It should not be very much surprising. Current situation can be attribute to the excess monsoon rains which has contribute to the sea water. Fresh water is lighter as compared to sea water which is heavier. Thus the lighter fresh water layer will float on the more dense heavy sea water. it is known that transition zone exists between fresh and saline water. Same is true in dug wells near to the coast which have fresh water on the top and any excess withdrawal leads toe water becoming brackish and saline. (Ghyben Herzbergs relationship can be referred)
