Apparently it is our excess coming in, for now at least we have some productive wells going and O.C. at least is pumping purified water back into the aquifers. This is what I have been told anyway. Take it with a grain of salt, like many things that end up on FR : )
Orange County processes its’ raw sewage, and then pumps the reclaimed water down into the local aquifers.
It also has a complex system of channeling water from the Santa Ana River into an array of settling ponds, where it then drains back into the same aquifers.