 It's one o'clock on Tuesday, April the fifth, so you must be watching Science at Soast. I'm your host, Pete McGinnis-Mark. And Soast means the School of Ocean Earth Science and Technology from the University of Hawaii. And every week we bring students or postdocs to the studio streaming live from beautiful downtown Honolulu to describe some of their research. And I'm really excited this week because Brittany Okuhuta from Earth Science Department and Brittany is doing something really relevant in Hawaii. It's looking at the groundwater resources or the hydrogeology. Her specialty lies in West Hawaii Island. So Brittany, it's wonderful to have you on the show. First of all, let me congratulate you. I believe that you are a newly minted PhD. So yet again, it shows that Manawa has trained another fine young scientist. And from the look of the background, you're joining us from the wilds of a glacier in Greenland. So I hope you're not too cold, but once again, welcome. Thank you. It's a pleasure to have you here. And as I said, looking at hydrogeology or groundwater resources, I think is another term for it. It's really important for all of us here in the islands. And you've been specializing on the Big Island. Is that correct? Yes, yes. All my research has been on the west side of the Big Island. OK, so does that mean that you've been based on the Big Island? Or are you still working here at Manawa? I still work in Manawa, but for my field work, myself and my field partner would go to Kona side about three to four times a year to collect water samples from wells, both monitoring and pumping wells. So we'll just go there for a couple of weeks at a time, collect all of our samples, and then bring them back to Manawa to sample everything and to do all the rest of our research. And that's because the laboratory is that you depend on a locate here on Oahu, even though you collect your data on the Big Island. Yeah, yeah. So we use a lot of in-house labs at UH in the Department of First Sciences. I even send my samples to the mainland for some extra analysis that we don't have on Oahu. OK, now I'm sure most of our viewers, at least those in Hawaii, recognize that water supply is really important. Can you give us a little brief introduction to where we get our water from? And perhaps we bring on the first slide and you can talk us through some of the aspects of that image. Sure. So this is a very simple schematic of our water resources in Hawaii. This is from a USGS port by Iuzuka et al. And so you can see that the source of our water essentially comes from our precipitation. And so every time it rains, that rain water will enter the ground surface and it'll make its way deeper into the subsurface. And then once that percolated water reaches the saturated aquifer, which is what you see like the water table labeled, that's where it becomes part of the aquifer system. So our groundwater resources are the main source of fresh potable water in Hawaii, where all of our basalts underground are completely saturated with fresh water. And it creates this freshwater lens that we're able to pump water from. And that fresh water lens will float on top of the salt water beneath it because fresh water is less dense than salt water. So it creates this nice lens that's not in contact with any other salt water or areas. And then all of that fresh water will make its way down the aquifer following gravity. And eventually it'll go out to the ocean and to the coastline where it will discharge as some rain groundwater discharge. And so it enters the ocean again, and then the ocean will evaporate, and it will start recycle all over again. And salt water. OK, and you mentioned saturated basalts. That's the volcanic rock from which not only the big island, but all of the Hawaiian islands have been constructed. Yes, yes. And most of the water that you're talking about is presumably pumped out of the ground as opposed to pulled out of streams or rivers and that kind of thing. Yeah, so in Hawaii, we don't really rely on any type of surface water from streams because it's easier to become contaminated when it's still at the surface. And so about 90% of our potable water supply is pumped from the ground. That way it's as fresh as it can be. And that presumably sets up your PhD research because the time interval between the rain falling and when you pump it out of the ground, as I understand it, can vary. So let's take a look at your field site in the second slide and start a discussion. How old is the actual rainwater that we're drinking when it's to the water board? It's the big one. Yes, so this is the big island. And the black outline areas are the five aquifer systems that I study. So I look at Waimea, Anihoomalu, Kiholo, Keoho, and Keala Kekua. And so that makes up West Hawaii. And then you can see that the green colors across the island represents the different volcanoes from all five volcanoes. And so the five aquifers that I look at are made up of three different volcanoes. There's made up of Mauna Loa, Mauna Kea, and Huala Lai. And so you can see that the aquifer boundaries generally follow the outline of the volcanoes. And that's because the aquifer boundaries are delineated based on surfacial geology. But we do know that some of these volcanoes erupted contemporaneously. And so there might be some interfering of lava flows beneath the subsurface, where we might have some Mauna Kea flows covering Huala Lai flows and vice versa. And so we do know that it's a little bit more complicated than just a simple line. And for the benefit of our viewers, can you just define what you mean by an aquifer? Is that the boundary within which any rainwater will go in a certain direction or how would you describe it? Yeah, so an aquifer boundary is generally defined as the area from which groundwater can be pumped sustainably. And typically you wouldn't see groundwater flow across aquifer boundaries. We kind of assume that this aquifer contains all of the groundwater that would flow into it and then flow out of it. So you can kind of think of it as like an obuah or a watershed in that sense, where everything should be self-contained in that area. But because we don't know exactly what the subsurface geology looks like, we can only give our best estimate of where this aquifer boundary is based on the surfacial geology. And so we assume that that's where they are. And an aquifer is a three-dimensional feature rather than just these black lines drawn on the map. You said that you had like Huwala Lai and Monaloh or Huwala Lai and Kohala. The lava flows the subsurface structure into finger. So it's presumably not easy to tell at the surface where you get your water from. That must be the detective story that you've been working on, right? Yeah, that's part of the big picture that we're trying to solve. Yeah, so what does the third dimension look like within a typical Hawaiian volcano? I think the third slide has a cartoon. Presumably it's not this simple, right? Right, right. So if we were to slice through the aquifers or through the island and then we would see the entire saturated aquifer where we have this blue area that's completely saturated with fresh water. And we can see that that was coming from the recharge, from the precipitation that falls into the water table. And then so for my research, I looked at the ages of groundwater. And so this shows how a very simple groundwater sample would flow through the aquifer. And so following this piston flow model, we believe that a groundwater sample is basically a packet of water. You can imagine it in a Ziploc bag and it's flowing through the aquifer. And so because it's in a packet, it doesn't come in contact with any other water molecules around it. And then that will eventually make its way to a pumping well where I would sample that water. And then that shows that time that it's in that packet in the aquifer is the age of the water sample. But water doesn't have a date stamp associated with it like tree rings. I can quite understand. You can understand the age. How do you tell the age of water? Was it shelf life, you know? Yeah, yeah. So what we can do is we can use geochemical tracers that are integrated into our aquifer systems in order to date it. So there are a bunch of different tracers that have been released into the atmosphere. Similar to the tree rings where we looked at like the carbon, the radio carbon of tree rings that can also get its way into our aquifers because when it's raining, the precipitation will mix with the atmosphere and that will eventually make its way into the aquifer. And so when we sample all of our water, we can send those samples to labs that look at those specific geochemical tracers. And yeah, so like this plot shows the various tracers that we can use. And by measuring that, we can look at that. There's a lot of stuff on this particular garden. Please help the viewers understand what we're looking at. Yeah, so water can be very young, less than 10 years old. It can also be very old and it can go up to a million years old. And so this plot shows the different tracers that you can use depending on how old the water is. So the red tracers are used typically to sample very recent waters that are less than five years old. If you wanna look at more modern waters that were released in anthropogenic times, you would typically use the yellow tracers that go up to about 70 years old. And if you wanna look at old water that would go up to about 50,000 years old. You could use the tracers that are highlighted in green and that's where the radio carbon falls in for the tree rings. And then if you have very old water that goes over a million years old that you could use the tracers that are highlighted in blue. That's okay. And you've got some letters to the right of each of those colored ellipses. No need to go through them all but just tell us a couple so that we can tell what it is that you're using. Yeah, so for my research, I'm looking at the modern waters. And so we're looking at the yellow portions. And so specifically I used the tritium helium which is the 3-H, 3-H-G-E. I used sulfur hexafluoride which is the S-F-6 and I used the chlorofluorocarbons which are the C-F-Cs. And so all three of these tracers can date modern waters that fall between less than 70 years old generally. Okay, and I have heard of tritium because of the nuclear explosion tests in the 60s. Is that setting your radiometric clock for you or does it occur continuously? Yeah, so tritium can be naturally produced but we generally do focus on the nuclear bond testings that released a bunch of tritium into the atmosphere. And so starting from about the 1950s, 1960s we had a big peak of tritium in the atmosphere. And so that gives us a date that we can compare our samples to in our groundwater samples. And so that way we can determine if we have this much tritium in our sample that decayed from the time that the bombs were detonated then we can have it. Presumably we should sort of allay any viewers' fears that plopes in Kono drinking radioactive water. These are minuscule traces. Yes, they're very minuscule. Yeah, it's almost so small of a concentration that it's pretty soon we'll no longer be able to be used to date groundwater in Kono. And perhaps people have heard of, say, looking at oxygen isotopes in Greenland ice cores and things that it's the same basic principle, right? Yeah. The clocks get set during when it's raining, then it's underground and various isotopes change and stuff like that. Yeah, exactly. So the tritium will decay to helium. And so we can measure that the amount of tritium and the amount of helium to determine how long it's been there. How do you collect your water samples? Do you get a bucket and go to the well head or what's the... You've got, in the next slide, you've got a variety of things. Yeah. And explain what CFCs are as well. Sure. So CFCs are a man-made compound that are primarily used for refrigeration and electrical purposes. But so they were released as early as the 1930s into the atmosphere when we started to produce all of the CFCs. However, we found that CFCs act as a great house gas. So when the Montreal Protocol was adopted, we could no longer produce CFCs. And so this gives us a very distinct historical concentration of how CFCs look at our atmosphere. And then we can use that history to match the samples that we collect in order to determine the age. And so both CFCs and tritium use some type of historical context in order to date our groundwater samples. And so for the CFCs, all of my samples are collected at the well head. So we drive up to the well ports and we can plug in all of our devices. And so CFCs have to be collected in these glass bottles that you can see in the picture. And what I have to do is I have to sample them in the bottle within another container because I can't have the sample come in contact with the atmosphere. And so by allowing it to overflow into itself like that, I'm only having water in there and no atmospheric bubbles are entering. And then for the tritium, it's the same thing, but I collect them in these copper tubes that are used to analyze in the labs. And so both are quite tedious in the way that they have to be collected. And there's a very specific method for both. And the tritium samples, because of how long they're about, I say about four feet of copper tubes, so they get pretty heavy once I finish collecting everything. And the volume you're working with, it's not buckets full, but it's not a thimble full. Yeah, so by the time I've done collecting after my two weeks of sampling, I have a lot of water. Right, sure. What depth do you take the water from? Does it matter? How deep below the ground surface you collect each sample? Yeah, it does matter, but most of my samples are coming from pumping wells. So I don't have control of how deep I'm sampling from. It's just wherever the pump is located. And typically pumping wells are drilled just below the water surface because it's very expensive to drill very deep. So once they reach the water table, they'll go made about 10 to 20 feet deeper than that. And that's where they will start to pump their freshwater. So generally it's around mean sea level, I would say for a lot of more inland wells. So in Kona, like you have some pretty high elevation, so those pumps can be located hundreds of feet below ground elevation. I see, presumably if one was to go deeper beneath the surface with other wells, you might find older water, different chemistry and that sort of thing. Yeah. So in slide six, you've got some attempt to show how things change as a function of the year. Yeah, so as I mentioned before, all of these traces that I'm using have very unique histories. And because they were released during anthropogenic or human times, we can keep track of how much of each tracer was released into the atmosphere. And so this kind of shows all of the different traces that I use where we see the tritium plot that has a very high peak. And that was the time when the nuclear bombs were being tested. And then you see that that drastically declines again once all of that stops. And so we can compare our relationship between the historical tritium concentrations and our measured samples. And same for the CFC curves, where you see three curves that are going slightly up and then declining at a gradual pace. That shows how the CFCs were released because of all of the industrial purposes, but then they are now starting to decline since they're no longer in production. So perhaps it's ironic, but fortunate for you that there were these nuclear tests and before we banned CFCs because they are the two main traces I think you use for recent water studies, yeah. Yes, so they're both kind of heading out of their time scale. And it's becoming more and more difficult to detect those tracers in our waters. Good job getting the PhD when you did rather than 50 years from now, right? Now, let's look at the geographic distribution of some of these wells. In the next slide, I think you've got a map, two maps. Yes. And this is the Kona coast with Huala Lai after the left. Yes. So where I have the black triangle, that's the peak of Huala Lai. And then the lines again delineate all of my aquifer boundaries. And so these two plots basically show the measured ages of all of my well samples. And so on the left side, I have my CFC ages and you can see that I sample across all five aquifers and I think we tend to forget how big the big island is, but this area is almost as big as Oahu really. And so it's a lot of ground to cover. And so you can see that the size of each marker shows how old that water is. And just looking at either the green dots or the purple dots, there's a variety of ages there, right? Yes, so we can go as young as 30 years and we can go up to like 75, 80 years in age. And you tend to see that as we move closer to the coastline, our ages become a lot younger. We have a lot smaller dots near the ocean. And then when we move further inland, we have much larger dots. So our ages become a lot older compared to our coastal samples. Is that because of the rain obviously falls all over West Hawaii? Upslope, are you getting water safe that fell on minor lower high elevations whereas lower down just comes from Hawaii? Yeah, so we do believe that some of the high level water is coming from the slopes of Maulalua and Maulakea and that it's making its way down the aquifers. But then when you're closer to the coastline, you do have that more local rainfall and that will make that younger age in the coastline samples. Okay, okay. Now you had maps with two different techniques. Did I tell the same story or is it completely different? Yeah, so generally we do see very similar trends between the two. I do have less treaty of samples than CFC samples, but we do see that both show young ages for the coastal samples and older ages as we move further inland. And when I actually plotted the ages against each other in a XY plot, I do see a very good correlation between the two ages. That's slide eight, I think. So let's take a look at slide eight. Yes. And I hate having graphs on this program because how do you interpret it? Help us, please. Yes, so on my X axis, I have- That's the one going across the button. I have the CFC 12 age and on the other side, I have the tritium ages. And so all of these markers show us the age that I measured using both of those tracers. And because they fall on that nice 45 degree line, that means that they match each other generally. And so the CFC tracer tells me the same story that the tritium tracer says. And so because I use these two different techniques, I can almost like validate my findings. And I can say like, yes, I do have this very consistent story throughout. So just in general, but why would the average viewer worry how old her water was? Can you put your research into a context for border water supply or the average user? Yeah. So for specifically like these modern waters where you can use CFCs and tritium, it's believed that modern waters are more likely to transport contaminants such as fertilizers or wastewater because the rain is entering the aquifer during times of anthropogenic activities. And so a lot of times if we need to focus on water cleanups, we wouldn't need to focus on the younger waters that are more likely to transport that nutrients. And same for if we have very old waters, if we're pumping that very old water, we have to remember that it would take that much more time to replenish our aquifers. So if you have water that's a thousand years old, we can't expect that aquifer to replenish in just 10 years. That sounds really relevant, but you know, particularly say on the Kona coast where water supply is an issue or in other places where septic tanks contaminate groundwater, looking at the age, it must be really gratifying to work on something that's so relevant to people here in Hawaii. Yeah, it's really great to kind of like talk story with everybody that we meet. Yeah, and you know, you've just got your PhD. Can I ask are you going to continue this kind of works? It seems really important. Yeah, I would like to continue this work. I definitely want to stay in Hawaii to continue doing my research. Since this is whole for me, I'm very passionate about it. I would like to either stay within like the USGS or with it at UH Manoa to kind of continue studying Hawaii's groundwater and just like how sustainable it can be. And is the Big Island the place to go and do this kind of work because you've got tall mountains or how about Oahu or Maui? Yeah, so Oahu is very important to study because of how densely populated we are. We're always pumping, but there's also a lot of research that's been done on Oahu and so the west side of the Big Island is definitely like the new place to study because it's not as highly researched as it is on Oahu. They only started pumping these wells in the 1990s. So there's a lot to still learn. Yeah, well, I wish you success, Brittany. And I'm sorry to say that we are running out of time. So let me just remind the viewers you have been watching Science at Soast. I have been your host, Pete McGinnis-Mark and my guest today has been Brittany Ahukta, who has recently gained her PhD in the Earth Sciences Department at UH Minoa. So Brittany, thank you very much. This is a really important topic that you've been describing today. Thank you for coming on the show. And good luck with your future career. Thank you. All right, and thank you for watching the show. Until next week, it's goodbye for now. Thank you so much for watching Think Tech Hawaii. If you like what we do, please like us and click the subscribe button on YouTube and the follow button on Vimeo. You can also follow us on Facebook, Instagram, Twitter and LinkedIn and donate to us at thinktechhawaii.com. Mahalo.