Deposits on surfaces of water distribution systems can act as a sink and a source for contaminants in the water phase. An intruding contaminant will interact with the deposit and possibly be sorbed and accumulated into the more or less complex matrix of the deposit. Thus, in a contaminated system, not only the water phase is of importance but also the deposits at the inner surfaces, consisting of mineral precipitations, corrosion products, organic material and microbiol biofilms.