If we do not draw water every day there is a danger of the well being polluted. It is even worse if the well had not been maintained, i.e. that salvation has not been the foundational belief that is both believed and acted upon daily. I think this picture refers to many wells and is the reason why the gospel is not being preached with power. We are offering people polluted water. The spiritual analogy is pretty clear.
Salvation is a continuous process.
We build up the side of our wells with the bricks of salvation bought dearly for us by remembering Christ s suffering on the cross and the blood price He paid to redeem us. The cement holding the sides together is our living faith. Any cracks in our faith by way of compromise cause the water to become polluted with the earth of worldly cares. Undealt with, these lead to the blocking of the well. In practice this means living in the light and dealing with sin and unbelief on a daily basis, dying daily to self and preaching the good news in word and deed.