In "The Blessing Tobacco," where does David get a drink on his way home from his grandmother's, and what is the significance of this?

David is so thirsty on his way home that he stops to drink from the river, even though he knows it is polluted. That is, David's circumstances are such that even the things that are meant to nurture and sustain him--like water--are potentially dangerous to him. The source of the river's pollution--and the danger to David's health--is not the reservation itself, but the white world's "poison" represented by the mill north of the river.