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The questions that we ask as scientists shape everything that follows. They can lead us to see the world in new ways, or mundane ones. They can spur the development of new approaches, or the recycling of established ones. They can focus our attention in useful directions, or leave us wandering aimlessly. In science, as in everything else, what you find will largely depend on what you were looking for. Thus, sometimes one can be helpful simply by framing questions, even if—or especially if—the answers are not yet clear. My purpose here is to identify questions that I hope will be productive ones. I make no pretence of presenting answers. These questions have emerged from long hours pondering hydrochemical data like those shown in Figures 1 and 2. The same kinds of data have also been pondered by many others in our community, so I make no particular claim of originality for the observations that follow. I …