## 2 Application of Membranes in Contamination Situations
### Groundwater Contamination  
Contaminants released into surface water or soil may continue to migrate and determine the quality of groundwater, which is often a source of drinking water. It should be noted that the amount and type of pollutants are influenced not only by the geological basis, type of land, vegetation, and human activity, but also by the hydrological system, water movement, or its intensity in the hydrogeological profile or hydrogeological unit. The movement of groundwater is often diversified, depending on the hydrogeological space and physical conditions, including speed, direction, hydraulic head, and pressure. All individual geochemical processes lead to the diversification of the chemical composition of groundwater. During circulation, mixing of groundwater of different composition occurs. This is a very important process that commonly takes place along the water flow paths, typical for the saturation zone, i.e., the rock layer in which the free spaces (crevices, pores) are completely filled with water. In the feed zone, along the streamline, groundwater mixes with successive portions of water percolating through the aeration zone [70; 71]. The chemical composition of groundwater in the drainage zone is usually the result of the composition of waters with different transit times, flowing from different distances, e.g., within a local or regional circuit [70; 71; 72]. The most important physicochemical reactions taking place in the groundwater environment include: dissolution-precipitation, oxidation-reduction, sorption, and ion exchange. Depending on the conditions under which they occur, these reactions may be reversible or irreversible. In the case of irreversible reactions, the composition of the water is usually determined not by the equilibrium state, but by the kinetics of the reaction. Many processes and reactions take place with varying intensity in different environments. Some of them are, under certain conditions, more important, e.g., for waters in the aeration zone and shallow waters of the saturation zone, they are chemical weathering and dissolution, and when describing the saturation zone, sorption and redox processes [73; 74]. In groundwater, electrokinetic and osmotic processes also take place, which consists of the selective migration of aqueous solutions through clay layers that behave as semi-permeable membranes. Such processes are commonly referred to as ultrafiltration or membrane processes. They are also referred to as osmotic processes, sieve effects, or ion filtration. Under certain conditions, they play an important role in shaping the composition of underground waters and deep waters [75]. For the migration of contaminants present in the water environment of the saturation zone, sorption processes occur relatively quickly (e.g., compared to the dissolution processes of most minerals) and the equilibrium stabilizes slowly during redox processes, which run much slower. In the deeper environments of the saturation zone, in conditions of difficult groundwater exchange, the influence of both groups of processes is usually much more visible than in the waters of the aeration zone [76]. The presence of gases such as O\({}_{2}\), H\({}_{2}\)S, CH\({}_{4}\), NH\({}_{3}\), and organic substances in a water-rock medium is of the utmost importance in most redox processes. However, the direction of oxidation-reduction processes in groundwater depends primarily on the content of oxygen, carbon, and organic substances as well as the forms (speciation) of sulfur, nitrogen, iron, and manganese. Most redox reactions take place with the active participation of microorganisms. The strongest reducing properties are exhibited by alkaline and alkaline earth elements. The role of oxidants can also be played by, inter alia, Fe(+3), Mn(+4), S(+6), and N(+5) [76; 77].