On August 1, 1993, at 0950 hours Pacific daylight time, a Cessna A150L, N5958J, and a Grumman 164B, N8407K, collided over the threshold on Runway 2, at the Ephrata Municipal Airport, Ephrata, Washington. Visual meteorological conditions prevailed at the time and no flight plan was filed by either pilot. Both airplanes were destroyed and the student pilot in the Cessna, and the certificated commercial pilot in the Grumman were seriously injured. Both flights originated at Ephrata for local flights. The student pilot was practicing touch-and-go landings, while the pilot in the Grumman was returning from an aerial application flight that had originated from Ephrata at 0915 hours. The student pilot reported that he had performed one touch-and-go without incident. After take off, the pilot announced his position on downwind, base and final. While on the base leg, the pilot stated that he saw the spray plane and thought that it was taking off from runway 11. The student did not see the spray plane again until just before the collision. The student reported that he did not hear any radio transmissions from the spray plane. The spray plane pilot reported that after the completion of an aerial application flight, if there was any herbicide left over, he had an agreement with the airport authority that he would spray next to the runway for weed control. The pilot reported that as he approached the airport from the north, he announced on 122.8 that he would be spraying on the west side of runway 20 and on runway 11. The spraying altitude was approximately 50 feet above ground level. The pilot reported that he did not see or hear the student pilot in the pattern. The collision occurred over the threshold of runway 2 at an elevation of approximately 50 to 75 feet. The UNICOM operator reported that she heard the student pilot announce his position while in the pattern, however, she did not hear the spray plane pilot at any time. A witness, who is a certificated flight instructor, was in an airplane in the run-up area for runway 2 at the time of the accident. The flight instructor reported that he heard the student pilot in the pattern make clear and accurate position announcements. The flight instructor first observed the spray plane when it was approximately 100 feet above ground level and flying almost directly over his airplane. At no time did the flight instructor hear position announcements or intentions from the spray plane.