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1.
The pattern of ground contamination across a site depends on the historical pattern of contaminant releases at the surface and the redistribution and the fate of contaminants below the surface. Using these concepts a new site assessment approach (assessment modelling) is proposed based on the development of three stochastic models: a model of the physical structure of the ground materials beneath the site; a model of the distribution of surface contaminant spills; and a model of the flow and transport of spilled material into the heterogeneous underlying ground to construct alternative, equally likely, present day contaminant distributions. Combining the models within a Monte Carlo framework can, in principle, improve the understanding of the potential for excess contamination across the site and improve decisions on remediation options and locations. A trial application has been undertaken in the UK using a particular site to assess the approach. The conditions at the site used for the trial and the first of the stochastic model developments, the geostatistical modelling of the soil heterogeneity, are presented in this paper. Non-parametric and parametric geostatistics have been employed to formulate the geostatistical models of the site soils using lithological information from 146 trial pits and boreholes. The approach to the soil modelling and the verification and validation of the results are described. The heterogeneity of the subsurface is complicated by the presence of made-ground, comprised of various inert building wastes, and the non-stationarity of the heterogeneity of the natural ground. This paper is the first of three describing the assessment modelling methodology and its trial application to the site.  相似文献   

2.
Effective long‐term operation of soil vapor extraction (SVE) systems for cleanup of vadose‐zone sources requires consideration of the likelihood that remediation activities over time will alter the subsurface distribution and configuration of contaminants. A method is demonstrated for locating and characterizing the distribution and nature of persistent volatile organic contaminant (VOC) sources in the vadose zone. The method consists of three components: analysis of existing site and SVE‐operations data, vapor‐phase cyclic contaminant mass‐discharge testing, and short‐term vapor‐phase contaminant mass‐discharge tests conducted in series at multiple locations. Results obtained from the method were used to characterize overall source zone mass‐transfer limitations, source‐strength reductions, potential changes in source‐zone architecture, and the spatial variability and extent of the persistent source(s) for the Department of Energy's Hanford site. The results confirmed a heterogeneous distribution of contaminant mass discharge throughout the vadose zone. Analyses of the mass‐discharge profiles indicate that the remaining contaminant source is coincident with a lower‐permeability unit at the site. Such measurements of source strength and size as obtained herein are needed to determine the impacts of vadose‐zone sources on groundwater contamination and vapor intrusion, and can support evaluation and optimization of the performance of SVE operations.  相似文献   

3.
Nearly all studies of site response under seismic excitation employ idealized site models in which uniform layers of homogeneous soils are separated by perfect interfaces. The impedance mismatch at each interface gives rise to wave reflections which strongly affect the calculated free surface response. In reality, these perfect interfaces may often not exist. Instead, a zone in which materials from both above and below are mingled may form a blurred interface between layers. This article examines the effects of such weathered zones. The zone is modelled by a linearly varying shear modulus so that no abrupt impedance change occurs. Significant differences in response of weathered and unweathered profiles are found.  相似文献   

4.
Amplitudes of seismic waves increase significantly as they pass through soft soil layers near the earth's surface. This phenomenon, commonly known as site amplification, is a major factor influencing the extent of damage on structures. It is crucial that site amplification is accounted for when designing structures on soft soils. The characteristics of site amplification at a given site can be estimated by analytical models, as well as field tests. Analytical models require as inputs the geometry of all soil layers from surface to bedrock, their dynamic properties (e.g. density, wave velocity, damping), and the incident bedrock motions. Field tests involve recording and analyzing the dynamic response of sites to artificial excitations, ambient forces, and actual earthquakes. The most reliable estimates of site amplification are obtained by analyzing the recorded motions of the site during strong earthquakes. This paper presents a review of the types and the generating mechanisms of site amplification, and the models and methods that are used to characterize them from earthquake records.  相似文献   

5.
Based on the studies of earthquake activity, tectonic movement, crustal shortening rate, fault activity, local stress field and historical characteristics of strong earthquake activities in Xinjiang, we divide the south part of Xinjiang into 4 seismotectonic zones, namely, the eastern segment of south Tianshan seismic belt, the Kalpin block, the Kashi-Wuqia junction zone, and the west Kunlun Mountains seismic belt. Using earthquake catalogues from Xinjiang since 1900, and on the basis of integrity analysis of earthquake records in different magnitude ranges, the seismicity state of different seismotectonic zones is analyzed quantificationally by calculating the mean value of annual strain energy release, annual rate of earthquakes with different lower limits of magnitude, b-value, and the parameter m of accelerating strain release model. The characteristic indexes of seismicity state for each of the seismic tectonic zones are then determined, which provide a quantitative basis for earthquake tendency analysis and judgment.  相似文献   

6.
Groundwater flows and travel times were analyzed under a landfill site characterized by a thick unsaturated zone, in an arid environment, using two-dimensional models. Transport in the unsaturated zone is important in arid environments with thick vadose zones and usually determines the travel time of groundwater from the landfill site to the regulatory compliance surface. The combined use of a parallel flow (Dupuit-Forchheimer) areal model and vertical cross-sectional model is explored at a site typical in the utility industry. Flow paths and travel times to the compliance zone are obtained for use in landfill site design or environmental impact assessments. Sensitivity analysis shows that boundary conditions are important factors influencing the entire analysis and thus need to be confirmed with field measurements.  相似文献   

7.
Like tree rings, high‐resolution soil sampling of low‐permeability (low‐k) zones can be used to evaluate the style of source history at contaminated sites (i.e., historical pattern of concentration and composition vs. time since releases occurred at the interface with the low‐k zone). This is valuable for the development of conceptual site model (CSM) and can serve as an important line of evidence supporting monitored natural attenuation (MNA) as a long‐term remedy. Source histories were successfully reconstructed at two sites at Naval Air Station Jacksonville using a simple one‐dimensional (1D) model. The plume arrival time and historical composition were reconstructed from the time initial releases that were suspected to occur decades earlier. At the first site (Building 106), the source reconstructions showed relatively constant source concentrations, but significant attenuation over time in the downgradient plume in the transmissive zone, suggesting MNA may not be an appropriate remedy if source control is a requirement, but attenuation processes are clearly helping to maintain plume stability and reduce risk. At the second site (Building 780), source concentrations in the transmissive zone showed an approximately a one order of magnitude over time, but apparently less attenuation in the downgradient plume. The source reconstruction method appeared to reflect site remediation efforts (excavation, soil vapor extraction) implemented in the 1990s. Finally, a detailed analysis using molecular biological tools, carbon isotopes, and by‐products suggests that most degradation activity is associated with high‐k zones but not with low‐k zones at these source areas. Overall, the source reconstruction methodology provided insight into historical concentration trends not obtainable otherwise given the limited long‐term monitoring data.  相似文献   

8.
Estimation of Site Effects in Beijing City   总被引：6，自引：0，他引：6  
— For the realistic modeling of the seismic ground motion in lateral heterogeneous anelastic media, the database of 3-D geophysical structures for Beijing City has been built up to model the seismic ground motion in the City, caused by the 1976 Tangshan and the 1998 Zhangbei earthquakes. The hybrid method, which combines the modal summation and the finite-difference algorithms, is used in the simulation. The modeling of the seismic ground motion, for both the Tangshan and the Zhangbei earthquakes, shows that the thick Quaternary sedimentary cover amplifies the peak values and increases the duration of the seismic ground motion in the northwestern part of the City. Therefore the thickness of the Quaternary sediments in Beijing City is the key factor controling the local ground effects. Four zones are defined on the base of the different thickness of the Quaternary sediments. The response spectra for each zone are computed, indicating that peak spectral values as high as 0.1 g are compatible with past seismicity and can be well exceeded if an event similar to the 1697 Sanhe-Pinggu occurs.  相似文献   

9.
Plovdiv is the second largest city in the Republic of Bulgaria. A large part of the city is located on Holocene alluvial sediments and the oldest neighborhoods are situated on syenitic rock outcrops. We believe that local site effects may be an important contributor to the destruction caused by earthquakes. The primary objective of this study was to estimate quantitatively the local site effects in the central area of Plovdiv in terms of fundamental site frequency and amplification factor. Another important objective was to see how these correlate with the geological structures underlying the city. Measurements of the seismic noise at more than two hundred regularly placed points were made in the central area of the city. The H/V spectra were then calculated and analyzed to determine the spatial distribution of the fundamental site frequency and the amplification factor. The results exhibit very good correlation with the local geology. They were also compared with an intensity map from the strong 1928 Plovdiv earthquake. The comparison clearly demonstrates that the local site effects were the main factor in the destruction of buildings—the zones where the most damage was observed are also the zones where we have low fundamental site frequencies and high amplifications. Similarly the areas with high fundamental site frequencies and low amplification factors cover the neighborhoods where less damage has been observed. This study may form a basis for a more comprehensive and systematic microzonation study in Plovdiv.  相似文献   

10.
In this paper, a numerical sensitivity analysis of the site effect on dynamic response of pipelines embedded in some idealised soil deposits resting on a halfspace covering a wide range of soil profiles encountered in practice and subjected to vertically propagating shear waves, is presented. The power spectrum of the lateral differential displacement between two distant points due to the site effect is formulated analytically by using an analytical amplification function of a viscoelastic inhomogenous soil profile overlying either a compliant halfspace or a bedrock, represented by a more realistic continuous model. Also, Kanai-Tajimi spectrum parameters are estimated and expressed analytically from the soil profile model. Finally, results in the form of stochastic response spectrum of pipelines, for different key soil and pipeline parameters, are given and discussed.  相似文献   

11.
INTRODUCTIONThe Zhangjiakou-Penglai fault zone has drawnextensive attentionfromseismologists and geologistssince it was determinedinthe1980’s(Zheng Binghua,et al.,1981).Ma Xingyuan,et al.(1989)consideredit asthe north boundaryof North China sub-block.Int…  相似文献   

12.
Site response analyses must take into account the nonlinear behavior of soils. This is typically achieved using an equivalent linear approach or using numerical analyses with an appropriate constitutive model. In this work a family of hypoplastic models is proposed for use in site response analyses. These nonlinear models use a rate-type tensorial equation and are capable of reproducing plastic deformation of soils for cyclic loading under both drained and undrained conditions. A methodology for the calibration of hypoplastic parameter for dynamic loading is proposed. The hypoplastic constitutive models are implemented in a finite element code and the site response of the Lotung downhole array site is used to validate the use of hypoplastic models for site response analysis. The hypoplastic models reproduce accurately site response at the Lotung site. The advantages and disadvantages of the hypoplastic models compared to other models are discussed.  相似文献   

13.
Cleanup standards for volatile organic compounds in thick vadose zones can be based on indirect risk (transport to ground water) when contamination is below depths of significant direct risk. At one Arizona Superfund site, a one-dimensional vadose zone transport model (VLE-ACH) was used to estimate the continued transport of VOCs from the vadose zone to ground water. VLEACH is a relatively simple and readily available model that proved useful for estimating indirect risk from VOCs in the vadose zone at this site. The estimates of total soil concentrations used as initial conditions for VLF.ACH incorporated a variety of data from the site. Soil gas concentrations were found to be more useful than soil matrix data for estimating total soil concentrations at this arid-zone site. A simple mixing cell model was used with the VLEACH-derived mass loading estimates from the vadose zone over time to estimate the resulting changes in ground water concentrations. For this site, the results of the linked VLEACH/mixing cell simulations indicate it is likely that the federal MCI. for TCE will be exceeded in underlying ground water if remedial action on I he vadose zone is not pursued.  相似文献   

14.
Dye dispersion in the surf zone: Measurements and simple models   总被引：1，自引：0，他引：1  
To examine the spatial and temporal effect of low-volume land-based runoff on beach contamination, discrete batches of dye were released at the shoreline at three beaches in Santa Monica Bay in 2000 (Malibu Creek, Santa Monica Canyon and Pico–Kenter drain). Dye concentration was measured at the shoreline 25, 50 and 100 m alongshore from the dye release point for up to 40 min after dye release. The shoreline concentration time series are characterized either by approximately exponential decay in concentration after passage of the dye patch maximum concentration or by persistent low concentration up to 30 min after passage of the initial dye patch front. In the absence of detailed measurements of physical conditions, several simple advection–diffusion models are used to simulate shoreline concentration time series for an idealized surf zone in order to probe the roles of alongshore current shear and rip currents in producing the observed characteristics in dye concentration time series. Favorable qualitative and quantitative comparison of measured and simulated time series suggest alongshore current shear and rip currents play key roles in generating the observed characteristics of nearshore dye patch dispersion. The models demonstrate the potential effects of these flow features on the extent and duration of beach contamination owing to a continuous contamination source.  相似文献   

15.
The modeling of thick vadose zones is particularly challenging because of difficulties in collecting a variety of measured sediment properties, which are required for parameterizing the model. Some models rely on synthetic data, whereas others are simplified by running as homogeneous sediment domains and relying on a single set of sediment properties. Few studies have simulated flow processes through a thick vadose zone using real and comprehensive data sets comprising multiple measurements. Here, we develop a flow model for a 7-m-thick vadose zone. This model, combining the numerical codes CTRAN/W with SEEP/W, includes the measured sediment hydraulic properties of the investigated vadose zone and incorporates the actual climate and subsurface conditions of the study site (precipitations, water-table elevations, and stable isotope data). The model is calibrated by fitting the simulated and measured vertical profiles of water content. Our flow model calculates a transit time of 1 year for the travel of water through the 7-m vadose zone; this estimate matches stable isotope-based results obtained previously for this site. A homogeneous sediment domain flow model, which considers only a single set of sediment properties, produces a transit time that is approximately half the duration of that of the heterogeneous flow model. This difference highlights the importance of assuming heterogeneous material within models of thick vadose zones and testifies to the advantage gained when using real sediment hydraulic properties to parametrize a flow model.  相似文献   

16.
Fletcher–Ponnambalam presented a new model for considering the balance equation of the storage volume of the reservoir using indicator functions. For stochastic inflows, the two storage moments of this balance equation, namely, the mean and variance, calculated using a random release policy were found to be quite accurate unlike any known models. Significantly, this model required no discretization of storage volumes or releases. In this paper, this work has been extended to two new cases: for multireservoir systems which require further consideration of the stochastic releases and for arbitrary distribution of inflows using the Beta-equivalent Kumaraswamy distribution which has a simpler form than Beta. The randomized release policies are easy to use even in a multireservoir problem. The Parambikulm-Aliyar Project from India is used as a case study. Results show accurate predictions of mean of storages in the multireservoir case but show the need for further improvement in the standard deviations of storages. The optimal benefits and the policies obtained are shown to be at least as good as obtained using Monte Carlo-based methods.  相似文献   

17.
Available water resources are often not sufficient or too polluted to satisfy the needs of all water users. Therefore, allocating water to meet water demands with better quality is a major challenge in reservoir operation. In this paper, a methodology to develop operating strategies for water release from a reservoir with acceptable quality and quantity is presented. The proposed model includes a genetic algorithm (GA)-based optimization model linked with a reservoir water quality simulation model. The objective function of the optimization model is based on the Nash bargaining theory to maximize the reliability of supplying the downstream demands with acceptable quality, maintaining a high reservoir storage level, and preventing quality degradation of the reservoir. In order to reduce the run time of the GA-based optimization model, the main optimization model is divided into a stochastic and a deterministic optimization model for reservoir operation considering water quality issues.The operating policies resulted from the reservoir operation model with the water quantity objective are used to determine the released water ranges (permissible lower and upper bounds of release policies) during the planning horizon. Then, certain values of release and the optimal releases from each reservoir outlet are determined utilizing the optimization model with water quality objectives. The support vector machine (SVM) model is used to generate the operating rules for the selective withdrawal from the reservoir for real-time operation. The results show that the SVM model can be effectively used in determining water release from the reservoir. Finally, the copula function was used to estimate the joint probability of supplying the water demand with desirable quality as an evaluation index of the system reliability. The proposed method was applied to the Satarkhan reservoir in the north-western part of Iran. The results of the proposed models are compared with the alternative models. The results show that the proposed models could be used as effective tools in reservoir operation.  相似文献   

18.
《Geofísica Internacional》2014,53(2):183-198
As a result of a gasoline spill in an urban area, Electrical Resistivity Tomography (ERT), Electromagnetic Profiling (EMP) and Volatile Organic Compounds (VOC) methods were used in order to define the contamination plume and to optimize the drilling and soil sampling activities. The VOC anomalies (recent contamination) indicated that a gas station located at the study site is an active contamination source. The mature contaminated zones defined by ERT and EMP methods corresponded with low resistivity anomalies due to degradation process of the hydrocarbons contaminants. The ERT, EMP and VOC results were integrated on a map, allowing the final configuration of contamination plumes and the optimization of drilling and soil/free-product sampling. Laboratory analyses of free-product samples suggest the existence of more than one contamination event in the site, with the presence of recent and degraded-hydrocarbon contaminants classified in the gasoline range. This study shows the advantages of joint application of ERT, EMP and VOC methods in sites with active contamination source, where the existence of recent and mature contaminants in subsoil is assumed.  相似文献   

19.
The U.S. Environmental Protection Agency has established several methods to delineate wellhead protection areas (WHPAs) around community wells in order to protect them from surface contamination sources. Delineating a WHPA often requires defining the capture zone for a well. Generally, analytical models or arbitrary setback zones have been used to define the capture zone in areas where little is known about the distribution of hydraulic head, hydraulic conductivity, or recharge. Numerical modeling, however, even in areas of sparse data, offers distinct advantages over the more simplified analytical models or arbitrary setback zones. The systematic approach discussed here calibrates a numerical flow model to regional topography and then applies a matrix of plausible recharge to hydraulic conductivity ratios ( R / K ) to investigate the impact on the size and shape of the capture zone. This approach does not attempt to determine the uncertainty of the model but instead yields several possible capture zones, the composite of which is likely to contain the actual capture zone. A WHPA based on this composite capture zone will protect ground water resources better than one based on any individual capture zone. An application of the method to three communities illustrates development of the R / K matrix and demonstrates that the method is particularly well suited for determining capture zones in alluvial aquifers.  相似文献   

20.
This paper presents the results of the application of the Ground Penetrating Radar (GPR) method, or Georadar, in outlining a zone of contamination due to solid residues at the waste burial site of Rio Claro in the state of São Paulo, SE Brazil. A total of eight GPR profiles with 50- and 100-MHz antennae were surveyed. Six profiles were located within the landfill site and the remaining two were outside. The main objective of the GPR survey was to evaluate the side extension of contamination. A Vertical Electric Sounding (VES) survey was performed at four points within the site in order to investigate the vertical extent of the contamination plume and to define the bottom of the landfill. Two additional VESs were done outside the landfill with the purpose of determining the top of the ground water table and the geoelectric stratigraphy of the background. From the interpretation of the GPR profiles, it was possible to locate the top of the contamination plume and to infer that it was migrating laterally beyond the limits of the waste disposal site. This was observed along the profile situated close to the highway SP-127, which was about 20 m from the limit of the site. The signature of the contaminant appears as a discontinuous reflector that is believed to be a shallow ground water table. The discontinuity is marked by a shadow zone, which is characteristic of conductive contaminant residues. The contamination did not move far enough to reach a sugar cane plantation located at approximately 100 m from the border of the site. In the regions free from contamination, the ground water table was mapped at approximately 10 m of depth, and it was characterized by a strong and continuous reflector. The radar signal penetrated deep enough and enabled the identification of a second reflector at approximately 14 m deep, interpreted as the contact between the Rio Claro and the Corumbataí formations. The contact is marked by the presence of gravel characterized by ferruginous concretes, which cause the strong amplitude reflection in the GPR profile. Within the landfill site, the quantitative interpretation of the VES results showed the contamination zone. The base of the landfill varies between 11 and 15 m deep. Outside the landfill site, the VES results showed no indication of contamination and allowed the determination of the top of the ground water table and the contact between the Rio Claro and the Corumbataí formations. The results of GPR and VES showed a good agreement and the integrated interpretations were supported by local geology and information from several boreholes, about 17 m depth, on average. The bottom of the landfill reaches a maximum of 14.5 m depth.  相似文献   

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