There are two kinds of elements that didn't appear on the periodic table until after 1892. what kinds are they and why do you think it took so long to discover them?

The two elements are Argon and Helium .

They are noble gas  and are not reactive

Explanation:

The two kinds of element are argon and helium. These two element were discovered in 1892 by a Scottish chemist Williams Ramsay.

These two elements are known as noble gas or inert gas  because of how hard they find it to react with other elements.

Elements back then were discovered through their reactivity with other elements but It took so long to discover these elements(argon and helium) because of their unreactive nature . Combine with the fact that they were odorless, tasteless and colorless it was more hard to discover them.

aquifers are bounded above and below by impermeable rock. the driving force of groundwater movement in confined aquifers is pressure, rather than gravity.

it is the most commonly accepted one