Well-screen and well-head clogging by hydrous ferric oxides

In the production of drinking water from groundwater by aeration and rapid sand filtration (RSF), iron(II) may be removed in two ways: at pH > ~8 by homogeneous oxidation and precipitation, and at ~6.0 < pH < ~7.5 by combined heterogeneous and biological iron(II) oxidation and precipitation. In line with this distinction, wells may become clogged by the accumulation of hydrous ferric oxide (HFO) precipitates in two ways: at “high” pH (pH > ~7) by development of HFO flakes resulting in minimal clogging of the screen slots, and at “low” pH (pH < ~7) by massive HFO precipitates and biomass resulting in extensive clogging of the screen slots. As HFO precipitation continues, both processes result in extensive well-head clogging, i.e. fouling of submersible pumps, collector lines, etc. While well-screen clogging may be monitored conveniently, no general method is available for monitoring well-head clogging. Recognition of both chemical clogging types is important for effective well construction, operation and maintenance. During rest, HFO precipitation processes continue, further blocking pumps and water lines. If a vertical hydraulic gradient is present over the height of the well screen, HFO precipitates may accumulate on the well screen and enter the aquifer. Consequently, the behavior of the well is important not only during operation, but also during rest. These findings are illustrated by numerous observations on well fields exploited by the drinking water utilities in the Netherlands.