A Holistic View of Geochemical Models

As will become clear in later discussions, this definition leaves something to be desired, in the sense that it suggests that a model is just a bunch of equations, whereas in fact we make a distinction between computer programs and models. A model is produced by a human being aided by computer programs. He or she must choose what data to use, what computer program options are appropriate, and must select the results which seem reasonable, and reject those which do not. 

Because the model uses observational data as input and produces other data which also represent possible observations, it can be said to represent or mimic nature. For example, a groundwater flow model uses measured or assumed parameters (e.g., permeabilities and storage coefficients) as input, and produces calculated water velocities, which may or may not be measurable. Models of complex natural situations are thus not only abstractions, but simplified abstractions of nature. In an effort to mimic nature more and more closely, they become more and more complex, often incorporating other models to deal with specific aspects of the overall situation. Thus, for example, a model of water chemistry will probably include a model of activity coefficients (see 3.4.2). 

Questions naturally arise as to the accuracy of predictions made by models. Quantum mechanical models of molecular processes are capable of fantastic accuracy. Models of planetary motion based on Newton s laws of motion are sufficiently accurate to put men on the moon and bring them back. Thermodynamics itself is a model of energy relationships, which Einstein once said is the only theory he was sure would never be overthrown. Unfortunately, hydrological and geochemical models deal with much more complex processes, and are less accurate. 

Natural systems, both pristine and anthropogenically perturbed, are complex. Many processes attaining to the movement and distribution of contaminants are coupled or have feedback loops. 

Chemical Reactions For a given amount of mass at a fixed point and time, chemical reactions determine the partitioning of the contaminants among different phases, and the dominant species in the aqueous phase. 

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