With each issue, Trib+Water brings you an interview with experts on water-related issues. Here is this week's subject:
Kevin M. Befus is a Mendenhall postdoctoral scholar with the U.S. Geological Survey. He received his Ph.D. from the University of Texas at Austin's Jackson School of Geosciences. Working with M. Bayani Cardenas, a professor in the Department of Geological Sciences at UT-Austin and an international team of researchers, Befus helped to map the world's groundwater supply.
Editor's note: This interview has been edited for length and clarity.
Trib+Water: Can you explain a little bit about the project, and why it’s necessary to map groundwater and know the amount of groundwater?
Kevin M. Befus: The main reason why it’s important to study groundwater in the first place is that about a third of the world gets their drinking water, the water they use for growing crops from groundwater. Groundwater is an incredible resource and very important for the water we use.
So how this study specifically fits into that is that we are currently using groundwater to support our crops, support industry, and so how much groundwater is used can affect how much water then flows to rivers and affects ecosystem health.
There have been recent studies where groundwater depletion, overuse of groundwater, has been mapped out and shows areas where we’re using too much groundwater to the point where we could affect the local or even larger scale hydrological cycle.
This study is trying to get into how does the time scale of groundwater flowing affect the renewability or sustainability of these groundwater resources. So we didn’t address the sustainability or renewability directly, but really the big question is how much of this young groundwater is there? And then that’s linked.
Shallow groundwater is … where the groundwater is most closely tied to the earth’s surface processes. It’s closely tied to the climate and water use.
Trib+Water: Had something like this ever been done before, in terms of a large scale map?
Befus: Pretty much before this study there have only been continental estimates, or maybe by country. We calculate the volume of groundwater in this paper … The last time that was done with this sort of rigor was in 1974, and so the man who did that also used that volume and said that we know more or less how much water is coming into the ground, so how much recharge there is, and so you take that volume and divide it by the renewal rate and you get a time scale — what the residence time of that water is. He did that for different countries, but not nearly at this resolution and not with the extra 40 years of information and research that has been done. That’s sort of the large global scale of groundwater ages.
There have been a lot of studies where both modeling and the chemical data have been used to find out how old the groundwater is in specific places, at most regionally, but never globally.
Trib+Water: How much groundwater is there?
Befus: We calculate the volume to be about 23 million cubic kilometers of total groundwater storage ... The total amount of surface fresh water, in terms of lakes and rivers, any sort of fresh water at the surface, that’s 100,000 cubic kilometers. So it’s 23 million cubic kilometers groundwater versus 100,000 of surface water, so this is a huge amount of water that is underground.
It’s not like we have rivers underground. In some places, like in Texas, there are caves so you actually do have essentially underground rivers through these caves, so that’s one type of groundwater. Probably I would say most of this groundwater we’re talking about is in the spaces between rocks, in the pores of rocks.
Trib+Water: Out of that groundwater, how much is actually accessible to humans?
Befus: We can drill really deep wells, and that’s really what determines how accessible it is. The definition of an aquifer is a water bearing area of rock that is economically viable for extracting water. Really, it depends. Are we willing to pay to extract the water?
What we are saying here is about 6 percent of groundwater is renewed or replenished on a 100-year time scale. If we want to care about how old the water is, then we can just, oh, we can maybe use 6 percent of the total volume of ground water, but that doesn’t have anything to do with how that’s going to affect ecosystems that rely on that groundwater. Rivers or lakes rely on that groundwater. Ours is less of a value judgment of whether it’s a resource that should be used, but this is one of the attributes of that groundwater, of whether it could be used, or should be used. It’s one piece of the puzzle. It’s not the whole puzzle itself.
Trib+Water: So would you say the availability of groundwater is a limited resource?
Befus: I would say that groundwater is a finite resource. It’s a giant resource. We have a lot of this water, especially compared to rivers and lakes. Underground, it’s an incredibly good resource. It reacts to climate more slowly than rivers and lakes, so during a drought, groundwater is a great source of water, depending on if it’s clean…
But it’s also very variable in space. Even just the amount of rain, how much it rains somewhere, is very variable. We have dry areas. We have wet areas. So we’re showing that groundwater, we have some places where there’s a lot of this young groundwater and other places where groundwater is a lot older. We have places where groundwater is moving more slowly, and may be less of a renewable resource in the places where there is less young groundwater.
Trib+Water: Do you see this having an impact on the U.S., particularly Texas in terms of the longterm availability of water and how we consume it?
Befus: Our study, we actually don’t take into account any sort of pumping or any use of the groundwater. It’s under "natural conditions." When you start to pump groundwater or if climate changes and you have a lot of wet years or decades, it will affect how much replenishment of the groundwater system there is. So these numbers we show on our maps will change as time goes on. The models we used are in steady state. The chemistry that we have does incorporate the variability in the groundwater systems over the past 50, 60 years, but the amount of this young groundwater will change and is 100 percent dependent on how much we pump.
Actually, the more we pump the more young groundwater there is because you pull water toward the well faster. Young groundwater itself is not a metric of the sustainability because, if it were, we would just pump and pump and pump like crazy and make more young groundwater, and in the end we would not run out of this groundwater but we would affect all of the systems that rely on this groundwater.
For Texas, Texas does rely a lot on groundwater, especially out in West Texas where we have all of the agriculture, and then San Antonio gets all of its municipal water from karst aquifer, so like the cave groundwater systems. I don’t think our study directly relates to any of these.
Trib+Water: What was the research team's process for this project? How do you go about mapping something that spans continents and is stored below the surface?
Befus: The base where we started out on this project was that the head, Tom Gleeson, he’s been doing a lot of large scale, global projects with groundwater. He brought together a bunch of data sets, a bunch of published papers to create essentially a map of the hydrologic properties of the Earth.
He made a map of the permeability of the shallow subsurface for the whole world, which is one of the main parameters that controls how groundwater flows. So he made a map of that, then he recently updated that and also produced a map of the porosity, how much space there is in these rocks for the water to exist. We used that to calculate this total volume of groundwater.
From there, really we were motivated to try and understand the renewability and the sustainability of groundwater, but the more we thought about it, the more we realized that question was too big, that we couldn’t do that at this time with the information that was available.
Before I was involved in this project, he contacted Scott Jasechko, who is a professor at the University of Calgary. Scott Jasechko is an incredible geochemist so he finds papers and brings in original data of the tritium. So the tritium measurements that we used in this paper, he amassed all of that data, some of which he collected himself, and found which data could help him understand what percent of the water in a specific groundwater sample, from a specific well, how much of that water was young. From that he got over 3,000 data points from around the world at different depths … He made the first estimate of the volume of young groundwater on Earth, but he had to collect all these data and use it for one estimate with an uncertainty range.
So they thought, wouldn’t it be great if we could actually model this spatially — if we use what we know about how groundwater flows, use the mathematical equations behind it to try and get more of a distributed calculation. Primarily because the date that Dr. Jasechko used was biased toward aquifers, so places where we want to drill wells, was where his data came from. There’s a large portion of the Earth that wasn’t covered by that data.
I was brought in, sort of fortuitously from my perspective, because my group at the University of Texas, we’re physical hydrologists, so we do a large amount of field data collection of our own, but we’re also modelers, so we do a lot of numerical stimulations. Tom Gleeson came and gave a talk at UT, and through his visit he learned that we were doing all of these groundwater models and so he thought, “Maybe these guys can help move this project along and figure out a way to do this for the world.”
About a month or so after he came to give a talk, he contacted my advisor, who is Dr. Cardenas at UT, and from there we had to try and figure out what was possible — how we could possibly try to run these groundwater age simulations for the world. So that was my last couple years of my Ph.D. at UT.
Trib+Water: What are the plans for this information from here?
Befus: Our main point of this research is to really show that groundwater is a finite resource. We can just stick a bunch of wells in the ground, we can stick a bunch of straws in the ground and suck up a bunch of water, but in the end it still is a finite resource…
We hope that, one, people start thinking about how old groundwater is, and really that’s related to how quickly groundwater flows. Groundwater flows pretty slowly, right? Only 6 percent of this shallow groundwater, within two kilometers, is modern.
So, really, not all groundwater is the same. We have a lot of groundwater but in some places water moves more quickly, so it may be more renewable there, not all places in the world are the same. This is a first attempt at really mapping this movement.