Complex sentence: Some tap water samples were also contaminated with ammonia at levels that were 6-18 times higher than the allowed level, and arsenic levels that were two to three times higher than acceptable World Health Organization guidelines.
Split into simpler sentences: Some tap water samples were also contaminated with ammonia at levels that were 6-18 times higher than the allowed level. Arsenic levels were two to three times higher than acceptable according to the World Health Organization guidelines.