Selection of a Computation Process

Even models of one type may have different solutions. It depends on the modeling purpose, number of interacting media, properties of studied components and imderground water. First of all, two major directions in solving model equations should be identified analytical and numerical. The selection of one or another depends mostly on the number and nature of considered components. 

Analytical solutions are those whose precision depends only on the accuracy of the initial data. They do not contain errors associated with the approximation due to simplification of the computation process. This approach is applicable to the simplest models, which are often represented by a restricted number of relatively simple equations. These may be solved without specialized program software. Analytical solutions, as a rule, are used in modeling of processes with minimum participation of chemical reactions, in particular in the analysis of distribution of nonpolar components, radioactive decay, adsorption, etc. Under such conditions for modeling often are sufficient equations of advective-dispersive mass-transport, which are included in the section Mixing and mass-transport . 

Numerical solutions are those which use approximation methods, i.e., replacement of some functions by the other ones, simpler but less accmate. In particular, in practice of hydrogeochemical balance computations are often used equations like this  

The numerical solution is commonly used at modeling of interaction between polar components in heterogeneous systems, especially when the temperature, pressure, rock composition and flow velocity are unstable. It makes the computation process much more complex and much more long and as a rule is performed utilizing special program software. 

Of conceptual significance in selecting methods and means of their computation for base models is the way to determine the ion activity coefficients in water. This selection depends first of all on the ionic strength of the studied solution and its temperature. Debye-Hiickel hmiting law (equation (1.69)) may be used only for fresh water with ionic strength less than 0.01. For fresh and brackish solutions with ionic strength of less 

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