Ground Water Models Scientific and Regulatory Applications (1990) / Chapter Skim
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4 TRANSPORT
4 TRANSPORT
Pages 113-159
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From page 113... ...
The purpose of a mode} that simulates solute transport in ground water is to compute the concentration of a dissolved chemical species in an aquifer at any specified place and time. Numerical solute transport models were first developed about 20 years ago.
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From page 114... ...
Mineralogic variability may be very significant and may affect the rate of reactions, and yet be essentially unknown. There are very few documented cases for which deterministic solute transport models have been applied successfully to ground water contamination problems involving complex chemical reactions.
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From page 115... ...
Most applications of transport models to ground water contamination problems documented to date have been based on this conventional formulation, even when the porous medium is considered to be anisotropic with respect to flow. The consideration of solute transport in a porous medium that is anisotropic would require the estimation of more than two dispersivity parameters.
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From page 116... ...
The scale dependence of dispersivity coefficients (macrodispersion) is recognized as a limitation in the application of conventional solute transport models to field problems.
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From page 117... ...
TRANSPORT 117 Sand, Gravel, Sandstone Limestone, Basalt · Granite, & Schist 100 Oh llJ In 10 z At o 1 .
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From page 118... ...
Because there is not yet a consensus on how to describe, account for, or predict scale-dependent dispersion, it is important that any conventional solute transport mode} be applied to only one scale of a problem. That is, a single model, based on a single value of dispersivity, should not be used to predict both near-field (near the solute source)
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From page 119... ...
Geological Survey, Ralston, Va., written communication, 1989. larger scales of miles.
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From page 120... ...
One is the flow equation, from which velocities are calculated, and the other ~ the solute transport equation, which describes the chemical concentration in ground water. If the range in concentration throughout the system is small enough that the density and viscosity of the water do not change significantly, then the two equations can be decoupled (or solved separately)
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From page 121... ...
Fewer parameters need to be defined to compute the head distribution with a flow mode} than are required to compute concentration changes using a solute transport model. A mode} of ground water flow is often calibrated before a solute transport mode} is developed because the ground water seepage velocity is determined by the head distribution and because advective transport ~ a function of the seepage velocity.
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From page 122... ...
dependent on ground water flow. An illustrative example is the analysis of the Love Canal area, Niagara Fails, New York, described by Mercer et al.
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From page 123... ...
NONCONSERVATIVE SOLUTES The following sections assess the state of the art for modeling abiotic transformations, transfers between phases, and biological processes in the subsurface. Descriptions of all of these processes are provided in Chapter 2.
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From page 124... ...
rA = rate of mass accumulation of species A (MAT- ~ ~ Y = volume of water in the system being modeled (~3) k = a rate coefficient that depends on the mechanisms of the reaction, the temperature, and other environmental conditions and has units giving MAT-i for FA x, y = reaction-order exponents
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From page 125... ...
Table 4.2 provides an assessment of the kinetic relationships for the abiotic transformations. Two mechanisms, radioactive decay and acid/base processes, are well understood and have well-defined databases.
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From page 126... ...
126 GROUND WATER MODELS TABLE 4.1 Summary and Evaluation of Thermodynamics of Abiotic Transformation Mechanisms for Aqueous Species Mechanisms/Reaction Form Status Thermodynamic Relation Codea Not applicable Radioactive decay, P ~ D + nuclear particle Oxidation/reduction. N+ + R > N + R+ E = E°+ RT in Acid/base processes, {H }{A } = K {H4} Precipitation/dissolution, C+ + A- = CA(solid)
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From page 127... ...
Geochemical Models In reality, many different abiotic transformations occur simultaneously. Equations describing all the different reactions and all the participating chemical species must be solved together because one chemical species can participate in several different reactions of the same type or of different types.
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From page 128... ...
The actual transformations that occur and the times and distances over which they occur are not specified by geochemical models; research in this area is badly needed for predictive modeling. Incorporation of Abiotic Transformations into Solute Transport Modeb Geochem~cal codes can be used independently of transport codes to provide estimates of species mobility.
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From page 129... ...
The linking of geochemical models, in their present forms, into solute transport models is difficult because of the computing demand. Therefore most of the currently available geochemistry models may be inappropriate for solute transport modeling, even if kinetics can be included.
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From page 130... ...
One of the unique aspects of solute transport modeling for subsurface waters is the very large amount of solid surface area to which the water is exposed. Surfaces often behave as reactants in the types of reactions described above.
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From page 131... ...
Assessment of Kinetics As noted previously, phase transfers are usually modeled using equilibrium reaction models. Kinetic expressions containing a km parameter multiplied by some sort of concentration difference normady are used to describe filtration of colloids but, when necessary, can be used to describe most of the other transfers as well.
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From page 132... ...
Such research is necessary because the instantaneous equilibrium approaches are not appropriate when solute advection is significant, which occurs in highly porous media and near wells and trenches used for remediation. Incorporation of Phase Transfers into Solute Transport Models The incorporation of phase transfers into solute transport models is relatively straightforward, as long as the rate term is available.
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From page 133... ...
The first feature means that, in modeling of the biological processes, mass balances often are needed for required substrates, even when they are not the chemical species of interest. The second feature means that the reactions by the microorganisms and the mass balance for the microorganisms must be posed in terms of a solid phase that does not move with the water.
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From page 134... ...
The kinetic parameters are not well known for many of the organic chemicals that commonly pollute ground water. The Ks parameter seems to vary widely (e.g., from as low as about 1 ,ug/} to hundreds of milligrams per liter)
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From page 135... ...
One of these materials is "growth rate limiting" and must be modeled if the amount of active biomass is to be described. For attached biomass, the linkage of limiting-substrate utilization can be made by solving a mass balance equation on cell mass for a limiting electron donor, often called the primary substrate (Rittmann and McCarty, 1980; Saez and Rittmann, 1988~.
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From page 136... ...
The last situation is known as co-metabolism. The rate of secondary-substrate utilization is determined by its intrinsic kinetic parameters, its concentration, and the amount of active biomass, which is controlled by the longterm availability of primary substrate.
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From page 137... ...
First, the mechanisms of clogging are not yet well enough understood to allow formulation of quantitative expressions. Second, clogging of the pores alters the flow paths; thus water flow and solute transport models must become
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From page 138... ...
must travel through the unsaturated zone and arrive in sufficient quantities at the water table to be an environmental or a health
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From page 139... ...
because of the long-term "bleeding~ of contaminants from the unsaturated zone overlying the water table. It is evident that the role of the unsaturated zone, either as a buffer or as a source, must be carefully evaluated in assessing ground water contamination and in selecting remedial measures.
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From page 140... ...
Here it is sufficient to recognize that when the soil water flow is transient, solute transport is also transient. The advective velocity (v)
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From page 141... ...
Because water flow in the unsaturated zone is episodic, so is contaminant transport. This feature is illustrated schematically in Figure 4.5 for vertical, downward leaching of nonsorbed and sorbed contaminants in a sandy soil as a result of rainfall over a 1-yr period.
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From page 142... ...
and one nonsorbed contaminant (3) through the root zone as a result of rainfall shown in (a)
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From page 143... ...
outputs are then used as inputs to a mode} that simulates pesticide behavior in the saturated zone (see Figure 4.8~. Unsaturated Flow and Transport in Structured Soile The classical Richards equation for transient water flow and the advective-dispersive solute transport mode!
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From page 144... ...
Episodes of pesticide loadings to shallow water table located beneath a citrus grove.
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From page 145... ...
The impact of such preferential flow on solute transport Is further detertn~ned by the rate of diffusive mass transfer into the soil matrix and the sorptive properties of the macropore and matrix regions. While the impacts of preferential water flow on subsurface hydrology have been more thoroughly investigated, only recently have efforts been initiated to investigate the influence of macropore flow on solute transport in structured soils.
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From page 146... ...
146 -25 (D __ ~ Z i_ - |
From page 147... ...
147 eq 4= _ P ~ ~q ~ ~ q,)
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From page 148... ...
of the vadose zone also includes two fluid phases air and water but this case has been treated previously. Codes that simulate the two classes of problems exist but have not been so heavily involved in water quality regulation or litigation as standard ground water flow and miscible solute transport codes.
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From page 149... ...
These resource management questions are resolved through fluid flow simulation and do not require solute transport simulations. Organic FIllid Contamination The migration and fate of organic compounds in the subsurface are of significant interest because of the potential health effects of these compounds at relatively low concentrations.
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From page 150... ...
If not remediated, the migration of an immiscible organic liquid phase is of interest because it could represent an acute or chronic source of pollution. Movement of the organic liquid through the vadose zone is governed by the potential of the organic liquid, which in turn depends upon the fluid retention and relative permeability properties of the air/organic/water/solid system.
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From page 151... ...
The problem of salinity as a miscible contaminant in ground water is addressed with standard solute transport models. In reality, seawater is miscible with fresh water, and the front between the two bodies of water Is really a transition zone.
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From page 152... ...
The former category of models addresses the far-field problem of chronic miscible contamination. Standard ground water flow and solute transport codes can be applied to these organic compound contamination problems.
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From page 153... ...
The equation set is coupled by the fluid retention and relative permeability relationships of the multiphase system. Miscible displacement of trace quantities of an organic fluid can occur within the water and gas phases.
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From page 154... ...
While the seawater intrusion problem is restricted to saturated porous media, organic fluid migration often occurs in the unsaturated zone. Consequently, fluid retention and relative permeability properties are required for the air/organic/water/solid system.
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From page 155... ...
For example, in the real, fully three-dimensional environment, a heavier-than-water organic fluid can move vertically through the soil profile and form a continuous distinct fluid phase from the water table to an underlying impermeable medium. Ground water will simply move around the immiscible organic fluid as though it were an impermeable object.
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From page 156... ...
1985. Transport of immiscible Guide within and below the unsaturated zone: A numerical model.
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From page 157... ...
1983. Simulation of solute transport in a chemically react~ve heterogeneous system: Model development and application.
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From page 158... ...
Pp. 27-48 in Pollutants in Porous Media, The Unsaturated Zone Between Soil Surface and Groundwater, B
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From page 159... ...
Water Resources Monograph 5, 2nd ed. American Geophysical Union, Washington, D.C., 180 pp.
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