On September 1, 1999, at 0615 hours Pacific daylight time, a Bell 206B, N59551, lost engine power about 1/2-mile after takeoff and force landed in a field in Delhi, California. The helicopter, operated by Bettencourt Flying Service, Inc., Delhi, was substantially damaged during the hard landing. The commercial pilot was not injured. The local aerial application flight was conducted under the provisions of 14 CFR Part 137 and originated from a private airstrip at 0605. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that the night before the accident, he washed the helicopter and then stored it for the night. He stated that on the morning of the accident, he completed the preflight inspection, during which he drained fuel from the airframe fuel filter and the fuel sump. He reported that he didn't note any water or contaminants in the fuel. The pilot stated that he started the engine and, after 5 minutes of warm-up, he lifted off and performed a pass down the runway before departing to the southeast. He reported that he was still in the initial takeoff climb, about 200 to 300 feet agl and about 1/3-mile off the end of the runway, when he heard the engine begin to unspool. He checked the rotor and engine tachometer and noticed that the rpm was decreasing. The pilot stated that the throttle was fully opened and he verified that the fuel valve was open. He was over a grove of almond trees as he began an emergency descent. He reported that he had to alter the angle of descent to make it past the trees, which resulted in further decrease of main rotor rpm. The pilot stated that the low rpm warning horn was sounding as he impacted the ground. He estimated that his speed on impact was approximately 20 to 30 miles per hour. The landing gear collapsed and the main rotor blades severed the tail boom. The helicopter came to rest on its left side. A Federal Aviation Administration inspector from the Fresno Flight Standards District Office responded to the accident site and examined the aircraft and powerplant. He stated that the throttle and governor linkage appeared to function normally. He inspected the turbine wheel through the exhaust stack and noted no abnormalities. The engine rotated freely. The inspector removed the airframe fuel filter and noted that approximately 90 percent of the fluid in the filter was water. Further, approximately 90 percent of the fluid found in the mechanical fuel pump screen and about 30 percent of the fluid from the Ceco fuel system screen was also water. He reported that the fuel cap did not appear to seal properly; the cap could be moved in the opening even when locked. He stated that it appeared that the shaft seal was worn in the cap and the cap locking mechanism was not adjusted properly. When the fuel cap was removed, the inspector noted that the bottom of the fuel tank opening was deformed in a dimensional shape that corresponded to the fuel nozzles used by the operator to refuel the helicopter.