water table vs groundwater level

water table and groundwater level both are nouns.

Word	Noun	Adjective	Verb	Adverb
water table	Yes	No	No	No
groundwater level	Yes	No	No	No

As nouns, water table and groundwater level are synonyms defined as:

water table and groundwater level: underground surface below which the ground is wholly saturated with water

Other synonyms of water table include water level.

water table (noun)	groundwater level (noun)
underground surface below which the ground is wholly saturated with water	underground surface below which the ground is wholly saturated with water