Boreholes or dug wells and shafts are used to recharge shallow phreatic aquifers where surface layers have a low permeability and spreading methods are not feasible. Abondened (dug-) wells that have run dry, due to falling water tables resulting from over-exploitation, are often used for this purpose. The use of existing structures is advantageous because it reduces costs.  During reverse drainage, the source water flows to a network of perforated drainage conduits, after which it infiltrates into the soil. This technique is most appropriate where land is expensive, because it has a negligible effect on surface land use.