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Pollution Distribution in Aeration Zone

#1724


Physical and Chemical Hydrodynamics of Pollution Distribution in Soil Aeration Zone: Computer Simulation and Structure Models

Tech Area / Field

  • ENV-WPC/Water Pollution and Control/Environment

Status
3 Approved without Funding

Registration date
16.12.1999

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Supporting institutes

  • Kazan State University / Scientific Research Institute of Mathematics and Mechanics, Russia, Tatarstan, Kazan

Project summary

Unsaturated zone located between ground surface and the ground water table (GWT) plays a crucial role in the problem of ground water quality.

Pollution caused by chemical plants, mineral mining, oil extraction, agricultural activity and emergency conditions, as a rule, penetrates into ground water [aquifers] from the ground surface. Before the pollutants reach the ground water table, fluids pass through the unsaturated zone where at least two phases, water and air, are present in the pore space. Flow within the unsaturated zone is controlled primarily by interplay between the gravity and capillary forces. Therefore heterogeneity of soil, its complex structure strongly affects the contaminants fate. Thus, the propagation follows primarily along a network of highly conductive channels (layers or fractures etc.), while the fluid accumulation (holdup) takes place mainly in less permeable elements such as porous blocks, low-permeable layers, and above impermeable streaks. Interaction of the downward fluid flux with low permeable inclusions causes its lateral spreading. Accumulation and spreading processes together determine the buffer (protective) properties of the unsaturated zone and, eventually, specify the pollution-affected area and intensity of pollution penetration under the ground water table. Thus, an adequate description of flow through unsaturated zone is important per se and as an essential element of an integrated mathematical model of contaminant penetration and transport in ground water. In the latter case modeling of the unsaturated flow serves to set appropriate boundary conditions at the GWT in the model of flow within saturated zone.

Modeling of flow and contaminant transport within the unsaturated zone is traditionally based upon equations of two- or three phase multicomponent flow through porous and/or fissured porous media. They involve many material functions and properties such as relative permeabilities and capillary pressure functions etc. These functions are assumed to be specified at each location, and to be correlated with local soil properties. Usually, they are known with significant uncertainty [ambiguity].While general theoretical framework of contaminant flow modeling is well established, the commonly used straightforward approach has an intrinsic weakness. It is almost impossible to imagine a real-life situation for each all necessary input data could be acquired with reasonable accuracy. Any pollution event is site-specific and rather sensitive to fine details of soil structure and properties. As a result, huge efforts are involved in acquiring necessary input data and assessment of the results of simulations. The flow reaches the level of complexity which hardly can be overcome by a direct attack, but requires new approaches based upon detail understanding of mechanisms and interactions involved on a range of lengthscales. Any progress in more concise description of flow through unsaturated zone based on better understanding of specific soil structure and flow scenario becomes especially valuable.

The primary objective of this Project is to develop necessary apparatus and computational tools to perform predictive simulations [computer experiment] and to study in detail essential features of contaminant propagation across the unsaturated zone for typical scenarios. The results will be subsequently used as “construction blocks” for large-scale modeling of contaminants propagation through unsaturated zone. Basically, all scenarios to be studied involve an interplay between capillarity and gravity-driven flow in non-homogeneous soil with two-phase or three-phase saturation. While the complete list is still subject to refinement, some typical scenarios and topics are listed below.

1. Interaction of propagating “slug” of contaminant with a property interface, including effects of permeability and capillary pressure contrasts. It can be argued, that such interactions control accumulation and lateral spreading of contaminants penetrating from the ground surface.

2. Role of fractures and highly-conductive paths in propagation and spreading:


· Modeling contaminants propagation through fractures and fracture networks including interactions with porous matrix due to capillary imbibition and diffusion.
· Development and validation of lumped models of flows through fractured media.
· Development of theoretical approaches to concise description of multiphase flow and transport in heterogeneous soils.

3. Study of a liquid contaminant accumulation, spreading and penetration across the capillary fringe and the ground water table.

4. Study of capillary hysteresis effects upon contaminant propagation and retention within the unsaturated zone.

5. Incorporation of the submodels (or “scenario models”) discussed into an integrated computer code for large-scale (‘site-scale’) modeling.

6. Study of specific mechanisms of fracture system development and Karst initiation in partially dissolvable carbonate rocks due to action of chemically active solutions.

7. Study of parameter identification techniques (such as history matching and ground penetrating radar data analysis) for model tuning.

It is expected that the project will result in developing new physical and mathematical models of mass transport in the unsaturated zone incorporating structural effects and corresponding algorithms and programs allowing to predict contaminant propagation through unsaturated zone and across GWT.

New physical and mathematical models of primary pollution in the unsaturated zone will be developed in the framework of the project. Their novelty will consist in taking into account real geometric structure and buffer properties of this zone.

Efficient numerical methods and algorithms will be proposed for models under development.

The project will result in a program package development intended for simulating primary pollution in the unsaturated zone and being a necessary component for computing chemical ground water pollutio.


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