Hydrogeology or geohydrology is the part of hydrology that deals with the occurrence and movement of water beneath the earth's surface (groundwater).
Hydrogeology is a complex subject, as the chemical and physical interactions between soil and water are intricate and difficult to quantify. Although the basic principles of hydrogeology are not complex or difficult to understand, the quantification of some of the most important aspects of this science are very complex, multi-component systems that are difficult to determine through direct measurement, and are therefore determined through groundwater modeling which presents questions as to the accuracy of these techniques.
For example, it may be desirable to determine the conditions of flow for a chemical plume from a spill or leak, such as produced by a leaking storage tank. If the contaminant reaches the groundwater then the contaminant may be dissolved in and carried with it. It is possible to determine the concentration in the water by sampling of wells. It is also possible to determine the direction and speed of the groundwater flow by measuring the water table elevation in several wells and using Darcy's law:
v = k /n x (Ha - Hb)/D
where v = velocity,
k = permeability factor, depending on soil type,
n = porosity, depending on soil type,
Ha, Hb = Hydraulic head (groundwater level) in wells A and B,
D = horizontal distance between wells A and B
((Ha-Hb)/D is called the 'hydraulic gradient').
However, the speed of the contaminant is not likely to be the same as the speed of the groundwater due to adsorption and organic matter content of the soil, dilution and attenuation, all depending on the nature of the contaminant. So, to determine where that plume may be headed, and where it will be in the future, requires mathematical groundwater modeling. Simple models may be two dimensional (in a horizontal plane) only. Other, more complex three dimensional models based on extensive field data, may require considerable computing time. These models are then usually set up using a finite element method (FEM).