Not GAM related, but this reminds me of once finding soil inside the fuel tanks of a Piper Tomahawk at Essendon during a preflight fuel drain. Refused to fly it and the owner said they would sort it out. Turns out they did not for quite some time. Always thought it highly suspicious - the perfect contaminants to block the filter causing an engine failure. It would be difficult to differentiate the cause amongst the burnt out debris field. An insurance claim waiting to happen. Still have a photo sent to me from the hour hungry instructor who worked for the mob when they finally flew it to Moorabbin for inspection - a half a shovel load in each tank.