Filling a borewell by sand

Due to extra depth about 25 feet water is salty so we want to fill this extra depth by sand …our tubewell is 425 feet wholelength casing with cement pipe 10 inch diameter arond the casing 10mm gravel is filled there is no rock whole point is done by rotary machine…inside the casing we can fill black sand that cant passed water but i want that due to pressure no salty water is come up from outside the casing through 10mm gravel…i am from rajasthan state nagaur district merta city teshil plz suggest advice and any expert related to this…i am an engineering student and my uncle have rotary machine(if any use of rotary machine in this technical issue)
my conact