Water is a solvent that is absolutely essential to sustain life. It is one of the world’s most precious resources. The uniqueness consists in its polarity, high boiling point, and some other important properties. Several of the water parameters, for example, alkalinity or redox potential, were discussed in this chapter. The environmental significance of these basic parameters is enormous because they are affecting the stability and mobility of substances in the ecosystem, and other parameters are either directly determined or strongly influenced by them. In addition, further important geochemical aspects, presented, for example, in the graphical form, i.e., the pepsilon-pH diagram (Pourbaix diagram) or redox ladder, were thoroughly explained in this chapter. Subsequently, the in situ chemical reduction (ISCR) techniques were discussed with the special emphasis on the nano zero-valent iron (nZVI). The nZVI reactions and thermodynamics behind them were also reviewed here.