On July 21, 2011, about 1926 central daylight time, an Air Tractor Inc AT-502, N1516G, experienced a total loss of engine power following the completion of an aerial application of a field near Fergus Falls, Minnesota. The pilot subsequently made a forced landing to a field along a highway. The certificated commercial pilot was uninjured. The airplane sustained substantial damage to the fuselage and wings. The airplane was registered to and operated by Farmers Union Oil Co. (DBA Cenex Airspray) under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed at the time of the accident. The flight originated from Fergus Falls Municipal Airport-Einar Mickelson Field (FFM), Fergus Falls, Minnesota, about 1810. The pilot stated that the airplane was fueled from a 300 gallon tank used to shuttle fuel. The pilot stated that there was 120 gallons of fuel aboard the airplane at the time of takeoff, and the fuel gauges indicated that the fuel tanks were 1/3 full. The pilot stated that the fuel consumption rate was approximately 50 gallons per hour. The pilot stated that he departed FFM and performed two aerial applications on two different fields before returning to FFM. He stated that his cruise altitude was 600 feet above ground level for the return. About 4-5 miles south of FFM, the airplane started losing engine power. He turned on the fuel boost pump and igniters to continuous operation. About 1 mile south of the FFM, the airplane experienced a total loss of engine power. The pilot then attempted a forced landing to a highway but had to abort the landing because of a vehicle that was turning onto the highway. The pilot landed into an area of 2 foot deep water located on the east side of the highway due to a powerline on the west side of the highway. The airplane nosed over during the landing. The airplane remained inverted until it was examined at FFM. The airplane was examined by a Federal Aviation Administration airworthiness inspector from the Minneapolis Flight Standards District Office on August 2, 2011, at FFM. Prior to the airplane being placed in an upright position, the inspector removed the drain plugs from the firewall fuel filter. The main fuel supply line to the engine produced no fuel. Removal of the fuel line through the fuel/oil heat exchanger produced less than 1/2 ounce of fuel. When the engine was removed from the firewall, there was a mixture of water with some oil that drained out of the hot section, but there was no fuel. The airplane was then uprighted and water drained out of the fuselage section. The operator had a repair station examine the fuel control unit (FCU) and the fuel pump. The FCU exhibited corrosion damage on the drive section, main housing, and water beads were noted in the bellows (air) chamber. The fuel pump exhibited water contamination in the fuel bowl and drive spline.