Tests are being done to determine whether Osceola’s drinking water supply was contaminated by a small diesel fuel spill yesterday. Workers re-fueling the Lakeside Riverboat and Casino near Osceola spilled about 15 gallons of diesel into West Lake, which is the sole drinking water source for Osceola. Jim Stricker of the Iowa Department of Natural Resources says it was the first time the boat had been re-fueled since its opening two months ago.Stricker says workers quickly used special “booms” on the surface of the water to soak up most of the spilled diesel.Stricker says it will be several days before test results are available, although he says there’s no immediate threat to Osceola’s drinking water.Stricker says some of the particles in diesel fuel are considered cancer-causing, and if it is found in unsafe levels in West Lake, Osceola will have to revamp its water treatment plant by adding carbon filtering.