From the net:
If your well suddenly starts to pump fine sand, this may indicate that the well is filling with sand. Typically the well pump is set so it’s a minimum of ten twenty feet higher than the base of the well. When the pump is down near the base of the well, sand or grit and sediment can be drawn in. In old wells the well shaft can fill up with fine sand and silt so much that the pump may start to suck in sand from the bottom.
Other sources for sand in water may be the fact that well screen is becoming degraded and it’s allowing sand or sediment in from the gravel pack around the well screen. When the well is drilled, it is lined with a steel, iron, or PVC plastic which is called a casing. The casing is the well shaft which is installed by the well driller. The well casing has spaces that allow water to penetrate the well from the surrounding ground water and at the same time keep out sand and grit. This is called the well screen. Over time the well screen can become degraded and corroded and allow silt and sand to get into the well.
In some wells the well pump can be too big a pump for the well and pull sand in from the surrounding acquifer. Sand can be very hard on the well pump and may rapidly wear out the pump, valves as well as fill up the bottom of the well. In any case, a sudden presence of sand or grit is not a good symptom and the source or cause should be established.
If see a lot of sand in your water it is best to contact your well driller to discuss the problem and discuss possible ways to fix this problem. Sometimes the well contractor can pull up the pump 10 to 20 feet in addition to end the problem of the sand. In some situations, a new casing may be advised. There are also special screens that the well contractor can install over the pump to keep out sand, although these are not feasible for some wells if the casing is very old or the well diameter too small. If it is not feasible or possible to repair, a new well may be recommended."