Unfortunately, a comprise (where both sides sacrifices a few of their needs) is needed.  If the issue is the underground brackish water, an option is to capture the water that is above ground. The tip of Lanai (Lanai Hale) is cold and holds  a lot of moisture.  A concept like (Mist Tree) may work.  Mist tree captures the moisture and turns the moisture into water.  moisture is abundant on Lanai Hale and will not affect the ecosystem.   
Looking for solutions..........