With simple disinfection, there is no assurance that the chlorine will get to the bottom of the well or into the aquifer around the well. Water is flowing from the bottom of the well up to the pump intake, minimizing the chance that any portion of the well below the pump intake will be exposed to the chlorine. When used for treating existing water supply systems, it should be used only for the first treatment. If simple chlorination is unsuccessful at disinfecting the existing well, the bulk displacement method of well chlorination should be used.