Researchers have gotten a good — and unique — look at the country’s groundwater system. And it shows that system is more interconnected than scientists previously thought.
Laura Condon, an associate professor in the Department of Hydrology and Atmospheric Sciences at the University of Arizona, where she builds large models of hydrologic systems, worked on this research, along with scientists at Princeton.
Condon joined The Show to talk more about things she found that were surprising.
Full conversation
MARK BRODIE: Is it fair to say there were some things that you found that were surprising to you?
LAURA CONDON: Yeah, so it's really well known that groundwater and surface water are connected and we know that subsurface flow paths are really important, but they are very difficult to observe just because of the cost of drilling wells and taking measurements.
And so what was really cool about this work is that we could build this model of the entire country and see how things were happening across scales, so not just across a single watershed, but across watersheds. And so it was really, I think, surprising just to see the extent of the connections and how many really long flow paths exist in our groundwater system.
BRODIE: Yeah, let's, pardon the pun, drill down on that a little bit, ‘cause I'm curious, so does this basically mean that water from different parts of the country, groundwater from different parts of the country, is maybe more interconnected than we thought it was?
CONDON: Yeah, for sure it does. I mean we're not talking about groundwater going from coast to coast. There are still barriers to this at the very large watershed level and based on geology and mountain ranges and stuff like that. But it means that within our kind of smaller subbasins and things like that, that water is routinely crossing what we often think of as watershed and subwatershed boundaries.
So specifically we … showed that it’s really common to have groundwater flow paths that are longer than 50 kilometers, and even longer than 100 kilometers is not unheard of.
BRODIE: What are the implications of that?
CONDON: I think the big implication of it is just to show that our systems are more connected than we think, and that when we try to do things like do water balances on small basins and think about the ins and the outs, that there's likely a lot of water that's crossing through the subsurface that is not obeying those boundaries, and that can kind of mess up how we think about our accounting in watersheds if we're not careful.
BRODIE: Well, so I'm wondering what that might mean, for example, for some places in Arizona where groundwater pumping has been happening at faster rates than that water is being replenished. Might this have an implication for those particular areas either on the positive or the negative?
CONDON: Yeah, in places like Arizona and the western U.S. in general, we already have deeper groundwater systems, and so one thing that's actually just cool is just looking across the U.S. from the east to the west at how those groundwater systems look different. And because we already have this huge history of groundwater development, we've already disconnected a lot of our groundwater systems, and we have deeper and longer flow paths already. And so what that means is that our systems are slower to respond and that we have a lot less connections between groundwater and surface water than we used to.
BRODIE: When you say slower to respond, does that also mean slower to recharge?
CONDON: Yeah, absolutely, and recharge is a complicated topic, but recharge is really depending on the intensity of rainfall we get, not just how much rainfall we get, and in a lot of places, like Arizona, depending on where you are, we have very, very little natural recharge that's occurring.
BRODIE: I'm also curious what this might mean for issues and maybe concerns about pollution from one place to another if groundwater systems are more interconnected than we maybe thought they were.
Is it possible that problems in one area might wind their way into another area without people necessarily thinking about that?
CONDON: Yeah, absolutely. This is something we worry about a lot when we think about groundwater contamination. “How far can it spread?” and “Where all can it go?” And we really show that there's a lot of connections happening over long distances, and what's critical about what we're showing is that these are connections that we traced back from surface water bodies. So this is groundwater actually making its way back to surface water, and so you can think about groundwater contamination not just spreading underground, but groundwater contamination also making it back to surface water bodies.
BRODIE: How do you see this research fitting into conversations both maybe at the state level and also the national level about dealing with an overextended Colorado River and dwindling groundwater supplies and just general water scarcity? How does this research and what you found fit into those conversations?
CONDON: Well, I think in general, we just really want to raise awareness about the importance of groundwater, and the fact that you can't just view watersheds from the surface. You can't just think of it as, “oh, here's the snowpack, here's the water in the river and we're good.” Actually 99% of our unfrozen freshwater is underground.
Groundwater is huge, and even if we're really concerned about what's, say the stream flow in the Colorado River, the important thing to know is that a lot of that stream flow has spent some of its journey underground, snow melts and can infiltrate and then reappear in the river.
So if you want to understand our rivers and how they work, you have to understand how they're connected to groundwater, and this becomes increasingly important as we look to the future and think about groundwater levels that could be changing, because that can change how much groundwater actually makes it to the stream.
BRODIE: Yeah. So now that you have this data, what would you like to do next? Like, what's the next thing you'd either like to try to figure out or maybe use this information to answer the next question?
CONDON: Yeah, so we're really excited about the models that we've been able to build. This really large scale national model that we're able to do this particle tracking on, supported this work. Moving forward, we have a lot of exciting opportunities to start to do more transient scenarios like, “OK, what would this look like in the future if this changed, if things got warmer, if we changed the snowpack, if we change land cover like with large scale wildfires?” And to see how those kinds of changes might reshape the flow paths in our watersheds.
What we have right now, what we presented in this research, is kind of like this background, natural this is what the flow paths are in general, but there's a lot of questions about how these can evolve and change in the future, and over what time scales that will happen, and that has big implications for our surface water systems. So that's what I'm really excited about going forward.
