It was a well. Every well is a spring, but every spring is not a well. Any water that rises from the ground, and can be drawn for use, is a spring: but where it is ready at hand, and on the surface, it is called a spring only; where it is deep and low down, it is called a well, not a spring.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Tr. xv. c. 5