Dave,
I agree that it is a smaller area that will really be affected. This 1600km2 will still lose 5000ML/day to evaporation.
Your concern about brine pollution is absolutely valid. The brine will be 60% saltier than the gulf water and will need to be discharged in a way that ensures mixing. This may be achieved by pressure injection into the gulf from a series of holes in the discharge pipe, to prevent a homogenous hypersaline solution which would sink.
Good luck with it all.

Rojo; I'm sure BHP will try to mix the brine with Gulf water, but how effective will that be? If their brine is 60% saltier than the water in the Gulf, and then they mix 1 volume of brine with, say, 5 volumes of Gulf water the resultant mix will still be 10% saltier (if my calculations are correct) than Gulf water and will still go to the bottom.