagrass and coral ecosystems. During dry season, biologically available nitrogen is 3-9 times higher than the maximum recommended for coral reefs, while during wet season values are 2-6.4 times the maximum. Biologically available phosphorous is also high, 1-8 times the maximum during dry season, 2-13 times during wet season. In some sites the concentration of pathogenic bacteria is above the limits set by law for primary and secondary contact. It is urgent to improve the management of sewage discharge, the main polluting source of San Andres coastal waters.  相似文献   

Spatial and Temporal Dependence of Temperature Variations Induced by Atmospheric Pressure Variations in Shallow Underground Cavities     

Frederic Perrier  Jean-Louis Le Mouël  Patrick Richon 《Pure and Applied Geophysics》2010,167(3):253-276

Pressure-induced temperature (PIT) variations are systematically observed in the atmosphere of underground cavities. Such PIT variations are due to the compressibility of the air, damped by heat exchange with the rock surface. It is important to characterize such processes for numerous applications, such as the preservation of painted caves or the assessment of the long-term stability of underground laboratories and underground waste repositories. In this paper we thoroughly study the spatiotemporal dependence of the PIT response versus frequency using vertical and horizontal profiles of temperature installed in an abandoned underground quarry located in Vincennes, near Paris. The PIT response varies from about 20 × 10^?3°C hPa^?1 at a frequency of 2 × 10^?4 Hz to 2–3 × 10^?3°C hPa^?1 at a frequency of one cycle per day. An analytical expression based on a simple heat exchange model accounts for the observed features of the PIT response and allows for correcting the measured time series, having standard deviations of about 10^?2°C, to residual variations with a standard deviation of about 2 × 10^?3°C. However, a frequency-dependent attenuation of the response, corresponding to a reduction in amplitude with a factor varying from 2 to 3, is observed near the walls. This effect is not included in the simple analytical expression, but it can be accounted for by a one-dimensional differential equation, solved numerically, where temperature variations in the atmosphere are damped by an effective radiative coupling with the rock surface, complemented by a diffusive coupling near the walls. The TIP response is observed to remain stable over several years, but a large transient enhancement of about a factor of two is observed near the roof at one location from July to October 2005. In a cavity located below the Paris Observatory, an additional contribution is identified in the PIT response function versus frequency for frequencies smaller than 2 × 10^?5 Hz. This contribution can be described using a modified analytical expression that includes the effect of heat diffusion into the surrounding rock. Using this expression, in this case also, the temperature time series can then be corrected, giving a residual standard deviation smaller than 1.6 × 10^?3°C. Transient temporal variations of the PIT response are observed in all sites, with possible nonlinear components in the PIT. Such effects are not properly understood at this stage, and limit the reduction of time series to standard deviations of the order of 2 × 10^?3°C, and consequently limit the search for new transient or seasonal temperature signals, for example due to the presence of tiny heat sources in the cavity or to geodynamical effects.  相似文献   

Space- and Time-Dependent Probabilities for Earthquake Fault Systems from Numerical Simulations: Feasibility Study and First Results     

Jordan Van Aalsburg  John B. Rundle  Lisa B. Grant  Paul B. Rundle  Gleb Yakovlev  Donald L. Turcotte  Andrea Donnellan  Kristy F. Tiampo  Jose Fernandez 《Pure and Applied Geophysics》2010,167(8-9):967-977

In weather forecasting, current and past observational data are routinely assimilated into numerical simulations to produce ensemble forecasts of future events in a process termed “model steering”. Here we describe a similar approach that is motivated by analyses of previous forecasts of the Working Group on California Earthquake Probabilities (WGCEP). Our approach is adapted to the problem of earthquake forecasting using topologically realistic numerical simulations for the strike-slip fault system in California. By systematically comparing simulation data to observed paleoseismic data, a series of spatial probability density functions (PDFs) can be computed that describe the probable locations of future large earthquakes. We develop this approach and show examples of PDFs associated with magnitude M > 6.5 and M > 7.0 earthquakes in California.  相似文献   

Amorphous silica mobilization by inter‐rill erosion: insights from rainfall experiments          下载免费PDF全文

Wim Clymans  Eric Struyf  An Van den Putte  Christoph Langhans  Zhengang Wang  Gerard Govers 《地球表面变化过程与地形》2015,40(9):1171-1181

Amorphous silica (ASi) carried in suspension by rivers is an important component in the global Si budget. Water erosion processes in cultivated catchments are likely to drive ASi delivery to the river system. However, no studies have investigated the controls on ASi mobilization by erosional processes in croplands. Rainfall experiments were performed on split fields (i.e. a part conventionally ploughed and a part under reduced tillage) to simulate ASi mobilization by inter‐rill erosion in croplands, and identify its dependency on soil, field and rainfall characteristics. The ASi content of the soil and the inter‐rill erosion rate were determined as the major controls on ASi mobilization. Variables such as tillage technique and crop type did not have a consistent direct or indirect effect. Inter‐rill erosion is clearly selective with respect to ASi, indicating association of ASi with the fine soil fraction and with soil organic carbon. Our experiments demonstrate that erosion increases due to human perturbation will increase the delivery of reactive Si to aquatic systems. We estimate that globally, c. 7% of all reactive Si that enters aquatic systems is derived from erosion of agricultural soils. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Insights in the exhumation history of the NW Zagros from bedrock and detrital apatite fission‐track analysis: evidence for a long‐lived orogeny     

Stéphane Homke  Jaume Vergés  Peter Van Der Beek  Manel Fernàndez  Eduard Saura  Luis Barbero  Balazs Badics  Erika Labrin 《Basin Research》2010,22(5):659-680

We present the first fission‐track (FT) thermochronology results for the NW Zagros Belt (SW Iran) in order to identify denudation episodes that occurred during the protracted Zagros orogeny. Samples were collected from the two main detrital successions of the NW Zagros foreland basin: the Palaeocene–early Eocene Amiran–Kashkan succession and the Miocene Agha Jari and Bakhtyari Formations. In situ bedrock samples were furthermore collected in the Sanandaj‐Sirjan Zone. Only apatite fission‐track (AFT) data have been successfully obtained, including 26 ages and 11 track‐length distributions. Five families of AFT ages have been documented from analyses of in situ bedrock and detrital samples: pre‐middle Jurassic at ～171 and ～225 Ma, early–late Cretaceous at ～91 Ma, Maastrichtian at ～66 Ma, middle–late Eocene at ～38 Ma and Oligocene–early Miocene at ～22 Ma. The most widespread middle–late Eocene cooling phase, around ～38 Ma, is documented by a predominant grain‐age population in Agha Jari sediments and by cooling ages of a granitic boulder sample. AFT ages document at least three cooling/denudation periods linked to major geodynamic events related to the Zagros orogeny, during the late Cretaceous oceanic obduction event, during the middle and late Eocene and during the early Miocene. Both late Cretaceous and early Miocene orogenic processes produced bending of the Arabian plate and concomitant foreland deposition. Between the two major flexural foreland episodes, the middle–late Eocene phase mostly produced a long‐lasting slow‐ or nondepositional episode in the inner part of the foreland basin, whereas deposition and tectonics migrated to the NE along the Sanandaj‐Sirjan domain and its Gaveh Rud fore‐arc basin. As evidenced in this study, the Zagros orogeny was long‐lived and multi‐episodic, implying that the timing of accretion of the different tectonic domains that form the Zagros Mountains requires cautious interpretation.  相似文献   

On the repeatability and consistency of three-component ambient vibration array measurements   总被引：1，自引：0，他引：1  

Brigitte Endrun  Matthias Ohrnberger  Alexandros Savvaidis 《Bulletin of Earthquake Engineering》2010,8(3):535-570

Ambient vibration measurements with small, temporary arrays that produce estimates of surface wave dispersion have become increasingly popular as a low-cost, non-invasive tool for site characterisation. An important requirement for these measurements to be meaningful, however, is the temporal consistency and repeatability of the resulting dispersion and spatial autocorrelation curve estimates. Data acquired within several European research projects (NERIES task JRA4, SESAME, and other multinational experiments) offer the chance to investigate the variability of the derived data products. The dataset analysed here consists of repeated array measurements, with several years of time elapsed between them. The measurements were conducted by different groups in different seasons, using different instrumentations and array layouts, at six sites in Greece and Italy. Ambient vibration amplitude spectra and locations of dominant sources vary between the two measurements at each location. Still, analysis indicates that this does not influence the derived dispersion information, which is stable in time and neither influenced by the instrumentation nor the analyst. The frequency range over which the dispersion curves and spatial autocorrelation curves can be reliably estimated depends on the array dimensions (minimum and maximum aperture) used in the specific deployment, though, and may accordingly vary between the repeated experiments. The relative contribution of Rayleigh and Love waves to the wavefield can likewise change between repeated measurements. The observed relative contribution of Rayleigh waves is generally at or below 50%, with especially low values for the rural sites. Besides, the visibility of higher modes depends on the noise wavefield conditions. The similarity of the dispersion and autocorrelation curves measured at each site indicates that the curves are stable, mainly determined by the sub-surface structure, and can thus be used to derive velocity information with depth. Differences between velocity models for the same site derived from independently determined dispersion and autocorrelation curves—as observed in other studies—are consequently not adequately explained by uncertainties in the measurement part.  相似文献   

Towards spatially distributed quantitative assessment of tsunami inundation models     

John Davis Jakeman  Ole M. Nielsen  Kristy Van Putten  Richard Mleczko  David Burbidge  Nick Horspool 《Ocean Dynamics》2010,60(5):1115-1138

This paper presents a framework and data for spatially distributed assessment of tsunami inundation models. Our associated validation test is based upon the 2004 Indian Ocean tsunami, which affords a uniquely large amount of observational data for events of this kind. Specifically, we use eyewitness accounts to assess onshore flow depths and speeds as well as a detailed inundation survey of Patong City, Thailand to compare modelled and observed inundation. Model predictions matched well the detailed inundation survey as well as altimetry data from the JASON satellite, eyewitness accounts of wave front arrival times and onshore flow speeds. Important buildings and other structures were incorporated into the underlying elevation model and are shown to have a large influence on inundation extent.  相似文献   

Geoarchaeological prospection of a medieval manor in the dutch polders using an electromagnetic induction sensor in combination with soil augerings     

David Simpson  Alexander Lehouck  Marc Van Meirvenne  Jean Bourgeois  Erik Thoen  Jelier Vervloet 《Geoarchaeology》2008,23(2):305-319

In archaeological prospection, geophysical sensors are increasingly being used to locate buried remains within their natural context. To cover a large area in sufficient detail, an electromagnetic induction sensor can be very useful, measuring simultaneously the electrical conductivity and the magnetic susceptibility of the soil (e.g., Geonics EM38DD). In this study, an 8 ha field containing a Medieval manor was mapped in a submeter resolution, using a mobile sensor configuration equipped with a GPS. As different soil features can yield analogous responses, the interpretation of geophysical maps can be ambiguous. Therefore, soil auger observations were laid out along two perpendicular transects to provide vertical profiles across the sensor measurements. This information greatly enhanced the interpretation of the anomalies obtained by the sensor. Both natural and anthropogenic features were delineated, which clearly presented a moated site along a former tidal channel. © 2008 Wiley Periodicals, Inc.  相似文献   

Recovery scenarios for deep over-exploited aquifers with limited recharge: methodology and application to an aquifer in Belgium   总被引：2，自引：0，他引：2  

M. Van Camp  K. Walraevens 《Environmental Geology》2009,56(8):1505-1516

Recharge of deep-seated aquifers must be provided through leakage from overlying geological formations and can be very limited. Although hydraulic characteristics of these aquifers may be favorable for extensive exploitation, the renewability of the groundwater resource may be very restricted and only significant on long time scales. Over-exploitation of such aquifers leads to steadily declining piezometric levels and water balancing and steady state conditions are not to be expected on the short term. Recovery of such systems is very difficult, also because of the socio-economic dependencies of the water resource and the long time it requires to replenish the system. Management plans for restoration should be based on the transient hydrodynamical behaviour. A recovery plan should be developed based on results of a groundwater flow model. In a first step must be simulated how long it will take for the aquifer system to fully recover by natural recharge. As this will most often be a very long time it can be cancelled out as a realistic scenario. An alternative objective is to be formulated which could be achieved in the near future on a time scale of years or decades. Using model simulations, reduction schemes of exploitation rates shall be quantified that will allow to reach these defined goals, such as raising piezometric levels above the top of the aquifer itself or above the top of the overlying geological unit. The methodology is illustrated with an example of a deep hard rock aquifer in Belgium where piezometric levels have dropped below the top. The objective here was to raise the levels again above the top within the next 50 years. However, this is not accomplished by balancing aquifer recharge and discharge, but is reached by a lateral redistribution of the water over the aquifer extent. A full recovery of this aquifer would require at least a century with only a very limited amount of exploitation.  相似文献   

Sustainable groundwater extraction in coastal areas: a Belgian example   总被引：1，自引：0，他引：1  

Alexander Vandenbohede  Emmanuel Van Houtte  Luc Lebbe 《Environmental Geology》2009,57(4):735-747

Water extractions in coastal areas have to deal with salt water intrusion and lowering of hydraulic heads in valuable ecosystems. Therefore, sustainable management of fresh water resources in these areas is crucial. This is illustrated here with two water extractions in the western Belgian coastal plain which extract groundwater from a phreatic dune aquifer. One water extraction faced problems with salt water intrusion, while lowering of hydraulic heads was an issue for both. To remedy the salt water intrusion, it was found that decreasing the extraction rate was the only solution. To offset this and to increase hydraulic heads around both extractions, it was decided to artificially recharge the aquifer of the second extraction with tertiary treated wastewater. By taking these interventions, the combined production capacity of the water extractions was increased with 56% whereas 27% less water was extracted from the dune aquifer itself. Extraction history and the effects of interventions are illustrated for both water extractions with water quality data and fresh water head observations. A more detailed insight in groundwater flow and fresh–salt water distribution in the aquifer is provided by simulating the evolution of the water extractions with a 3D density dependent groundwater flow model.  相似文献