Diffusion : the spreading of a species dissolved in the water phase by the Brownian motion of the ions (molecules). Dispersion : the spreading of a species dissolved in the water phase by local variations in the water velocity. Adsorption/desorption : interaction of species dissolved in the water phase with the solid matrix. This process can be physically based or chemically based, reversible or irreversible. Chemical reactions : reactions of species dissolved in the water phase with other species, resulting in the occurrence of different species altogether. Biodegradation : the degradation of species dissolved in the water phase by bacteria. Radioactive decay : the degradation of species by radioactivity. Concentrations of species in the water phase C i (including pure water itself) are defined as the mass of the species per unit volume: kg/m 3 , g/l, mg/l, etc. The density of a multi-component fluid, consisting of N components, is then given as: Soil Processes and Current Trends in Quality Assessment 34 1.3. Groundwater flow Groundwater flow is described by Darcy's law. Darcy's law is in principle the form of the momentum balance (Navier-Stokes equation), averaged over a large number of pores. It also follows from a balance of forces on water flowing through a porous medium. 4 dependence is only important at high concentrations. E.g. in case of seawater intrusion, or in deep saline aquifers which are sometimes used to store waste or to produce energy. In these deep aquifers salt concentrations can be as high as 300 g/l, resulting in a water density of 1200 g/l (giving a salt mass fraction of 0.25). Water density fluctuations will also play a role in the subsurface storage of heat. Water viscosity is a function of pressure, temperature and composition. This influences the hydraulic conductivity (see next section). The dependence on the temperature is by far the most important. Hence, this dependence must be taken into account in the analysis of subsurface storage of heat. 1.3 Groundwater flow Groundwater flow is described by Darcy's law. Darcy's law is in principle the form of the momentum balance (Navier-Stokes equation), averaged over a large number of pores. It also follows from a balance of forces on water flowing through a porous medium. Substitution of equation (7) in equation (4), assuming that the density Ï is constant then gives Darcy's law in terms of the groundwater head h: 1 z z z z