Is clear water pumped from a deep aquifer a pure substance?

Yes

Explanation:

As pure substances are substances that are formed directly by elements and water is directly formed by the union of two elements which is Oxygen and hydrogen, H2O is the formula and that is a pure substance as it is conformed mainly by only these two elements, so the question would be Yes, clear water pumped from a deep aquifer is a pure substance.

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.