there is a service point (at least on our A/C ) just below the aft service door,for ground water to a holding tank for potable water.....have the schematic but cant post it......
note....dont think that was your question after re-reading it.....on occasion the water comes from the aft lav sink....I know this because during pre-post flight inspections,Ive taken a "hit"......hopefully this answers your question