On September 10, 1997, at 0920 central daylight time, a Grumman G-164A, N84PG, owned and operated by Edwards Aerial Service under Title 14 CFR Part 137, impacted terrain following a loss of engine power while maneuvering near Hillsboro, Texas. The commercial pilot was not injured and the airplane sustained substantial damage. Visual meteorological conditions prevailed for the local aerial application flight and a flight plan was not filed. The flight originated from a private agricultural airstrip at 0820. Telephone interviews, conducted by the investigator-in-charge (IIC), revealed that the airplane had been stored in a hangar for approximately 2 weeks with the fuel tanks 3/4th full. The pilot performed a preflight, loaded the hopper with a defoliant, and departed for the flight. During a downwind swath, the engine lost power and the pilot turned the airplane into the wind for the forced landing. The airplane struck a dirt terrace during the landing roll, the left main gear collapsed, and the airplane nosed over coming to rest inverted. The left wing and vertical stabilizer received structural damage. A mechanic and the operator reported to the IIC that debris was found in the fuel gascolator and the carburetor. Structural damage occurred to the vertical stabilizer, rudder, landing gear, propeller, and wing. In a telephone conversation, conducted by the IIC, the pilot reported the fuel contamination from fueling the airplane at the Hillsboro Municipal Airport. On the Pilot/Operator Aircraft Accident Report the pilot recommended "don't buy fuel at Hillsboro Muni[cipal] A.P." [airport]. During several telephone conversations with airport personnel and on the enclosed documents, the IIC gleaned that the airport has two 12,500 gallon fuel storage tanks for the purpose of refueling aircraft. Airport personnel use the Velcron filter system. Fuel testing recommendations from the manufacturer and the FAA were utilized with negative test results. Four of the airplane operators, who base their airplanes at the Hillsboro Municipal Airport and purchase fuel at the airport, reported that they had experienced no contamination with fuel purchased at the airport.