No idea on why there is so much sand. [SUp hear in Rhode Island we are supposed to drive the 6" casing into the bedrock. Sometimes it doesn't get that far and ground water will wash in to the shaft carrying sand, silt, and other contaminants into the well. Other times there can be fissures in the bedrock that not only bring water into the well shaft but rinse sand and silt in with it also. If the sand is coming in above the pump it may be coming in from ground water as you get lots of heavy rains. This may sound weird but we once had a customer who's well would get filled in with sand and every year we would have to pump it out. If we weren't careful and didn't pump the water far enough away it would rinse back into the well taking more sand with it. This may be happening to you. As with any hole it hard so see inside, never mind one that deep so these are just guesses. Good luck.