Plume transport models

Much research has been applied toward understanding the ways that various animals use odor plumes. It is not within the scope of this book to exhaustively survey the resulting literature. One aspect of this research has direct bearing on the use of plumes to locate sources of chemical molecules such as explosives. As they are studying their animal subjects, these researchers construct two kinds of mathematical models to assist them. 

One of these models the plume itself, attempting to provide a predictive capability for describing the characteristics of the plume in a measured environment. Knowing the motion characteristics of the medium—air or water—the models seek to predict the concentration and dimensions of the plume at distances downstream. Of course, the nature of the problem quickly leads to use of statistical descriptions. This provides a model quite adequate for constructing tracking algorithms. 

An example of this work is that of Farrell and co-workers [34], They present a rather complex model to attempt to account for the effects of fluid motion and turbulence in three different levels of scale, relative to the plume. They begin with classical equations of motion, but by breaking their particle velocity vector into three components related to the three scales of interest, they are able to introduce appropriate statistical descriptions for the components. The result is a model that retains both the diffusive and the filamentary nature of the plume. 

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Several types of computer models have been developed for estimating the expected concentrations of the chemicals of interest as they move away from the source. Soil transport models attempt to estimate the expected concentration at the surface above buried sources. Plume transport models attempt to estimate the concentrations within a plume, along with its shape and position. A different form of model is designed to guide a search pattern for employing a sensing system to trace a plume. 

Gaussian Plume Model. One of the most basic and widely used transport models based on equation 5 is the Gaussian plume model. 

The initial direction of transport of pollutants from their source is determined by the wind direction at the source. Air pollutant concentrations from point sources are probably more sensitive to wind direction than any other parameter. If the wind is blowing directly toward a receptor (a location receiving transported pollutants), a shift in direction of as little as 5° (the approximate accuracy of a wind direction measurement) causes concentrations at the receptor to drop about 10% under unstable conditions, about 50% under neutral conditions, and about 90% under stable conditions. The direction of plume transport is very important in source impact assessment where there are sensitive receptors or two or more sources and in trying to assess the performance of a model through comparison of measured air quality with model estimates. 

Residual oil impact estimates by modeling provided a severe test of GRID s capacity since the CMB impact estimates were small (less than one-quarter yg/m ) and the physical basis of the model inherently limits it s ability to predict point source plume transport. Since Initial comparisons (Figure 5) showed GRID estimated impacts to be overpredicted at all sites relative to CMB estimates, further improvements to the data base were suggested. Overall, annual model verification results for all sources were relatively poor with the dispersion model predictions consistently underestimating both the CMB-derlved estimates and the measured TSP mass data. 

To understand the behavior of the movement of the contaminant in ground-water, people solve Eq. (1) forward in time. In solving this equation forward in time, one assumes that the plume is originated from somewhere and will travel through the porous media due to advection and dispersion. The conventional procedure to solve Eq. (1) is to use finite difference or finite element methods. For simple cases, closed-form solutions exist. Quantitative descriptions of the processes forward in time are well understood. Multidimensional models of these processes have been used successfully in practice [50]. Numerical solute transport models were first developed about 25 years ago. When properly applied, these models can provide useful information about transport processes and can assist in the design of remedial programs. 

Transport and dispersion was evaluated without any form of tuning by comparing a simulation of the ETEX-1 release to the official measurements of surface concentration. To facilitate comparisons with models evaluated during ATMES 11 (Atmospheric Transport Model Evaluation Study) an identical statistical methodology was employed (Mosca et al. 1998). Background values were subtracted so that only the pure tracer concentration was used. Measurements of zero concentration (concentrations below the background level) were included in time series to the extent that they lay between two non-zero measurements or within two before or two after a non-zero measurement. Hereby, spurious correlations between predicted and measured zero-values far away from the plume track are reduced. 

When the effects of time variations in release rates are included, pulse (or instantaneous) and continuous (plume) emissions are the two most common time-variant inputs in transport models. The classic example of a pulse release is a hazardous waste spill. The steady release of contaminants into groundwater from a subsurface contaminant and the continuous release of volatile solvents from an air-stripping tower are examples of plume emissions. 

Laboratory experiments, transport modeling, field data, and engineering cost analysis provide complementary information to be used in an assessment of the viability of an MNA approach for a site. Information from kinetic sorption/ desorption experiments, selective extraction experiments, reactive transport modeling, and historical case analyses of plumes at several UMTRA sites can be used to establish a framework for evaluation of MNA for uranium contamination (Brady et al, 1998, 2002 Bryan and Siegel, 1998 Jove-Colon et al, 2001). The results of a recent project conducted at the Hanford 100-N site provided information for evaluation of MNA for a °Sr plume that has reached the Columbia River (Kelley et al, 2002). The study included strontium sorption-desorption studies, strontium transport and hydrologic modeling of the near-river system, and evaluation of the comparative costs and predicted effectiveness of alternative remediation strategies. 

MULTIMED Unsaturated zone/groundwater MULTIMED was developed as a multimedia fate and transport model to simulate contaminant migration from waste disposal units. Release to either air or soil, including the unsaturated and the saturated zones, are possible interception of the subsurface contaminant plume by a surface stream are included. 

Chemical transport and transformation have been a part of environmental science and engineering for decades. Air pollutant plume dispersion modeling and surface water quality stream modeling are mature elements of EC, commonly termed chemical fate and... 

Contaminant Transport Modeling. A major difficulty in the calibration of any two-dimensional contaminant transport model is relating the two-dimensional simulated plume to the real three-dimensional plume. A model based on Equation 4 can simulate two dimensions in cross section or areal view. An areal view was selected for the problem considered here. Use of a two-dimensional areal view model implies that the contaminant is uniformly spread out through the entire saturated thickness of the aquifer. However, in the field the aldicarb plume is only around 10 feet (3m) thick while the aquifer is around 70 feet (21 m) thick. Moreover, the concentration data were collected from wells having 3 ft (0.91 m) well screens and hence are representative of only a small fraction of the total aquifer thickness. It was decided to calibrate the model to concentrations representative of the center of the plume vertically. That is, the model was calibrated to maximum measured concentrations in each well nest. As a result, the loading rate to the model is inflated over probable field values. The model assumes the load to the model is distributed over the full aquifer thickness, when in the field the zone of maximum concentration is probably no more than 3 feet thick. Therefore, the probable loading rate in the field is roughly 3/70 or 4% of that used to calibrate the model. 

As we have shown in Chapters 6 and 7, the migration of an acid plume from acid mine drainage is a very complex process. However, only A -based models have been used to simulate the acid plume and metal transport at this site for regulatory reporting. See Chapter 10 for a criticism of AVbascd transport models. As we contend in this book, accurate model simulations are only possible if we understand what geochemical reactions have occurred and are occurring in the aquifer. 

The coupled reactive transport model is designed to simulate the acid plume migration under this cover and attenuate reclamation plan. An 800 m strip along cross-section A-A (Figure 6.2) was discretized into 200 cells (Figure 10.4). Each cell is 4 min length. The time step is 0.08 year. 

Table 10.3. (Continued from previous page.) phreeqc input for a reactive transport model to simulate plume migration at the Bear Creek site.

Zhu, C., Hu, F. Q., and Burden, D. S., 2001a, Multi-component reactive transport modeling of natural attenuation of an acid ground water plume at a uranium mill tailings site. J. Contaminant Hydrology, v. 52, pp. 85-108. 

Gaussian Plume Model. One of the most basic and widely used transport models based on equation 5 is the Gaussian plume model. Gaussian plume models for continuous sources can be obtained from statistical arguments or can be derived by solving ... 

Transport, microbial and chemical processes relevant in the natural attenuation of contaminants in aquifers are complex and take place on different scales of magnitude from below pore scale up to macro scale. Exploration, identification and quantification of these processes are difficult in natural heterogeneous aquifer systems and often numerical models are used to interpret measured concentrations and to predict the future behaviour of a contaminant plume. Many model approaches therefore currently exist to describe reactive transport processes in contaminated aquifers (overviews Steefel and... 

The wide spread detection of dissolved organic compounds in groundwater is a major environmental concern, and has led to greater emphasis on incineration and waste minimization when compared with the land disposal of solvent-containing wastes. The movement and environmental fate of dissolved organic solvents from point sources can be approximated by the use of computer-assisted, solute-transport models. These models require information about the composition of leachate plumes, and site-specific hydrogeological and chemical... 

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