All aquifers can be considered to fall on a continuum between porous mediasystems and conduit systems (Fig. 1.1). In homogeneous porous mediaaquifers (Fig. 1.1a), groundwater flows through gaps between the sand grains.In heterogeneous porous media aquifers (Fig. 1.1b), systematic variation inthe size of the sand grains leads to the existence of preferential flow zones. Atthe other extreme, in purely fractured media (Fig. 1.1e) groundwater flowsonly in conduits, and the aquifer matrix between the conduits is impermeableand has no porosity. In fractured porous media water is also stored in theaquifer matrix between the conduits. In some cases, the matrix permeability isnegligible (Fig. 1.1d), although in other cases it can contribute significantly toflow (Fig. 1.1c). In reality, most fractured rock aquifers are of the fracturedporous media type. Models of groundwater flow, however, usually assumeeither homogeneous porous media or purely fractured media. Furthermore,models of groundwater flow in purely fractured systems usually assume thatfractures are planar and parallel and many also assume that the fractures areidentical. While these assumptions are unlikely to be true in reality, theyprovide a useful starting point for our understanding of groundwater behaviourin fractured rocks