What strikes me as most significant is that water drawn from the "high-level aquifer" is already brackish and approaching the point of not being drinkable.  That most likely indicates that the Ghyben-Herzberg freshwater lens under Lanai is relatively small and thin -- probably an ongoing natural condition (since Lanai sits somewhat in Maui's "rain shadow") and partly due to historic long-term depletion for agriculture.

It's an obvious fact of nature that one can't continue drawing more fresh water from this lens than it receives from natural "recharging" processes, without depleting it.  Excess withdrawal causes the "mixing zone" of brackish water to rise upward, and eventually freshwater wells start turning brackish.  

That the high-level aquifer water is brackish is great cause for concern -- and fiddling with the definition of "potable" so it can continue to be used for golf course irrigation is turning a blind eye to the problem and will only make it worse and more intractable.