In examining water samples preserved in glass jars over the last century, you notice a steadily declining ph. what is the most likely general explanation?

There are many factors that can affect pH in water, it can be natural or man-made. Water samples preserved in the jar for a long time then suddenly be exposed to outside environment will exhibit steadily decline in ph. The general explanation is because the water interacted with carbon dioxide molecules.  This creates H2CO3  lowering the pH value but the reaction is usually minimal as H2CO3 has a low solubility constant (Henry’s Law).  

no i don't feel that humans have oversubscribed some medications