groundwater

Tracking the Fallout and Fate of Fukushima Iodine-129 in Rain and Groundwater

Matt Herod December 21, 2015

A recently published paper (by myself and colleagues from uOttawa and Environment Canada) investigates the environmental fate of the long lived radioisotope of iodine, 129I, which was released by the Fukushima-Daichii Nuclear Accident (FDNA). Within 6 days of the FDNA 129I concentrations in Vancouver precipitation increased 5-15 times above pre-Fukushima concentrations and then rapidly returned to background. The concentrations of 129I reached were never remotely close to being dangerous, however they were sufficient to distinguish the impact of the FDNA on the region.

Subsequent sampling of groundwater revealed slight increases in 129I concentration that were coincident with the expected recharge times. This suggests that a small fraction of the FDNA-derived 129I may have been transported into local groundwater after infiltrating through soils.

Sample map showing location of all three precipitation sampling locations as well as the location of both wells used for groundwater sampling. The surface expression of the Abbotsford-Sumas Aquifer is shaded.

What is iodine-129 and where does it come from?

Iodine-129 is the longest lived isotope of iodine with a half-life of 15.7 million years. It is radioactive and occurs everywhere throughout the environment. It is produced in three ways. The first two are natural and the third is by the nuclear industry.

The natural production of 129I occurs in the atmosphere and in soil/rocks. The atmospheric production happens when a cosmic ray proton hits a xenon-129 nucleus and removes a neutron, replacing it and creating an iodine-129 nucleus. The production in soil and rocks happens when a uranium-238 nucleus spontaneously fissions and one of the halves it releases has a mass of 129 ala, iodine-129.

The anthropogenic production occurs because when uranium fissions in a nuclear reactor sometimes one of the parts is 129I. This anthropogenic production is by far the largest source in the environment as substantial amounts have been released by nuclear fuel reprocessing. This 129I that has been released can trace a host of environmental processes and inform us about what happens to 129I or the much more dangerous, 131I. The current levels of 129I are much too low to pose a health threat to humans or the environment, but do allow 129I to be used as an environmental tracer.

129I from Fukushima is present in Vancouver, B.C. rain

The purpose of this research was to discover the fate of 129I in the released by Fukushima, which although a small amount, was isolated in time and space. We measured the 129I deposition in rain and its subsequent movement though soils and see if it reached groundwater. The results tell us about the impact of Fukushima, how 129I moves, where it is attenuated, and how quickly contaminants in this aquifer move from the ground surface to the water table. This knowledge can then be applied to understand 129I behavior in other settings such as nuclear waste repositories and watersheds or it can be used to learn about the behavior of other types of contaminant in this aquifer and how vulnerable it is to contamination.

The results in rain show an increase in 129I concentrations of up to 220 million atoms/L*. This increase was seen ~6-10 days after the emission from Fukushima began and are 5-15 times higher than rain samples collected before Fukushima. Following this increase 129I concentrations returned to background with a few weeks. This agrees with other studies monitoring the fallout of Fukushima derived radioisotopes [Wetherbee et al., 2012]. Furthermore, atmospheric back trajectory modelling shows trajectories for air parcels arriving in Vancouver from over the Pacific ocean and Japan.

Variation in the concentration of 129I and the 129I/127I ratio in precipitation from Vancouver, Saturna Island and NADP site WA19 over time. The time range that each NADP sample integrates is displayed using horizontal error bars. 1σ error is contained within data points if not visible. The dashed vertical line shows the date of the FDNA relative to samples.

We also calculated the mass flux of 129I from Fukushima. That is the actual quantity of 129I that was deposited on the region in grams, or in this case in atoms/m2. This was calculated by simply multiplying the concentration of 129I in rain by the amount of rain that fell. We found that only about 15% of the annual 129I deposition in the Vancouver region could be directly linked to Fukushima affected rain events. The total mass deposited by Fukushima was ~0.0000000000002 (2 x 10^-13) grams. This is a negligibly small quantity with respect to radioactive risk.

Despite the fact that the deposition of 129I from Fukushima was infinitesimally small it was still measurable. Therefore, the question became where did it go and can we learn about local groundwater resources using 129I as a tracer?

129I variation in groundwater may be due to Fukushima

The results in groundwater show very small 129I concentration increases. Two different wells were sampled. The first had a recharge time, which is the time it takes for water to move from the water table to the well screen, where it is sampled, of 0.9 years and the second had a recharge time of 1.2 years [Wassenaar et al., 2006]. The exact time it takes for water and dissolved contaminants to travel through the unsaturated zone was unknown. However, the sediments of this aquifer are very coarse and are known for their ability to rapidly transport contaminants, such as nitrate [Chesnaux and Allen, 2007]. Therefore, if we were going to see 129I from Fukushima this was an ideal location.

The increases in groundwater 129I concentrations were seen in two different wells (ABB03 and PB20) located close to one another. The two wells also had slightly different recharge times. The first was 0.9 years and the second was 1.2 years. The 129I anomaly in the first well occurred at 0.9 years and in the second well at 1.2 years. These 129I anomalies, which occurred exactly when the recharge age predicted they would, suggests that some of the 129I deposited by Fukushima was reaching the wells and causing these increases.

Temporal variation in the concentration of 129I in groundwater in ABB03 and PB20. The solid vertical line shows the date of the Fukushima accident and the dashed horizontal line shows the median of each dataset respectively. The 3H/3He ages from [Wassenaar et al., 2006] of groundwater in each well and their uncertainty is pictured as the solid arrow which is aligned with the 129I anomaly possibly caused by the FDNA. The dashed arrow covers a 40 day (0.11 year) time span and represents a possible vadose zone transport time.

In order to verify if it was possible for 129I to travel from the ground surface to the water table in the time required to produce the variations observed we modelled its transport time and attenuation through the unsaturated zone.

The time it took for 129I to reach the water table in the model was then added to the previously dated recharge time to get an estimate for how long it might take 129I from Fukushima to reach the wells we sampled. The results show that it is indeed possible for 129I deposited in rain to infiltrate through the unsaturated zone and reach the wells in time for us to detect it. However, this rapid transport assumes that certain flow paths exist to rapidly conduct 129I due to the heterogeneous lithology of the unsaturated zone. There is evidence of such flow paths [McArthur et al., 2010].

To summarize,

Within a week of the FDNPP accident elevated 129I concentrations were observed in precipitation. This agrees very well with work on other radionuclides in air filters and rain.

129I concentrations in rain returned to background within a few weeks. However, discrete pulses of elevated 129I occurred for another several months.

Elevated 129I concentrations were measured in two wells and corresponded with the expected recharge times indicating that 129I from Fukushima can be traced into groundwater.

Vadose zone modeling has shown that 129I can be rapidly transported to the water table and reach the well screen in accordance with groundwater ages.

We propose 129I transport is enhanced by preferential dispersion of 129I that exists due to the heterogeneous nature of the vadose zone.

This results in variability in groundwater 129I concentrations that preserve the variability in the input of 129I via washout with some dampening of the signal due to attenuation and dilution.

Conceptual model showing the possible transport pathways of Fukushima derived 129I which was deposited via precipitation. A fraction of this 129I was rapidly transported through a heterogeneous vadose zone via preferential flowpaths to groundwater where minor 129I variation was detected. The remainder was retarded or attenuated in the vadose zone during transport.

Thanks for reading, if you have any questions or concerns please leave a comment or send me an email to discuss further!

*Note: 100 million atoms/L of 129I is equivalent to an activity of 0.00000014 (1.4 x 10^-7) Bq/L. These quantities are extremely low level and only the most sensitive analytical methods in the world can detect them. This amount of radioactivity is several orders of magnitude lower than the natural background radiation produced by naturally occurring radionuclides in soil and the atmosphere. For more on naturally occurring radioactivity see here. Even a clean rainfall has about 1 Bq/L of tritium (radioactive hydrogen), which remains from atmospheric weapons testing in the 1960’s

Access the full paper here: http://onlinelibrary.wiley.com/doi/10.1002/2015WR017325/abstract

References

Chesnaux, R., and D. M. Allen (2007), Simulating Nitrate Leaching Profiles in a Highly Permeable Vadose Zone, Environ. Model. Assess., 13(4), 527–539, doi:10.1007/s10666-007-9116-4.

McArthur, S. A. Q., D. M. Allen, and R. D. Luzitano (2010), Resolving scales of aquifer heterogeneity using ground penetrating radar and borehole geophysical logging, Environ. Earth Sci., 63(3), 581–593, doi:10.1007/s12665-010-0726-9.

Wassenaar, L. I., M. J. Hendry, and N. Harrington (2006), Decadal geochemical and isotopic trends for nitrate in a transboundary aquifer and implications for agricultural beneficial management practices., Environ. Sci. Technol., 40(15), 4626–32.

Wetherbee, G. A., D. A. Gay, T. M. Debey, C. M. B. Lehmann, and M. A. Nilles (2012), Wet Deposition of Fission-Product Isotopes to North America from the Fukushima Dai-ichi Incident, March 2011, Environ. Sci. Technol., 46(5), 2574–2582.

Bubbling Merrily: Artesian Springs

Matt Herod December 7, 2015

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I recorded the video above on a recent field camp near Deep River, Ontario. This video shows a great example of a flowing artesian spring which is bubbling up at the headwaters of a creek. The water is freezing, crystal clear and totally delicious! The classic textbook on groundwater, Freeze and Cherry, puts the attraction of groundwater springs nicely when they say “Flowing wells (along with springs and geysers) symbolize the presence and mystery of subsurface water, and as such they have always evoked considerable public interest.”

There are two types of artesian springs. Those that are controlled geologically, which are commonly taught as the only variety of artesian system, and topographically, which are often overlooked.

Geologically controlled artesian springs/wells result from a specific combination of hydrogeologic conditions. Specifically, the aquifer must be under pressure, which is usually caused by a steep elevation gradient in combination with relatively impermeable confining layers such as clay. This is called a confined aquifer. Recharge to this aquifer occurs on top of a hill, where the aquifer outcrops. This water then infiltrates through the permeable sediments to the water table and into the confined aquifer. However, this does not explain why a spring or a well drilled into and artesian aquifer often bubbles up with water, like the video above.

A conceptual model of a confined artesian aquifer in which the recharge area is exposed at higher elevation and the aquifer sediments are bounded by two aquitards. Source

The reason for this is somewhat abstract and has to do with water pressure. In an unconfined aquifer the water table and the potentiometric surface, which is the abstract line dictated by the water level in the well, are generally synonymous and are defined by the point at which the water pressure is equal to atmospheric pressure. However, in confined aquifers where artesian conditions exist this becomes more complicated. The reason for this is that within the confined aquifer the water pressure is often greater than atmospheric. Imagine diving down in a lake and feeling the pressure of the water above you. Therefore, when this aquifer is drilled or a pathway to conduct water to the surface exists the water will want to flow upward towards that point where the water pressure and the atmospheric pressure are equal. This point can be above the ground surface and this leads to flowing artesian conditions. The figure below illustrates this concept nicely.

Source: Minnesota Department of Health,

In this figure the water level in the well on the right, which is connected to the confined aquifer, is distinct from the water table in the unconfined aquifer. The water is not flowing because the potentiometric surface is not higher than the ground level. In the other artesian well, which is flowing, the water flows up to the potentiometric surface, well above the ground surface. This is because that surface represents the point where the water pressure, which is the pressure of the water within the confined aquifer, and the atmospheric pressure are equal.

The other type of artesian spring are topographically controlled and often occur in valleys. The reason for this is that as water recharges at the top of hills this can locally raise the potentiometric surface if there is a steep valley nearby. Therefore, at the base of the valley the potentiometric surface can be higher than the ground surface causing water to discharge.

So which type is the one in the video? Let’s start by checking the topo map of the region. The spring is located at the red star, which based on the terrain map is actually pretty flat, certainly much flatter than the opposite bank of the Ottawa river.

Based on this map it doesn’t look like the spring is topographically controlled. There may be some local elevation that does not show up at the map scale, although I don’t recall there being that much. One thing to keep in mind about this location is that there is a lot of bedrock exposed. It is possible that some of this bedrock aquifer is over-pressured and water flowing through fractures in the bedrock is discharging as a flowing artesian spring. In my mind, after about 10 minutes of looking around, this is the most likely scenario. It may also be completely wrong, but without a more detailed look around it is difficult to say.

Artesian springs and springs in general really represent the importance of protecting our groundwater resources. It is critical that places such as this artesian spring be protected from contamination and development as they are very fragile and represent important sources of clean, safe water as well as habitat to a large diversity of local flora and fauna. If you know of any artesian springs in your area please comment below and let me know if they are protected or if they have been compromised by contamination or development.

Matt

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ATTA and the Curious Case of Krypton-81

Matt Herod September 14, 2015

Ok, so I took some license with the title. This isn’t really a curious case and neither Krypton-81 nor ATTA are actually people. In fact, Krypton-81 (81Kr) is a radioisotope of the noble gas krypton and ATTA, which stands for atom trap trace analysis, is the revolutionary technique that has made its analysis possible. I recently heard about developments with ATTA at the IAEA Isotope Hydrology Symposium and have been doing some reading about the method and its revolutionary application to the dating of both young and ancient groundwater.

Figure 1. Dr. Z-T Lu working on the ATTA system at Argonne National Labs. Used with permission.

81Kr has long been a bit of a dangling carrot for groundwater dating people like myself. 81Kr is a long lived radioisotope of Kr (half-life: 229,000 years) that is produced by cosmic ray interaction in the atmosphere with other krypton isotopes. This production results in about 5 atoms of 81Kr for every 10^13 atoms of the other Kr isotopes. This 81Kr then settles to the earth surface and is incorporated into groundwater recharge and can then used to date groundwater from 150 thousand to 1.5 million years old. The way this works is that once water reaches the water table no new krypton is added and the clock starts ticking as the 81Kr decays away. In order to use this method we assume that the initial concentration in the recharge is in equilibrium with the concentration of 81Kr in the atmosphere, which is well mixed. ATTA then measures the amount of 81Kr that is left in the water sample compared to the other Kr isotopes and an age can be calculated from the difference between this ratio and the intial ratio.

ATTA can also be used for the short-lived isotope krypton-85 (half-life: 10.8 years). 85Kr is produced by fission in nuclear reactors and is released during nuclear fuel reprocessing. The short half life of 85Kr makes it useful for dating recently recharged groundwater from 1 to 40 years old.

Figure 2. Dating ranges of 85Kr, 39Ar, 81Kr and other established radioisotope tracers. (Source). Used with permission.

The reason krypton is such a useful tracer for groundwater dating is that as a noble gas the interaction of Kr with soils, rocks and the biosphere is minimal whereas other tracers such as 36Cl, 14C or 3H are often subject to retardation during transport or inputs from multiple sources which makes extensive corrections necessary or renders them completely unusable for dating. Furthermore, very few reliable tracers exist in the range that Kr isotopes cover making them extremely useful. One isotope that I haven’t mentioned as much is argon-39, which can be used to date water from 50-1000 years old, is also a noble gas, and can also be measured with ATTA.

Measurements of krypton can also be used for dating of ancient ice cores as well. Atmospheric gases including Kr are trapped in air bubbles in the ice and therefore, using the same method as groundwater dating, an absolute age for an ice core can be obtained. There are several other applications for Kr dating as well such as dating of deep crustal fluids and brines.

Figure 3. Sampling ice cores for Kr analysis by ATTA. Photo: V. Petrenko. Used with permission.

The development of atom trap trace analysis was first reported in Science in 1999 and since then has undergone several substantial improvements primarily aimed at reducing the required sample size required for an analysis of Kr. ATTA (Figure 4) works by trapping Kr atoms with a laser which causes a slight and temporary change in their atomic structure which lasts for about 40 seconds. During this period the Kr atoms in the laser beam are focussed and slowed and then trapped in an MOT (magneto optical trap) where they are held in place for an average of 1.8 seconds. Once the Kr atom is in the MOT it fluoresces as it returns to its original state. This fluorescence is detected by a camera which is sensitive enough to detect the emission from a single atom (Figure 5)!

Figure 4. Schematic layout of the ATTA-3 apparatus. (Source). Used with permission.

One of the key features of ATTA is that this laser induced fluorescence within the MOT occurs uniquely for every isotope as the laser frequency is tuned specifically! This means that only atoms of of the desired Kr isotope are trapped. Furthermore, this means that ATTA is completely immune to interference from other elements, isotopes, isobars, or molecules. In essence nothing can confuse the detection of the 81Kr atom once it fluoresces and therefore there is no background of spurious detections that need to be corrected for. Among low-level analytical techniques this is unique and a really big deal! As a user of AMS, which is another low level analytical method that does suffer from these issues, this is statement is an eye-catcher.

Figure 5. A CCD image showing the response of an atom in the MOT. Used with permission.

Since its invention ATTA has evolved considerably. We are now on the 3rd iteration of ATTA and significant improvements have been made that make ATTA much more practical for routine use. Specifically, the amount of sample required for an analysis has been reduced drastically. The first version of ATTA could only be used for atmospheric measurements as the quantity of Kr needed was too large to be extracted from water. ATTA-2 required ~1000 kg of water to extract 50uL of Kr gas. Now, ATTA-3 only requires 5-10uL of Kr which can be obtained from only 100-200 kg of water or 40-80 kg of ice. This advancement means that ATTA is now usable for groundwater dating applications never before possible. This has been demonstrated by the use of ATTA to date groundwater in Egypt to around 500,000 years old and even older water in Brazil up to 800,000 years. Other dating methods have confirmed that ATTA measurements are accurate.

Now that the sample sizes required for an 81Kr or 85Kr analysis have been lowered so dramatically the method is even more useful to the geoscience community. One of the messages from Dr. Lu’s talk at the IAEA meeting was that this technique is open for business and the geoscience community is strongly encouraged to reach out for collaboration and discussion. Furthermore, it may also be possible to use ATTA to measure argon-39, calcium-41 and potentially lead-205, strontium-90 and cesium-137,135 at extremely low levels.

Note: During the writing of this blog I corresponded with Dr. Z-T Lu, one of the creators of ATTA. I would like to thank him for allowing me to use his personal photos in this post. Dr. Lu is now establishing a radiokrypton dating centre at the University of Science and Technology of China.

References

Lu Z-T, Schlosser P, Smethie WM, Sturchio NC, Fischer TP, Kennedy BM, et al. Tracer applications of noble gas radionuclides in the geosciences. Earth-Science Rev. 2014;138:196–214.

Chen CY. Ultrasensitive Isotope Trace Analyses with a Magneto-Optical Trap. Science (80-). 1999;286(5442):1139–41.

Du X, Purtschert R, Bailey K, Lehmann BE, Lorenzo R, Lu Z-T, et al. A New Method of Measuring 81Kr and 85Kr Abundances in Environmental Samples. Geophys Res Lett. 2003;30(20):2068. Available from: http://arxiv.org/abs/physics/0311118

Aggarwal PK, Matsumoto T, Sturchio NC, Chang HK, Gastmans D, Araguas-Araguas LJ, et al. Continental degassing of 4He by surficial discharge of deep groundwater. Nat Geosci. 2014;8.

Lu Z-T. Atom Trap, Krypton-81, and Saharan Water. Nucl Phys News. 2008;18(2):24–7.

Guest Post: Jeremy Bennett – Approaches to modelling heterogeneity in sedimentary deposits

Matt Herod September 10, 2015

Hello everyone. Great that you could make it out to my blog post. I would like to introduce you to some ideas about environmental modelling that I have recently discovered during my work. These ideas are from this paper by Christine Koltermann and Steven Gorelick back in 1996. Whilst the primary focus of their paper is on modelling hydrogeological properties such as hydraulic conductivity, I think there is crossover with other modelling too.

What I find the most interesting about this work are the words they used to describe modelling approaches, meaning the way the modeller sees the world. They break down modelling into three different approaches: structure-imitating, process-imitating, and descriptive methods. Over the next few mousewheel-scrolls I hope I can explain these ideas in simple terms so that they are easy to understand.

This paper discusses models that are spatially distributed – this means that we are trying to estimate values at different locations in space. In the following diagrams I have simplified things to one dimension to hopefully make things a bit clearer. It is also important to note that many models will combine elements of one or more of the following model approaches – often at different scales.

Descriptive methods

Descriptive modelling approaches are primarily conceptual – kind of like joining the data dots in Figure 1 to produce the circle. There might be no hard and fast rules here, although models may be based on years of experience and observation in the field. These models may not be so rigorous and possibly difficult to replicate in different environments.

A good example of descriptive modelling are geological cross sections. They are constructed using borehole data and similar lithologies at similar depths are assumed to be part of the same geological formation. More experienced practitioners will have better intuition for connecting the dots and interpreting the stratigraphic record. In many cases thes cross sections are a suitable model. However in some hydrogeological applications this level of modelling is insufficient as more information is required about the geometry of the formation, and perhaps variations in its hydraulic properties – something that is difficult to derive solely from descriptive methods.

Structure-imitating methods

Structure-imitating modelling approaches quantify observations of the thing to be modelled and use these rules to produce something that looks similar. The structure that is imitated could be the actual shape of the object to be modelled, or it could be something more abstract, such as the geostatistical structure of the observations. To demonstrate: In Figure 2 we have some data shown with black lines. We can then derive information about this data, say in this case the distance of each data point from the centre. From this structural information we can model the rest of the circle.

A well-known structure-imitating method is kriging. This method uses the geostatistical structure (i.e. mean and covariance) of a set of observations to estimate values of a variable at other locations. A typical criticism of kriging and other geostatistical methods is that defined boundaries between facies become indistinct and don’t look so geologically plausible. Many other methods have been developed, such as multiple-point statistics, to address these arguments.

Process-imitating methods

Process-imitating modelling approaches rely on the governing equations of a process to produce a plausible model. Governing equations describe the physical principles underlying processes such as fluid motion or sediment transport. This type of approach can occur both as forward or inverse modelling. Forward models require setting key parameters in the model (such as hydraulic conductivity) and then predicting an outcome, such as the distribution of groundwater levels. Inverse models start with the observations and try to fit the hydrogeological parameters to the data.

Our final circle model is in Figure 3. In this particular case we know the equation that gives us the circle. As with all process-imitating modelling approaches there is some kind of parameter input required (or forcing). Here we have assumed that the circle is centred about the origin, and our parameter input is the radius of the circle (4) on the right hand side of the equation. Thus we can model the circle based on the equation and a parameter input.

The classic process-imitating model approach in hydrogeology is aquifer model calibration. This is a relatively simple, but widely used, application where zones of hydraulic conductivity are created and adjusted to reproduce measured groundwater levels (hydraulic heads). Often these zones are tweaked using a trial-and-error process to get a better match (or reduce the error). Aquifer model calibration is considered a process-imitating approach because it attempts to replicate the governing equations of fluid flow within porous media. MODFLOW is a model from USGS that is often used in this type of modelling.

Thanks for making it all the way down here. My aim was to provide you with a couple of new words to describe modelling approaches in geosciences and beyond. If you are working in hydrogeology then this paper by Koltermann and Gorelick is definitely worth a read – it gives an excellent foot-in-the-door to hydrogeological modelling.

Reference

Koltermann, C. E., and Gorelick, S. M. (1996). Heterogeneity in Sedimentary Deposits: A Review of Structure-Imitating, Process-Imitating, and Descriptive Approaches. Water Resources Research, 32(9), pp.2617-2658.

About Jeremy

Jeremy Bennett is conducting doctoral research at the University of Tübingen, Germany. He is researching flow and transport modelling in heterogeneous porous media. Prior to his post-graduate studies in Germany he worked in environmental consultancies in Australia and New Zealand. Jeremy figures there is no better way to understand a concept than to explain it to others – hopefully this hypothesis proves true. Tweets as @driftingtides and blogs here.