Orange County California has just put a wastewater treatment plant into operation that sends the treated wastewater directly into a reservoir that is used for drinking water.  Ordinarily I would be more uncomfortable with the idea of drinking wastewater directly after treatment because of the potential disaster that could result from any kind of failure.  This system gets around that concern by pumping the treated wastewater into a reservoir that will dilute and filter contaminants that get through the plant.  As discussed in my book, drinking treated wastewater is routine for most of us.  The thing that makes discharging wastewater into groundwater different is that those using the groundwater for drinking water tend to have limited treatment and expect the groundwater to be clean.  One would want to see extremely careful monitoring of the system to make sure it works as advertised, but overall, it has clear advantages over simply importing more water to the southwest. 

Nonetheless, the plant begs the question of the sustainability of the southwestern water supply.  They have, in effect, substituted treated wastewater for imported water, which, on the surface, should make the system more sustainable.  Nonetheless it relies on non-renewable energy supplies (I presume) to replace the solar driven hydrologic cycle. (Approximately 30% of all solar energy goes into driving the water cycle.)  Over the long term, the southwest will need to engage in extreme water conservation including innovative approaches such as household grey water recycling to even begin to address the long term sustainability of the region.