From ‘Sources of Magic 13th Edition’

Depleted Mana Wells

After a millenia of normal use, or abuse, a ley line tapped well can run dry of magical power. This is usually preceded by a Leymoth infestation producing a collection of cocoons that will later replenish the Leymoth population in that area.

Normal procedure is to wait for all the cocoons to hatch, and then seal the dormant well.