PLT OF AN SMT TWIN RETURNED AND LANDED AFTER OBSERVING FUEL LEAKING FROM A FUEL TANK.

Narrative

I HAD MADE 1 ROUND TRIP AFTER WHICH I ONLY NEEDED THE FUEL TANKS ABOUT QUARTER FULL. ON PREPARATION FOR NEXT LEG; I ORDERED THE FUEL TOPPED OFF. I WAS PRESENT DURING FUELING; SUPERVISING THE LOADING OF ACFT. THE FUELER DID NOT INDICATE TO ME THAT THERE WAS ANYTHING WRONG WITH THE CAPS. BEFORE TAXI; I VISUALLY INSPECTED THE FUEL CAPS; THAT THEY WERE ON AND LOCKED DOWN. TAXI WAS NORMAL. ON TKOF; OUT OF THE CORNER OF MY EYE I SAW SOMETHING STREAMING BACK FROM THE L SIDE OF THE ACFT. I LOOKED AND SAW THAT THE CAP WAS LOOSE AND FUEL WAS ESCAPING IN A FINE MIST. I IMMEDIATELY REQUESTED TO RETURN TO THE ARPT. LNDG WAS NORMAL. SINCE THE FUEL WAS BLOWING AWAY FROM THE ENGS; I DID NOT REQUEST ASSISTANCE. ON RETURN TO THE GATE; I FIRST CHKED TO SEE THAT THE CAP HAD NOT JUST BEEN LOCKED IMPROPERLY. UNFORTUNATELY; THERE APPEARED TO BE A BAD SEAL. I DISEMBARKED THE PAX ; WENT OVER TO THE MAINT HANGAR AND RECEIVED A NEW CAP WHICH FIT TIGHTLY. I RETURNED TO THE GATE AND SPOKE WITH THE PAX ON WHAT HAD HAPPENED. THEY SEEMED SATISFIED WITH THE NEW CAP. WE DEPARTED AND THE REST OF THE FLT WAS NORMAL. MAINT WAS ADVISED OF THE PROB DURING THE SIT; GAVE ME INSTRUCTIONS; AND ATTENDED THE PROB ON MY RETURN.