My pump is almost 4 years old, work normal till recently I noticed sand mixed water flowing out of the delivery pipe. What should I do to make it normal again?
Try raising the pump a couple of feet. You may eventually need the well cleaned out.
If it is pumping sand there is a problem of course. The pump is generally hung about 10 feet from the bottom of the well ( Ex: if the well is 120 feet the pump would hung be at 110 feet). In your case it may have been closer to the bottom and picking up the sand. There is a possibility that recently another well has been dug in the area and disturbed sediment has gotten in the water. It sounds like you aren't using this as drinking water or the sand would have been dropped in the pressure tank and you wouldn't see it. If it is a seasonal use well ( like for your garden or lawn in spring through fall) this debris might wash out after a few uses.
