A lot of contaminants, like lead or arsenic, won't be obvious without tests, but some clues, like cloudiness, brown or yellow coloration, a foul odor, or an excessive chlorine smell, can indicate the need to investigate further.
许多污染物,如铅或砷,不经检测难以察觉,但一些迹象——如浑浊、棕黄变色、恶臭或过重的氯气味——则提示需进一步调查。