We were asked this question by a client who thought a 5,000-gallon underground oil tank could be emptied, steam cleaned and used to store potable water. The short answer is ‘once a fuel tank, always a fuel tank’.  It should never be used for fluids that come in contact or consumption by humans. Removing leachable compounds (contact toxins such as benzene and TCE at the PPM and PPB level) in the molecular grain structure of the base metal would be very difficult. The only way that you can safely convert such a tank is to clean by any means you choose; then validate by micro analysis that the residual levels of contaminates is certified below the current recommended levels.