CABIN ATTENDANT INFORMS FLT CREW OF B737-500 THAT LAVATORY FLUIDS SMELL AS IF CONTAMINATED WITH FUEL. GND INVESTIGATION DETERMINES THE SUSPICION TO BE ACCURATE.

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DURING CRUISE FLT AFTER DEPARTING ZZZ1; THE PURSER CALLED TO INFORM ME THAT THE FORWARD LAVATORY SMELLED OF FUEL AFTER THE TOILET WAS FLUSHED. SHE ALSO SAID THAT THE LAVATORY FLUID WAS A MURKY BLUE; WHICH I NOTICED AFTER USING THE LAVATORY PRIOR TO DEP (I ASSUMED THAT PERHAPS IT WAS A NEW TYPE OF FLUID). WHILE I ATTEMPTED TO CONTACT ZZZ1 VIA ACARS; THE PURSER CALLED BACK TO SAY THAT THE AFT LAVATORIES HAD THE SAME PROB. WE TEMPORARILY LOCKED THE LAVATORIES WHILE I CONTACTED DISPATCH VIA VHF TO ASK THEM TO CONTACT ZZZ1 OPS CONCERNING THE LAVATORY FLUID; AS I SAW THE LAVATORIES BEING SVCED DURING THE PREDEP EXTERIOR INSPECTION. THE PURSER ASKED A DEADHEADING PLT TO CHK THE LAVATORY; AND HE CALLED VIA THE INTERPHONE TO EXPLAIN HIS FINDINGS. HE SAID THAT THE LAVATORY FLUID WAS CERTAINLY DIFFERENT; BUT HE DID NOT FEEL THE SMELL WAS FUEL AND THAT THERE WAS NO SAFETY ISSUE. AT THIS POINT; I RE-OPENED THE LAVATORIES WHILE CONTINUING TO PURSUE THE ISSUE WITH DISPATCH; WHO AT THIS POINT HAD BROUGHT MAINT INTO THE PICTURE. DISPATCH CONTACTED ME A SHORT TIME LATER TO EXPLAIN THAT ZZZ1 HAD USED THE SAME CONTRACTOR AS ALWAYS; AND THAT THEY HAD BEEN USING THE SAME FLUID FOR OVER 1 YR. I THEN ASKED HIM TO GET BACK TO ME VIA ACARS AFTER DISCUSSING THE ISSUE WITH MAINT. UPON OUR DSCNT INTO ZZZ1; APPROX 140 NM FROM THE FIELD; DISPATCH CALLED VIA VHF TO INFORM ME THAT MAINT HAD CONTACTED ZZZ1 AND HAD DETERMINED THAT THERE WAS FUEL CONTAMINATION IN THE LAVATORY FLUID. WITH THIS NEW INFO; I ASKED THE PURSER TO LOCK ALL THE LAVATORIES; AND I INFORMED THE PAX THAT THE LAVATORIES WOULD BE UNAVAILABLE FOR THE DURATION OF THE FLT DUE TO FOREIGN SUBSTANCE CONTAMINATION IN THE LAVATORY FLUID. WE ENTERED THE PROB VIA MAINT CODE; AND DETERMINED THAT ZZZ2 WAS OUR BEST OPTION FOR LNDG (NEAREST IN POINT OF TIME) DUE TO THE SEQUENCING OCCURRING WITH APCH CTL (TERMINAL WX ISSUES). WE LANDED UNEVENTFULLY (WE DID NOT DECLARE ANYTHING); TAXIED TO THE GATE; AND CONVERSED WITH MAINT BEFORE DEPARTING THE PLANE.