What is the significance of the water in Ingmar Bergman's The Virgin Spring?

From Ingmar Bergman, Cinematic Philosopher by Irving Singer (p 55-56):

The miracle of the gushing water in the Virgin Spring alerts us to the
fact that everything we have seen is a reflection strictly controlled
by the aesthetic parameters of a religious legend. From the very first
shots of the movie…each scene and every event in the narrative has
beguiled us with its surface realism…Only later, when the couple finds
the body of the dead daughter and the father eventually repents his
violence, does the tone transcend the neorealism that has been
dominant until this point. Bergman establishes through the miraculous
spring the reflective import of the realistic images we have been
watching. We recognize then that they were more than merely realistic…

When he had finished murdering [the brothers and the boy], the father
looked at his two hands with dazed astonishment at what he had done.
After the miracle, he holds up his hand as the offending member that
performed the deed, and that he will now use to build the chapel. This
alerts us to the fact that we have perceived the unfolding of a fable
that is partly realistic but also designed to provide an explanation
of how our body, and specifically that much of it with which we
identify ourselves, can play its roles in a universal search for moral
and spiritual redemption.

So the spring is our cue to look deeper at the story. It also marks the point where redemption is possible and suggests that in our own lives, we should look beyond the events for the spiritual lesson.

Singer is a philosophy professor at MIT who has studied and written on the works of Bergman, Alfred Hitchcock, Jean Renoir, and Orson Welles. The Virgin Spring won the best foreign film Oscar for 1960.