Query on sandy water from borewell

How to filter sandy water coming from borewell?


I installed a borewell in rain water pit due to less water in the borewell. I drilled 12 mm holes in my casing up to 4 feet height and wrapped it with screen. This pit is filled with 40 mm mortar, 10 mm mortar and sand. Whenever there is rainfall, water level increases, but bore water is now turning into sandy (light brown) colour). How do I get rid of this problem?

Kindly suggest,