The health officials, investigating contamination of seven wells in Taleigao today indicated that it may be due to mixture of ammonia in ground water. Though the final laboratory report on the water samples of five wells, is expected on Friday, sources in the department indicated that it may be due to mixture of ammonia. Samples of two others were not taken as they are in close proximity with the four. Last week seven wells in Chincholem and four in Borbhat, from Taleigao were reported to be contaminated with dead fish floating over the whitish colour water giving a foul smell.