Hydrogeology: the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust. The study of the interaction between groundwater movement and geology can be quite complex. Groundwater does not always follow the surface topography; groundwater follows pressure gradients (flow from high pressure to low), often through fractures and conduits in circuitous paths. Taking into account the interplay of the different facets of a multi-component system often requires knowledge in several diverse fields at both the experimental and theoretical levels.