Is it permissible to continue working in a job that relies on falsifying reports concerning water purification—from which several states drink—and the salary is derived from that; is it lawful or unlawful?

It is not permissible to conceal the truth or to cooperate with those who cheat and deceive in their deception, even if it leads to termination from work, especially if the water is contaminated and harms people, in which case the person becomes complicit in the harm. The truth must be stated and efforts made to change the situation, because advising Muslims is obligatory, for "Religion is sincerity [or good counsel]: to Allah, His Book, His Messenger, and to the leaders of the Muslims and their common folk." The officials responsible for the deception must be advised, and if they do not respond, or if advising them would cause harm, then it is obligatory to inform those who can prevent this harm and ward off corruption from the people.