In chapter 5, Winnie runs away and meets Jesse Tuck in the woods.  He drinks from the spring, and she wants to drink, too, but he won't let her.  Then at the end of the chapter, Mae and Miles show up, and they decide they have to take her home with them because the worst possible outcome has arrived (someone has discovered the spring). 

Though nothing is stated outright in this chapter, it is revealed that the spring is significant to the Tucks, and they don't want anyone else to find out about it.  The reader may then infer the reason for this.