佳木斯台形变观测对东北地区几次浅震“场兆”异常的探讨

大地震在主破裂前往往行成地壳变形,高应力作用下进入峰值后的变形阶段,反映到地倾斜观测日均值曲线上,表现为固体潮形变趋势异常,这些异常能否指示远距离的地震,近年来不少研究者发现,远场前兆往往出现在对应力、应变变化反应灵敏的特殊构造部位。佳木斯台可能处于这种特殊的构造部位,对东北地区浅源地震短期前兆异常“场兆”的趋势性变化具有一定的意义。     

Influence of mass transfer characteristics for DNAPL source depletion and contaminant flux in a highly characterized glaciofluvial aquifer

The transfer of contaminant mass between the nonaqueous- and aqueous-phases is a process of central importance for the remediation of sites contaminated by dense nonaqueous-phase liquids (DNAPLs). This paper describes a comparison of the results obtained with various alternative DNAPL-aqueous-phase mass transfer models contained in the literature for predicting DNAPL source-zone depletion times in groundwater systems. These dissolution models were largely developed through laboratory column experiments. To gain insight into the implications of various representations of the local-scale kinetic as well as equilibrium DNAPL dissolution processes, aquifer heterogeneity and the complex architecture of a DNAPL source-zone, the aqueous-phase contaminant concentrations and mass fluxes arriving at a down-gradient compliance boundary are analyzed in a conditional stochastic framework. The hydrogeologic setting is a heterogeneous fluvial aquifer in Southwest Germany, referred to as the aquifer analog dataset, that was intensively characterized in three dimensions for hydrogeological parameters that include permeability, effective porosity, grain size, mineralogy and sorption coefficients. By embedding the various dissolution models into the compositional, multiphase flow model, CompFlow, the relative times predicted for complete depletion of a released DNAPL source due to natural dissolution are explored. Issues related to achieving environmental benefits through, for example, partial DNAPL-zone source removal via enhanced remedial technologies are also discussed. In this context, performance metrics in the form of peak aqueous-phase contaminant concentrations and mass fluxes arriving at a down-gradient compliance boundary are compared to each other. This is done for each of the alternative mass transfer models. A significant reduction in the fractional flux at a downstream location from the DNAPL source can be achieved by partial source-zone mass reduction; however, peak concentration levels at the same location remain much higher than the United States Environment Protection Agency (US-EPA) drinking water limits. Although groundwater quality was found to improve more rapidly for the equilibrium dissolution model, it is also shown that dissolution models that promote rapid DNAPL disappearance produce greater prediction uncertainty in the aqueous-phase flux reduction.     

Simulation of groundwater age evolution during the Wisconsinian glaciation over the Canadian landscape

The simulation of groundwater age (residence time) is used to study the impact of the Wisconsinian glaciation on the Canadian continental groundwater flow system. Key processes related to coupled groundwater flow and glaciation modeling are included in the model such as density-dependent flow, hydromechanical loading, subglacial infiltration, glacial isostasy, and permafrost development. It is found that mean groundwater ages span over a large range in values, between zero and 42 Myr; exceedingly old groundwater is found at large depths where there is little groundwater flow because of low permeabilities and because of the presence of very dense brines. During the glacial cycle, old, deep groundwater below the ice sheet mixes with the young subglacial meltwater that infiltrates into the subsurface; the water displacement due to subglacial recharge reaches depths up to 3 km. The depth of penetration of the meltwater is, however, strongly dependent on the permeability of the subsurface rocks, the presence of dense brines and the presence or absence on deep fractures or conductive faults. At the end of the simulation period, it was found that the mean groundwater age in regions affected by the ice sheet advance and retreat is younger than it was at the last interglacial period. This is also true for frozen groundwater in the permafrost area and suggests that significant parts of this water is of glacial origin. Finally, the simulation of groundwater age offers an alternative and pragmatic framework to understand groundwater flow during the Pleistocene and for paleo-hydrogeological studies because it records the history of the groundwater flow paths.     

Multicomponent simulation of wastewater-derived nitrogen and carbon in shallow unconfined aquifers. I. Model formulation and performance

One of the most common methods to dispose of domestic wastewater involves the release of septic effluent from drains located in the unsaturated zone. Nitrogen from such systems is currently of concern because of nitrate contamination of drinking water supplies and eutrophication of coastal waters. The objectives of this study are to develop and assess the performance of a mechanistic flow and reactive transport model which couples the most relevant physical, geochemical and biochemical processes involved in wastewater plume evolution in sandy aquifers. The numerical model solves for variably saturated groundwater flow and reactive transport of multiple carbon- and nitrogen-containing species in a three-dimensional porous medium. The reactive transport equations are solved using the Strang splitting method which is shown to be accurate for Monod and first- and second-order kinetic reactions, and two to four times more efficient than sequential iterative splitting. The reaction system is formulated as a fully kinetic chemistry problem, which allows for the use of several special-purpose ordinary differential equation (ODE) solvers. For reaction systems containing both fast and slow kinetic reactions, such as the combined nitrogen-carbon system, it is found that a specialized stiff explicit solver fails to obtain a solution. An implicit solver is more robust and its computational performance is improved by scaling of the fastest reaction rates. The model is used to simulate wastewater migration in a 1-m-long unsaturated column and the results show significant oxidation of dissolved organic carbon (DOC), the generation of nitrate by nitrification, and a slight decrease in pH.     

Multicomponent simulation of wastewater-derived nitrogen and carbon in shallow unconfined aquifers. II. Model application to a field site

A multicomponent reactive transport model as presented by MacQuarrie and Sudicky [MacQuarrie, K.T.B., Sudicky, E.A., this volume. Multicomponent simulation of wastewater-derived nitrogen and carbon in shallow unconfined aquifers: I. Model formulation and performance, J. Contam. Hydrol.] is applied to a well-studied wastewater plume in a sandy aquifer near Cambridge, Ontario. Domestic wastewater is released into the unsaturated zone via a drain field at a depth of about 0.8 m. The physical transport parameters for the model are obtained by simulating a non-reactive solute, while kinetic input data for the nitrogen and carbon reaction network are obtained from the literature. The model shows that the wastewater-loading rate has little influence on the moisture content in the unsaturated zone, thus oxygen diffusion in the air phase is an important transport mechanism. The model results are in general agreement with the field-determined moisture and oxygen profiles near the drain field. The simulation results show that oxidation of ammonium and dissolved organic carbon (DOC) goes to completion in the 1.5-m distance between the drain field and the water table, and that calcite dissolution limits the pH reduction to about 0.2 units. The model-predicted nitrate concentrations in the core of the plume are in the range of 20-25 mg N/l and are in good agreement with the field data. Overall, the results for the major reactive species from the model simulation agree well with the geochemical data obtained below the drain field and it is concluded that the major physical and biochemical processes have been correctly captured in the current model formulation.     

A pragmatic approach for estimation of source-zone emissions at LNAPL contaminated sites

When considering natural attenuation as a remediation strategy at a site contaminated by a light non-aqueous phase liquid (LNAPL), it is important to consider the emission of contaminants from the source zone. A quantification of source-zone emissions is essential both for comparison with down-gradient mass fluxes to provide an estimate of fractional mass flux reduction, as well as for estimating the source lifetime. Because the spatial distribution of LNAPL at a field site is strongly dependent on both the spill circumstances and the heterogeneity of the geologic materials, which can be problematic for in-situ determination, alternative methods for estimating source-zone emissions are needed. In this work, a three-dimensional multiphase flow and transport modelling approach is used to investigate the relationship between the lateral extent of an LNAPL body and the emission of contaminants to groundwater at a contaminated site. For simulations involving an LNAPL release in an aquifer comprised of heterogeneous porosity and permeability distributions that were generated geostatistically, it is shown that a simple linear relationship exists between the lateral extent of the LNAPL body in the capillary fringe and the emission to the aqueous phase. The parameters describing the relationship are found to be linear functions of the groundwater flow velocity and the vertical infiltration rate. This site-specific relationship provides a simple method to estimate contaminant emissions to groundwater at LNAPL contaminated sites.     

Numerical simulation of a fine-grained denitrification layer for removing septic system nitrate from shallow groundwater

One of the most common methods to dispose of domestic wastewater involves the release of septic effluent from drains located in the unsaturated zone. Nitrogen from such systems is currently of concern because of nitrate contamination of drinking water supplies and eutrophication of coastal waters. It has been proposed that adding labile carbon sources to septic distribution fields could enhance heterotrophic denitrification and thus reduce nitrate concentrations in shallow groundwater. In this study, a numerical model which solves for variably saturated flow and reactive transport of multiple species is employed to investigate the performance of a drain field design that incorporates a fine-grained denitrification layer. The hydrogeological scenario simulated is an unconfined sand aquifer. The model results suggest that the denitrification layer, supplemented with labile organic carbon, may be an effective means to eliminate nitrogen loading to shallow groundwater. It is also shown that in noncalcareous aquifers, the denitrification reaction may provide sufficient buffering capacity to maintain near neutral pH conditions beneath and down gradient of the drain field. Leaching of excess dissolved organic carbon (DOC) from the denitrification layer is problematic, and causes an anaerobic plume to develop in simulations where the water table is less than 5-6 m below ground surface; this anaerobic plume may lead to other down gradient changes in groundwater quality. A drain field and denitrification layer of smaller dimensions is shown to be just as effective for reducing nitrate, but has the benefit of reducing the excess DOC leached from the layer. This configuration will minimize the impact of wastewater disposal in areas where the water table is as shallow as 3.5 m.     

Flow and transport in fractured tuff at Yucca Mountain: numerical experiments on fast preferential flow mechanisms

Recent discovery of bomb-related ³⁶Cl at depth in fractured tuff in the unsaturated zone at the Yucca Mountain candidate high-level waste (HLW) repository site has called into question the usual modeling assumptions based on the equivalent continuum model (ECM). A dual continuum model (DCM) for simulating transient flow and transport at Yucca Mountain is developed. In order to ensure properly converged flow solutions, which are used in the transport simulation, a new flow solution convergence criteria is derived. An extensive series of simulation studies is presented which indicates that rapid movement of solute through the fractures will not occur unless there are intense episodic infiltration events. Movement of solute in the environs of the repository is enhanced if the properties of the tuff layer at the repository horizon are modified from current best-estimate values. Due to a large advective–dispersive coupling between the matrix and fractures, the matrix acts as a major buffer which inhibits rapid transport along the fractures. Consequently, fast movement of solutes through the fractures to the repository depth can only be explained if the matrix–fracture coupling term is significantly reduced from a value that would be calculated on the basis of data currently available.