This could be a reason for increased activity after a rain:
"
Ernest Thompson Seton wrote that so far as he knew, the groundhog does not drink water but, like the rabbit, satisfies the need for liquid with juices of food-plants, aided by their sprinkling with rain or dew.
[18] Schoonmaker said it is possible groundhogs enjoy the rain water that clings to plants because so far as he was able to determine, groundhogs do not drink and it seemed the juices of plants supplied the needed liquid."
I have never seen one directly consume water, but did see one swin across a shallow pond several years ago. Neat creatures.