Most of us learned as kids that the water we drink is the same water that the dinosaurs drank: that the water coming out of the tap or flowing in the rivers is not new, but rather ancient. That early lesson came to mind as I was standing on the bluffs at Taylor’s Falls, watching the storm-fed St. Croix roar between the palisades. Interpretive signs here describe the river valley’s eventful past, with lava flows pouring from a rift in the continent and torrents of glacial meltwater deepening the channel. But what about the water itself? Is this really the same water that the dinosaurs drank? What does that even mean? How old is the water in the St. Croix River or anywhere on the planet, for that matter?
I posed my questions to Calvin Alexander, University of Minnesota Professor Emeritus in the department of Earth and Environmental Sciences: a noted scholar and widely cited researcher on all things water. In his response to my request, Dr. Alexander said he’d be happy to discuss my question, warning me of his tendency to launch into hour-long lectures (which sounded good to me). “My research interests,” he said, “include measuring the age of groundwater and the age and formation of the solar system. The question you ask is wondrously complex and nuanced.”
He was no doubt being generous by not suggesting I might have paid better attention in middle school. But the resulting conversation was enlightening and has fundamentally changed the way that I think about water. I thought I’d invite Agate’s readers along for the ride as I take my curiosity into a wide-ranging conversation with Dr. Alexander.
The following is excerpted from a May 2024 interview.
CA: If I understand, your question is, “what is the age of the water in the St. Croix River?”
LA: Yes, and the broader question of whether that is any different than water anywhere else on earth.
CA: Do you want to start at the present or do you want to start at the beginning?
LA: At the beginning, please.
CA: All right. What is water? Water is H2O: a molecule with two hydrogen atoms and one oxygen atom. The hydrogen atoms in water are over 13 billion years old. All the hydrogen in the universe was created in the Big Bang over 13 billion years ago. The oxygen in the water was created in stars that lived and died before the solar system was formed, and the solar system was formed about 4 ½ billion years ago. So, the hydrogen atoms in the water molecules are over 13 billion years old; the oxygen atoms in the water are over 4½ billion years old.
LA: The amount of the water that is currently present on earth in various forms—in the atmosphere, as groundwater or surface water—has that remained constant or is there still a process by which any more is created? Have there been any new hydrogen or oxygen atoms since that time?
CA: To our knowledge there is no process that has added oxygen atoms to the solar system since the solar system was formed, and there is no process that has added hydrogen atoms to the solar system since the solar system formed. The hydrogen atoms in the water molecules that are currently on earth have not necessarily been on earth for 4½ billion years but have been in the solar system for 4½ billion years. When a piece of a comet hits the earth, it’s made mainly of water ice, and that adds atoms and molecules of water into the earth’s system that weren’t there 4 ½ billion years ago. Indeed, most models assume that the earth was at least partially formed before the comets added most of the water to the earth between 4.5 and 4.4 billion years ago. Yes, there are still meteorites that contain water and there are still pieces of comets that contain water that are hitting the earth’s atmosphere today, but it’s absolutely negligible compared to the amount of water that’s already in the oceans and in the earth and tied up in other ways.
LA: And it all comes from within the solar system.
CA: Yes, that’s right. Well, there are still extra-solar objects that come into the solar system—there was one a couple years ago that came through and just whipped by the sun and went right back out and it wasn’t part of the solar system—but again, those are miniscule compared to the amount that’s in the solar system itself.
LA: How does one date water? For example, is the water in the Mississippi any different than the water in the St. Croix or any other river on earth, in terms of its age?
CA: OK, first we have to discuss what we mean by “age.” I have spent a major portion of my scientific career determining what we call the “age” or “residence time” of various groundwaters. And what we mean by that concept is, “how long ago did these molecules fall out of the sky as rain or snow?” We measure the age of the water in an aquifer or the age of the water in a river as the length of time since the last time it fell out of the sky as rain or snow. Because it gets recycled: it’s been through millions of cycles as it falls out of the sky, it runs off into the rivers, goes into groundwater, runs into the rivers, runs into the oceans, it evaporates, flows over the continents, or falls back onto the ocean. The oceans are 70% of the surface of the earth; most of the rainfall over the earth lands in the ocean.
LA: That’s interesting; when you speak of residence time, it makes me think of Lake Superior, where I thought “residence time” referred to how long it takes for water to move through the system, for every bit of water in the lake to be replaced by quote-unquote “new” water.
CA: Scientists have different working definitions for what they mean by residence time. In that sense, it’s not being used to mean “the last time it fell out of the sky as rain or snow,” but rather a calculation based on the volume of the lake and the flow of rivers into and out of the lake’s basin. It’s never “every” molecule, of course, because they’re continually mixing. This mixing creates challenges when assessing the age of any body of water. Go out to the St. Croix, take a bucket of water. Every molecule in that bucket did not fall out of the sky at the same time. A lot of the water molecules fell out of the sky very recently; some of them fell of the sky a long time ago. So this concept of residence time is always an average.
Having said that, let me give you some examples. I just saw an article that the Soudan Mine State Park has re-opened to tourist travel up on the North Shore. We’ve done a lot of work there. At the bottom of the Soudan mine the rocks are 2.7 billion years old. (The concept of the age of a rock is simpler because a rock is a solid object that formed in a reasonably short length of time at some point in the past.) At the bottom of the Soudan Mine, water is dripping out of drill holes back into that 2.7-billion-year-old rock. We think that the water was trapped when the rock formed. So, I can take you to a place in Minnesota where I think there is water that is essentially 2.7 billion years old; that water fell out of the sky 2.7 billion years ago. Or we could look at the rain gauge in the back yard, and talk about water that fell out of the sky three hours ago. And there is everything in between.
When one measures the residence time of water in aquifers, one finds that there’s a whole range. There are places where the water that’s coming out of an aquifer fell out of the sky yesterday. There are places where it fell out of the sky a year ago, fifty years ago, a hundred years ago, a thousand years ago, ten thousand years ago, a million years ago. All these kinds of places exist. And again, what we’re measuring is usually an average of waters of different ages.
Now, realize, too, that the water molecules that are in the St. Croix River today may not always have been water molecules. They may very well have cycled through other kinds of oxygen and hydrogen containing molecules many times since the earth formed. You and I eat carbohydrates and breathe in oxygen and breathe out water molecules. We make water molecules. Right now, I’m talking to you, I’m breathing out water molecules that have only existed for minutes or hours as water molecules. The hydrogen and the oxygen in them have gone through this cycle many, many times. Sometimes slowly, sometimes rapidly. Got it?
LA: Yes.
CA: All right, back to the original question.
LA: (Laughing)
CA: How old is—what is the average age—of those water molecules in that bucket of water from the St. Croix River.
LA: Right.
CA: I can’t give you a hard and fast answer. I can tell you that most of the St. Croix River is surface run-off: it’s runoff from the snows that fell last winter, it’s runoff from the latest rainstorm and the mixture of those two changes depending on how long it’s been since the last rainstorm, and depending on how much snow we got last winter. But most of it is, from my perspective, rather recent. It has fallen out of the sky in the last months or maybe year or so. And it changes. If it’s flooding (because of a big snowstorm and/or rainstorm) the average age of the molecules in that bucket probably gets shorter. At the end of a long dry spell, the end of the summer—particularly a dry summer—you’re looking at water that was probably hanging out in swamps and in shallow aquifers for maybe years before it drained into the creek that drained into the stream that drained into the river that drained into the St. Croix. So the age of the bucket of water is a continually changing number. But on average, I suspect that the average age is on the order of a year to a few years since the last time those water molecules fell out of the sky.
LA: Could I ask a sideways question here? People speak of the importance of saving water. We need to save water. But if all of the water that’s has ever been on earth is still here in one form or another, we’re not really talking about saving water, are we? Are we talking about saving fresh water? Or saving clean water that’s usable?
CA: I like the last answer better than the previous one. Yes. When we talk about “saving water,” what we’re talking about is not using water frivolously but to only use the water for things that we need to do. Historically, there’s been an enormous amount of frivolous uses of water. So what we’re asked to do, in drought conditions, is usually to decrease frivolous uses. And, of course, the definition of “frivolous uses” varies from person to person and time to time.
LA: And the way it is used in one place determines the way it can be used in another place.
CA: People don’t like to think that they are drinking wastewater from somebody else, but that is exactly what they’re doing. If you live in Minneapolis or St. Paul proper, you’re drinking wastewater from St. Cloud and wastewater from the nuclear power plant in Monticello and from every farm upstream of us. And we use it and then run it through our sewage treatment plant and pour the treated water into the Mississippi River. Nobody right now would like to drink the output from that sewage treatment plant, but that is exactly what the people in St. Louis and New Orleans do, with a little dilution and treatment along the way.
Your definition of what you’re willing to drink depends on how thirsty you are. And many places in the world we are consuming more water than what falls out of the sky at that particular place that year. We can do that because maybe the local river comes from the mountains which get more snow, or we can do that because we’re over-pumping groundwater aquifers to supply that water, or we can do that because we desalinate the sea water. All those things require energy to do.
LA: Is there any qualitative difference between younger water vs. older water?
CA: I understand what you’re trying to ask. A few examples. PFAS, the forever chemicals made by the the 3M company, have only existed since about WWII. So if you are drinking water from an aquifer that’s residence time is older than, say, a hundred years, it shouldn’t have PFAS in it. One of the tools we use a lot to measure residency time of groundwater is the atmospheric nuclear testing that we and the Soviets and several other nuclear counties did in the 1950s and 60s, which put an enormous amount of radioactive hydrogen into the hydrologic cycle. That means that by measuring tritium (3H), the radioactive isotope of hydrogen in the water, we can measure the average residence time. We do the same thing with Carbon-14 dating. The water molecule doesn’t contain a carbon atom but groundwater typically has a considerable amount of dissolved bicarbonate in it and that’s one of the tools we use to measure the age. We also use all kinds of synthetic chemicals. I mentioned PFAS. Other tools that have proven very useful are herbicides and pesticides. In many cases we know very specifically when they went on the market. And because the plants and critters they are designed to regulate become accustomed to them, many of these chemicals have changed over the years. So we know that a certain chemical was sold and applied to Minnesota farms between 1955 and 1975, that didn’t exist before that and they stopped using it or it was outlawed at some date after that. If we take a water sample from a well and it contains those chemicals, we know something about how long the residence time has been. Of these dating methods, tritium is losing its usefulness because it has a half life of 12.33 years, and the 1960s were a long time ago, and most of that tritium has decayed away.
LA: How do these issues of age and residence time relate to the water from wells?
CA: When discussing water from aquifers, you’re always dealing with mixtures, and the water quality can be strongly degraded by the well’s construction. Many, particularly older, wells were not carefully constructed, and the well water can be degraded by surface runoff running up to the well and running down the outside of the well casing. It doesn’t take much of that to cause serious problems. But if the recharge area of an aquifer is known and the well has been carefully constructed and maintained, the age of the well water can be determined. There’s a huge range. I used the example of the Soudan mine, 2.7 billion years. There are deep aquifers that contain water that has been mainly isolated for thousands of years, tens of thousands of years. There are lots of deep aquifers in Minnesota with average residence times of 50,000 to 100, 000 years. That’s good news. Such old ground water fell out of the sky long before humans were having a serious effect on the hydrologic cycle, long before we had invented radioisotopes or nasty chemicals to contaminate it. It can be pristine, desirable water. (That’s partially behind the mythology of “pure mountain spring water”—which can be a total oxymoron. Because that pure mountain spring water might have fallen out of the sky during last winter’s snow storm.) In addition, the longer water has been underground the longer that water has had to dissolve contaminants from the aquifer rocks. In Minnesota, for example, contamination by arsenic, manganese, and radon can be problematic in old groundwater. The 2.7 billion-year-old ground water in the Soudan mine, for example, is a totally undrinkable calcium chloride brine, saltier than sea water, loaded with dissolved iron, which has to be treated as hazardous waste when pumped from the mine.
As I said, this is wonderfully complex. One can conceptually ask the question, when did that molecule fall out of the sky and how long did it take to get where it was sampled. But the problem that is always present—I’m sorry but I’m a chemist by training—is that the number of water molecules in a glass of water is on the order of 10 to the 24th molecules, which is a number we don’t even have a name for. And each of those molecules has a history. The average age can be discussed in a meaningful fashion. But one also needs to consider what the standard deviation of that number is, how much spread is there, and the other complicating factor, which is that, as we drill wells and start pumping water out of the aquifers, we are routinely pumping it out faster than it recharges. So that means we are drawing down the water in the aquifer, which one, means that you’re going to run out of water and/or two, you’re going to increase the recharge of surface waters, which will carry all the modern contaminants down into the aquifer being pumped.
LA: Can you give me examples of some of the older aquifers in Minnesota?
CA: That depends entirely on where you are in the aquifer. Under the Twin Cities, the deepest usable aquifer is the Mount Simon Sandstone and in many cases that is water that went into the aquifers probably before the last Ice Ages—until we started pumping it. Some of the oldest water I know of right now is accessible at the old Schmidt brewery in downtown St. Paul. There is a publicly accessible well there that is several hundred feet deep into the Mount Simon, and we’ve measured the age of that water and it comes out into the hundreds of thousands of years range. And that’s a nonrenewable resource that you’re pumping out of the ground.
LA: —because even though there will be more water that ends up down there, it won’t be absent all of those chemicals we’ve produced.
CA: Yes. In general, mountains are still uninhabited; not something you can farm on, not good places to build factories. But sometimes the places where you do mining—and mining is notorious for dumping really ugly stuff into water supplies—you have to understand what your local water cycle is.
LA: I want to circle back to something you said earlier. You said that the hydrogen and oxygen atoms that formed long ago are continually changing partners: sometimes part of a water molecule and sometimes part of something else. I understand that’s probably Chemistry 101.
CA: Water molecules are themselves involved in all kinds of chemical reactions. Our metabolism takes carbohydrates and oxygen and turns it into carbon dioxide and water. That’s what animals do. Plants do the opposite. Plants take carbon dioxide and water and produce carbohydrates with help from the sun. So we are continually doing chemical reactions that produce water and chemical reactions that consume water. The atoms don’t change. That’s why I said, it’s hard to talk about how long this particular water molecule has been in existence because the hydrogen and oxygen atoms in that water molecule may have been part of a whole bunch of other things since, well, half a billion years ago.
LA: So, what is on your mind these days, given the perspective you have gained from your many years of research?
CA: I’m 80 years old. When I was an undergraduate in the middle 1960s I took a course in environmental geology and one of the questions on the final was, “What is the carbon dioxide content of the earth’s atmosphere?” And the answer that got me credit on that question in 1962 was 315 parts per million. The current answer that would get me credit on a question like that is 425 parts per million. That’s a 35% increase during my lifetime. Global climate change is real and getting worse, and it’s nobody’s fault but ours. But relative to our conversation, and the reason I took off on this tangent, is one of the serious tools that might help. If you can capture energy from solar or geothermal or any other number of non-fossil fuel ways of doing that—part of the problem is to store it. And one of the ways to do that is to simply use that electrical energy to decompose water molecules to give you hydrogen and oxygen. You simply dump the oxygen back in the atmosphere, and then you use it wherever you are to burn the hydrogen to form water again. Hydrogen’s a good fuel. The earth has been doing this for 4 ½ billion years.
My wife and I are delighted now, to have granddaughters. And what’s going to happen during their lifetimes? The rate of change is not slowing down. I remember when I was 10 years old, my grandfather told me that, if he had suggested that people could go to the moon when he was my age, he would have been sent to an asylum. That was Buck Rogers stuff at the time, pure science fiction. And my parents had to make the decision about if they should let me and my two brothers take swimming lessons in the 1950s—because of concerns about polio. Then came the effective polio vaccines. We have to find a way through the climate warming challenge, and remember that good things have happened too; wonderful, previously unimaginable things have happened.
LA: Thanks so much for your time today, and for your body of work, which makes that next good thing more likely.
About Calvin Alexander
Calvin Alexander (E. Calvin Alexander, Jr.) was born and raised in Oklahoma. He graduated with a BS in Chemistry from Oklahoma State University in 1966. While an undergraduate at OSU he developed a sport interest in cave exploration. He graduated with a PhD in Chemistry from the University of Missouri at Rolla in 1970. His thesis project was on noble gas isotopes in meteorites and the early history of the Solar System. In 1970, he accepted a position as a post-Doctorial researcher in the Physic Department at the University of California at Berkeley, where he was part of the Apollo Lunar Sample Analysis effort. In 1973 he joined the faculty of the Geology and Geophysics Department (now Earth Sciences) at the University of Minnesota in Minneapolis, retiring in 2014.
One of his career-long interests has been Planetary Geology—both of other planets in the Solar System and of the Earth as a planet. It has been his great privilege to be alive and involved in Planetary Geology during the initial exploration of the Solar System. He is the curator of Meteorites at the University of Minnesota and has taught classes at several levels on Planetary Geology, meteorites, geochronology and cosmochemistry. His early sport caving morphed into a major research interest in Karst Hydrogeology in the mid-1970s. Much of his research and teaching in the last three decades has focused on how the human species interacts with karst hydrogeology. That has placed him on the interface between science and society—an interface that is intellectually exciting, challenging and can also be frustrating but is a critical place to be as our species faces unprecedented challenges in managing the Earth’s surface environment.
