There are a number of processes that affect the spread of contaminants in groundwater, making it really tough to predict the spread. These include the density, solubility, partitioning, dispersion, velocity of water, reactivity, decay, etc.
Contaminants that have a specific gravity of more than 1 (that is, heavier than water) tend to settle down, while the lighter ones (specific gravity less than 1) float above the water. The solubility of a contaminant will affect how much of it would be in the water and how much would be left out. Contaminants also partition between liquid phase and solid phase and this will affect how much of it will be in the liquid phase. The spread of contaminants can be strongly affected by the processes of advection (movement with bulk flow) and dispersion (can be 2-dimensional or 3-dimensional). A number of contaminants can react with water and can decay over time. All these will also affect the quantity available for spreading.
Since there are so many processes affecting the spread, we typically use computer software (Modflow is a good tool for it) for such calculations. Such efforts are typically coupled with field scale experiments to determine various parameters related to the contaminant and the groundwater.
Needless to say, the process is quite complex and time consuming.
Hope this helps.
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