Thomas Kwader expains a useful tool for proper abandonment of all unused wells in order to protect ground water resources.
Abandonment of screened wells by filling the casing from top to bottom with neat portland cement usually is an effective way to seal the borehole. If there is an opening along the annular space (outside of the casing), the opening should be sealed with cement grout or a flowable mixture of bentonite pumped through a tremie pipe.