KWR rapport - KWR 06.068

Chemical clogging processes in the filter slots of pumping wells and drains in unconsolidated aquifers


Well clogging is important because of decreasing capacity of wells due mechanical or chemical forces. In this study the focus is directed on chemical well clogging. Some chemical processes were considered in case of a real well to investigate their contribution to the clogging process.
A literature review revealed that chemical reaction and biochemical activity in the well can be a reason of clogging. The most widespread precipitations are
iron hydroxide, manganese oxide and aluminum hydroxide. The iron and manganese incrustations are the result of mixing two water types: oxic water (which contains oxygen , Fe(III), Mn(III), Mn(IV)) and anoxic water (without oxygen, but containing Fe(II) and Mn(II)). Aluminum hydroxide is a result of mixing of waters with different pH. Microbiological activity plays an important role in chemical clogging. Bacteria speed up the iron hydroxide oxidation and the produce slime, which clog the filter slots. All possible rehabilitation and prevention methods were analyzed in case of chemical clogging.
Field site Vierlingsbeek was studied to investigate the reason of clogging processes. Microscopic analyses of incrustation from the well screen indicate
that they are mainly composed of iron hydroxide (ochre). Chemical analysis by PHREEQC (beaker experiment) and PHT3D (steady-state pumping) revealed that two processes are needed: mixing and precipitation, kinetics. Result of models is the same as a result of microscopic analysis: ochre deposition due to mixing of oxic and anoxic water is a reason of clogging well.

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