Inverted wells in unconsolidated aquifers of limited saturated thickness

For unconsolidated aquifers of limited saturated thickness and adequate permeability, inverted wells can provide the discharge required for public water supply, industry or extensive irrigation. In an inverted well, the housing (casing) extends to the base of the aquifer with up to four screens projecting upwards; the pump can be located toward the bottom of the housing. Inverted wells have proved to be successful in the older alluvial deposits in the north-west of Bangladesh. Field measurements at a pumping site, where a conventional well was replaced by an inverted well, demonstrate how an inverted well provides a substantial discharge in an aquifer of sand zones even when the saturated thickness becomes less than 20 m. Measurements of groundwater heads at the water table and deeper within the aquifer system lead to a conceptual understanding of the flow processes associated with an inverted well. Since the pumping water level in the housing can be below the top of the screens, an investigation into conditions in the gravel pack indicate that there is a column of water in the gravel pack which is higher than the pumped water level. Comparisons are made between conventional and inverted wells. Although an inverted well costs more than a conventional well, its ability to provide a reliable discharge from an aquifer of limited saturated thickness is a significant advantage.