"how could there be a well! in the dark walk? that she had visited year after year? haughtily she said, 'nonsense.'" why is julia described as "haughty" in the quotation?

Because she is being arrogant

Explanation:

Julia has done the Dark Walk run since she was a little girls and she remembers it absolutely well, so she considered impossible and even more ridiculous that there could be a well in such dark and scary place, trying not to fall in what she thought was a trick she contemptuously said "Nonsense" as a way to tell them she was not being foolish.