In normal hydrochemically layered aquifers precipitation reactions mostly occur in the upper section of the screen, especially when the well pump has been installed in it. When incrustations, which hinder water entrance, have covered this section they continue to grow downward.
Bank filtrate wells
Wells that abstract groundwater adjacent to rivers encounter both iron rich groundwater and oxic river water. The sides of the river are often colmated (i.e. clogged with fine material and biofilm) which limits the amount of water which can infiltrate out through the sides. Because of this the majority of infiltration out of the river will come from the deeper riverbed. This indicates that the deeper sections of the well screen will be subject to increased growth of iron rich incrustations.
The spatial distribution of well incrustations is neither radially nor vertically regular. The screen surface which faces the direction of groundwater flow will be more incrusted. The uneven distribution of incrustations should be taken into account when planning a well rehabilitation.