Behavior of suspended and colloidal particles during artificial groundwater recharge

Transport phenomena in the subsurface environment are often treated on the basis of a two-phase system: an immobile solid phase and a mobile dissolved phase. Many organic and inorganic substances, e.g. PAH, biogen material and heavy metals, have a strong tendency to adsorb readily onto the solid phase, thus, considered to be virtually immobile. However, this solid phase, generally assumed to be immobile, could be mobile as colloidal or suspended particles in the aquifer. These particles can be assumed to be chemically similar to the surface of the aquifer material and hence may have a huge sorption capacity due to their large surface area. Thus, colloidal and suspended particle transport must be taken into consideration in describing and predicting the movement of substances. Particles are abundant in the subsurface environment. Particles are associated with geologic matrices or form due to geochemical reactions. The geochemistry and hydraulic boundary conditions of the aquifer determine the formation, mobilization and transport of particles. They can be mobilized by a hydraulic or geochemical gradient and reach a concentration up to 25 mg/l at the monitored test site “Insel Hengsen” located within the Ruhr valley, Germany.