EGU Blogs

Groundwater

New study shows nitrate leaches to groundwater for over five decades

November 12, 2013

A new and very interesting study just came out in the journal Proceedings of the National Academy of Sciences (PNAS) the other day titled “Long-term fate of nitrate fertilizer in agricultural soils”. This paper addresses some very interesting and extremely important questions using isotopic geochemical tools.

The question central to this paper is what happens to all of the nitrogen in the fertilizer that we spread on our crops? This question is a very popular one in the nitrate contamination field (haha, pun intended) and this paper attempts to look at the long term fate of this molecule, which is an aspect that is not very well understood. The paper looks at a field that had nitrate fertilizer labelled with the stable isotope nitrogen-15 spread over it for three decades to see what the fate of this nitrogen is? In other words, where does it go? Does it stay in the same place? Is it still available to plants or does it end up in the groundwater or surface water? In fact, the authors found that 61-65% of the applied fertilizer was used by plants, and 12-15% was still in the soil. Furthermore, 8-12% of the nitrogen added had entered the hydrosphere during the 3 decade observation period and the fertilizer remaining in the soil was still available to crops and to leak into groundwater over the upcoming fifty years.

Let’s take a quick step back though. Why is nitrogen, an element that is essential to the health of plants and one of the basic elements found throughout the the natural world being considered a contaminant? There are two main reasons that having excess nitrate, which is NO3, in the hydrosphere is a problem. The first is eutrophication. Eutrophication is a phenomenon that occurs when excessive nutrients enter a water body. The additional nutrients cause massive plant growth including algae. This leads to an occurrence called an algae bloom, pictured below, that covers the water surface, preventing other plants from photosynthesising and leading to their death. This mass die-off consumes all of the oxygen in the water body and leads to the death of the entire food web. In short nutrient overloading leads to the apocalypse.

Algae bloom in Florida. (Source)

The second reason that high levels of nitrate are a concern is because of a health problem associated with people and animals consuming excess nitrate called Methemoglobinemia, which is also known as blue baby syndrome. This disease affects the chemistry of the blood and prevents the transfer of oxygen from the blood to tissues. This occurs most often in babies under 6 months of age and can be caused by elevated nitrate levels in their drinking water. It is for this reason that the maximum allowable concentration of nitrate in most place is 10 ppm.

Ok, back to the study. So the authors found that there are three places where the added nitrate can end up over time and this finding is pictured in the graph below. What this figure is showing is that immediately after adding the fertilizer in 1983/84 the total load of N03 in the plants goes up 50% immediately, which is good for growing things. A further 30-40% is trapped in soil organic matter. The total of both these reservoirs roughly equals 100% of the expected NO3 load.

However, over time the amount in the plants slowly increases while the reservoir of labelled NO3 in the soil organic matter decreases at a much faster rate. This loss of NO3 from the SOM is being found in sampling lysimeters showing that it is leaching off the SOM and ending up in groundwater.  The NO3 that is trapped in the SOM mixes with the existing nitrate that is already there and is then slowly leached over the next several decades. In fact, the authors found that 0.4% of the labelled nitrate leached per year and during the study period 8-12% of the labelled fertilizer ended up leaching off the SOM into the groundwater! This may not seem like a lot but considering the massive volume of fertilizer spread in agricultural regions each year this can add up to a huge annual influx of NO3 to the hydrosphere and have serious environmental repercussions.

Cumulative budget of 15N-labeled fertilizer nitrogen based on mass and isotope balances for plants, soil organic matter (SOM), and nitrate in lysimeter outflows for Lys S (full symbols) and Lys W (empty symbols). Image from Sebilo et.al. (2013) and used according to PNAS copyright guidelines for educational purposes only.

The next thing the authors tested, which is pictured in the figure below, was to see how far into the future the labelled fertilizer will continue to leach to the groundwater. The data points that have already been collected are pictured and a decay function, such as exponential decay, is fitted to them. As you can see the decay function fits the existing data nicely and therefore can be used to predict what could happen in the future. The graph is showing that it will take another five decades for the values 15N in the soil to return to background i.e. pre-application of the 15N labelled fertilizer levels. Basically, this means that the single application of 15N enriched fertlizer in 1981 will have an influence on soil nitrate and hence leaching for 100 or more years.

Decay functions fitted to observed δ15N values of soil organic matter from Lys S (red) and Lys W (blue). The model suggests that it will take circa 100 y to reach the background δ15N values of circa +5‰ observed before tracer application. Image from Sebilo et.al. (2013) and used according to PNAS copyright guidelines for educational purposes only.

To summarize, the study by Sebilo et.al. found that the fertilizer that gets added to fields all over the world is not immediately taken up by plants. In fact, it sits around in soil organic matter and can slowly leach into groundwater over 50 or more years!  This is a major cause for concern and understanding the dynamics of nitrate in soil over long time periods will help us avoid some of the potential environmental or health problems that fertilizing could cause. Indeed, we now know that simply waiting for the nitrate we add to disappear naturally is not an option as the SOM provides a continuous source of nitrate.

Here is the link that alerted me to this article in the first place:

http://www.waterworld.com/articles/2013/10/nitrogen-fertilizer-remains-in-soils-and-leaks-towards-groundwater-for-decades-researchers-find.html

Reference:

Mathieu Sebilo, Bernhard Mayer, Bernard Nicolardot, Gilles Pinay, and André Mariotti
Long-term fate of nitrate fertilizer in agricultural soils
PNAS 2013 110: 18185-18189.

PDF: http://www.pnas.org/content/110/45/18185.full.pdf+html

Thanks for reading, especially if you read the whole thing!

Matt

AW# 60 – Radioactivity: What’s the use?

October 1, 2013
AW# 60 – Radioactivity: What’s the use?
Cherrenekoff radiation is a pretty way to demonstrate radiation. (Photo: Matt Herod)

I am very excited to be hosting the 60th Accretionary Wedge at GeoSphere! Sorry my own contribution is so slow in coming…it has been a busy month PhD wise. In fact, I expect it will be a busy year PhD wise since I am hoping to submit in the Fall of 2014 and I have got a LOT of writing to do. Anyway, in the call for posts I said:

For this wedge the topic will be momentous discoveries in geology or its sub-disciplines that you feel have altered or shaped our understanding of how the Earth works, or opened new doors into research that had never been considered before. The discovery you choose does not have to be universally recognized as momentous but should be in your opinion. It could be something that we take for granted every day, but is in actuality part of the underpinnings of our science.

The discovery I am going to discuss is one that a wide variety of different geoscience disciplines uses every day and one that is particularly near and dear to my heart, since my PhD. is about an application of this discovery: radioactivity and radioactive isotopes. The consequences of the discovery of radioactivity have been extremely far reaching in many fields, particularly the geosciences. For this post I am not going to describe what radioactivity is, but rather some of the fantastic applications and subsequent discoveries that have hinged on the initial discovery of radioactivity and radioactive elements. There are a lot, so for the ones I don’t talk about I’d love to see comments on.

Cherrenekoff radiation is a pretty way to demonstrate radiation. (Photo: Matt Herod)

Cherenkov radiation is a pretty way to demonstrate radioactivity. (Photo: Matt Herod)

The Earth is constantly in a state of change. The process that move and shape the Earth are doing so in front of our very eyes. If only we had a means to see it happen….oh wait. Radioactive isotopes can often be used as tracers of natural processes. Sometimes the isotopes are naturally occurring and sometimes they are added through human activities. Either way, we can use them to uncover Earth’s mysteries.

The first thing that everyone thinks about when discussing application of radioisotopes in geology is radioactive dating.  The basic principle of radioactive dating was discovered by Ernest Rutherford in 1905 and states that if we know the half life and the concentration of the decay product we can use that information to calculate how much of the parent isotope there was and then how old the material the daughter product was measured in is. This basic idea has spawned a wide variety of dating techniques using different isotopes with a range of half lives from very short to very, very long. Radioactive dating is the reason we know old the Earth is, when the dinosaurs lived and died, when ancient volcanoes erupted, what sort of tectonics took place on the early Earth, how long ago our ancestors lived and so much more. The problems to which radioactive dating can be applied are limited by the presence of a usable isotope rather than running out of questions. Indeed, there will always be more things to date using the wide varity of isotopes available. It is possible to date recent things using Carbon 14 or Tritium (Hydrogen), which both have fairly short half lives. The furthest back that I have seen dating methods go is for rocks from the Isua Greenstone belt, which were dated at 4.28 billion years using the samarium-neodymium isotope system (Note: This work was done by J. O’Neil a new prof at uOttawa). Either way, if it is new or old radioisotopes can date it.

Occasionally pollution can be useful. It goes without saying that pollution is bad. However, on occasion its presence can be used to solve scientific problems when nature does not provide a means to do so. One instance of this is using radioactive releases from nuclear fuel reprocessing to trace ocean currents in the North Atlantic ocean. The releases themselves are extremely low level, and are not dangerous to humans or the ecosystem in any way. However, they are easily detectable and therefore can be used. Fuel reprocessing releases a lot of iodine-129 and cesium-137, as well as some other isotopes. These isotopes are then released into the Atlantic ocean and circulate with currents. Using these isotopes to trace the depth profile of currents, where they move and how long it takes for them to circulate is a burgeoning field in oceanography research.

Not all radiotracers are pollution though. Ideally, we can use naturally produced radiotracers to tell us about the environment. For example, contaminated places aside, radon is produced by the decay of naturally occurring uranium-238. It can then be incorporated into groundwater or pass through the soil as a gas. One immediately obvious use of radon is in uranium exploration. If there is a higher concentration of uranium in the rocks more radon will be produced. Therefore, if soil gas sampling for uranium exploration finds elevated radon that could be an indication of a possible economic uranium enrichment. Another way of using radon is something that I have some personal experience with: tracing groundwater discharge into surface water. When groundwater comes in contact with uranium minerals it dissolves some of them or dissolves the radon gas directly. If this groundwater then discharges into a lake or a river for example we might expect to find higher radon at the discharge point. I did a lot of sampling for this one summer during my undergrad and we did find a few places where the radon concentrations were higher than background, which is an indication of groundwater discharge. We also canoed down an entire river with the instrument dangling out the side, sampling as we went. No radon was found in that instance, but it was really fun.

Radioactivity is also useful in the lab. In fact, I use the radioisotope iodine-125 almost every day as a tracer of a lab method I have been developing for the extraction of iodine-129 and 127 from organic materials. The process that I am using to extract iodine is combustion under a pure oxygen atmosphere and then trapping the iodine in a bubbler containing a hydroxide solution. However, it is often difficult to know if the extraction has been successful or not, particularly if I am playing with flow rates or temperature settings. The sample may be burnt, but did I capture the iodine? In order to test whether or not my combustion has been successful I add some iodine-125 to the sample before it is combusted. Then all I have to do at the end is see if my 125I, which is tested via gamma counting, is in my trap solution and I know if I got the other isotopes as well, because they all behave the same chemically. This line can also be used to extract other radiohalides from organic materials. In fact, as I type this a visitor from the Lamont-Doherty Earth Observatory is extracting chlorine-36 from vegetation and rat poop.

IMG_3801

My iodine combustion line. The paper describing the work done on this will be submitted very soon! (Photo: Matt Herod)

Cosmogenic isotopes are one of the most useful tools in the surficial geochemists arsenal and their use is very wide ranging. I wrote a very long post about cosmic rays and cosmogenic isotopes previously. See here. The big uses that I discussed for cosmogenics, which are radioisotopes that are produced when cosmic rays collide with particles in the atmosphere or directly with minerals to produce other isotopes that can then be used for a wide range of dating applications.

Exposure age dating is one of the principle uses of cosmogenic isotopes and several different ones have been employed to this end. Exposure age dating is when we use cosmogenic isotopes to date how long something has been exposed to secondary cosmic rays at the Earth’s surface. For example, a rock at the toe of a glacier may have just been exposed to cosmic rays a year ago and its exposure age date will tell us this. Another rock, 20 metres further from the glacier toe may have been exposed 5 years ago and this difference will be recorded. Basically, the clock starts ticking once cosmic rays are able to reach the rock or bone, or whatever material is being dated.

Carbon 14 is unquestionably the most used of any of the cosmogenic isotopes. 14C is produced primarily when neutrons collide with nitrogen in the atmosphere replacing a proton in the nitrogen nucleus and transforming it into carbon-14, although atmospheric nuclear weapons testing also produced a substantial amount of 14C. The primary use of 14C, or radiocarbon, is radiocarbon dating. Once radiocarbon is produced it enters the global carbon cycle and disperses throughout the environment. Since it is part of the carbon cycle it becomes incorporated into all living things, including people and animals. 14C continues to enter our bodies while we are alive. Once we die, there is no further addition of 14C and therefore, the clock starts ticking on its use as a dating tool. 14C has a 5,730 year half life and is therefore useful as a dating tool for biological materials up to ~45,000 years ago.

Sick of reading about how useful radioactivity is in geology? I don’t blame you. However, what I have presented here is just the tip of a very large iceberg. Other examples, include helium-tritium dating, helium ratios, tritium in natural systems, gamma logs for oil and gas exploration, and much, much more. It is all thanks to that very momentous discovery in 1896 by Henri Becquerel. If you have examples of the use of radioisotopes or radioactivity in geology please comment about it below.

The summary post for September’s AW will be up in a few days. So if you have a late submission you can still get it in.

Matt

Back to Basics on Groundwater

September 17, 2013
Back to Basics on Groundwater
A spring coming out of the shale near Red Creek. Yes, that water is black!!! (Photo: Matt Herod)

When many people hear the word groundwater they imagine a raging underground torrent of water flowing along a pathway called an aquifer. Well, sorry to disappoint you, but you could not be more wrong about how groundwater exists and flows. In this post we will discuss the very basics of groundwater science (hydrogeology) and flow.

What is groundwater?
As the name implies groundwater is simply water that exists underground. It is the opposite of surface water, which exists on the surface of the Earth such as lakes, rivers and oceans. Groundwater is an extremely important resource for industry, drinking water and other applications, however, it is generally quite poorly understood. The branch of geology that researches groundwater is called hydrogeology and is still a relatively new sector of the geological sciences.

As I have already mentioned groundwater exists underground. However, there are still lots of misconceptions about how people envision groundwater. Many see large underground lakes and rivers, and while those do exist, they represent an infinitesimally small percentage of all groundwater. Generally speaking groundwater exists in the pore spaces between grains of soil and rocks. Imagine a water filled sponge. All of the holes in that sponge are water-filled. By squeezing that sponge we force the water out, similarly, by pumping an aquifer we force the water out of pore spaces.

Notice that SpongeBob is full of pore spaces…I’m not sure if they are water-filled though. (Source: Wikipedia)

There are lots of terms in hydrogeology, most of which are very simple, but essential. Here are a few of the big ones and their meanings.

Porosity: Porosity is an intrinsic property of every material. It refers to the amount of empty space within a given material. In a soil or rock the porosity (empty space) exists between the grains of minerals. In a material like gravel the grains are large and there is lots of empty space between them since they don’t fit together very well. However, in a material like a gravel, sand and clay mixture the porosity is much less as the smaller grains fill the spaces. The amount of water a material can hold is directly related to the porosity since water will try and fill the empty spaces in a material. We measure porosity by the percentage of empty space that exists within a particular porous media.

File:Well sorted vs poorly sorted porosity.svg

Porosity in two different media. The image on the left is analagous to gravel whereas on the right smaller particles are filling some of the pores and displacing water. Therefore, the water content of the material on the right is less. (Source: Wikipedia)

Permeability: Permeability is another intrinsic property of all materials and is closely related to porosity. Permeability refers to how connected pore spaces are to one another. If the material has high permeability than pore spaces are connected to one another allowing water to flow from one to another, however, if there is low permeability then the pore spaces are isolated and water is trapped within them. For example, in a gravel all of the pores well connected one another allowing water to flow through it, however, in a clay most of the pore spaces are blocked, meaning water cannot flow through it easily.

Video showing how connected pores have high permeability and can transport water easily. Note that some pores are isolated and cannot transport water trapped within them. (Source)

Aquifer: An aquifer is a term for a type of soil or rock that can hold and transfer water that is completely saturated with water. That means that all it is simply a layer of soil or rock that has a reasonably high porosity and permeability that allows it to contain water and transfer it from pore to pore relatively quickly and all of the pore spaces are filled with water. Good examples of aquifers are glacial till or sandy soils which have both high porosity and high permeability. Aquifers allows us to recover groundwater by pumping quickly and easily. However, overpumping can easily reduce the amount of water in an aquifer and cause it to dry up. Aquifers are replenished when surface water infiltrates through the ground and refills the pore spaces in the aquifer. This process is called recharge. It is especially important to ensure that recharge is clean and uncontaminated or the entire aquifer could become polluted. There are two main types of aquifer. An unconfined aquifer is one that does not have an aquitard above it but usually does below it. The other type is a confined aquifer that has an aquitard above and below it.
Aquitard: An aquitard is basically the opposite of an aquifer with one key exception. Aquitards have very low permeability and do not transfer water well at all. In fact, in the ground they often act as a barrier to water flow and separate two aquifers. The one key exception is that aquitards can have high porosity and hold lots of water however, due to the their low permeability they are unable to transmit it from pore to  pore and therefore water cannot flow within an aquitard very well. A good example of an aquitard is a layer of clay. Clay often has high porosity but almost no permeability meaning it is essentially a barrier which water cannot flow through and the water within it is trapped. However, there is still limited water flow within aquitards due to other processes that I won’t get into now.
File:Aquifer en.svg
Water Table: The water table is a term that hydrogeologists use to describe an imaginary surface that usually exists underground. Below the water table all pore spaces are completely filled with water and above it they are filled with air. The water table is the boundary between these two zones called the saturated and unsaturated (vadose) zone. In order to imagine the water table it helps to imagine a layer that exists underground rather than a line as the water table is a surface that extends in every direction. The top of the water table is determined by water pressure. When water pressure in the pore spaces is the same as air pressure we are at the water table. The water table is subject to rise and fall depending on pumping of water out of the aquifer or other changes. Finally, if the water table and the surface of the Earth intersect we have a spring.
File:Water table.svg

Cross section of what the water table looks like as a line. Remember it is actually a surface that extends in every direction. Note that the water in the well only rises to the surface of the water table because air pressure and water pressure are equal at the water table. (Source: Wikipedia) 

Groundwater Flow

The study of hydrogeology is a very mathy one. There are lots of complicated equations, Greek letters, and funny squiggles. However, you don’t need an advanced degree in math to understand the basics, but it helps to know a little bit. Basically all ground water flow can be described by a single simple equation. Sure, there have been lots of modifications made to fit specific conditions and circumstances, but it all comes back to the same basic principles outlined in a single equation. That equation is called Darcy’s Law (cue dramatic music).

Henry Darcy – “The father of hydrogeology”

Darcy’s law states that the velocity water flows is dependant on the material which it flows through and the hydraulic gradient, which is the difference in water level between two points of measurement divided by the distance between them. In mathematical terms it looks like this:

Darcy’s Law

Q is the discharge or the amount of water that flows out of a given material over a set amount of time.

K is called the hydraulic conductivity and is a property of every material that tells us the speed any liquid moves through a given material. It is directly related to the porosity and permeability of the the material and the density of the liquid in question. For water we don’t need to worry about the density though, just the porosity and permeability.
(h1-h2)/l is usually represented by the letter i and is called the hydraulic gradient. It is the difference in water level between the two points of measurement divided by the distance between them.
Here is a graphical representation of the properties that make up Darcy’s law:

Graphical representation of Darcy’s Law in a hypothetical porous medium with two points of measurement (h1 and h2) and a hydraulic conductivity of K. (Matt Herod – 2011)

So now we understand some of the basic principles governing groundwater flow, but we haven’t discussed why it would flow from one place to another. We all take it for granted that groundwater is not stationary and moves, but why is that? The answer is shockingly simple, and lies in the fact that everything in nature is in a constant struggle to find balance.

Water flows from areas of high energy to low energy in an attempt to distribute that energy evenly throughout the water table. In this case energy is not a synonym for electricity, but energy in all forms, such as pressure or concentration differences. In the case of the water table the driving force is usually differences in pressure and elevation along the surface of the water table which lead to water flow. In hydrogeological terms these energy differences are referred to as hydraulic head, which can be measured at any point in the water table. It is helpful to imagine the weather when we think of groundwater flow. We all know that wind moves from areas of high air pressure to low air pressure bringing weather changes and temperature fronts in with it. Groundwater behaves the same and moves from places with high hydraulic head to low hydraulic head the same as the wind.

Obviously there is much more to discuss in the field of hydrogeology. The big topics being this like contamination or freshwater resources. However, in order to discuss those topics properly it is crucial to have a solid grasp on groundwater terms and basics. Please feel free to to comment if you have any suggestions for future posts on groundwater. Thanks for reading.

Matt

Note: This is a re-post from my old, less visited geo-blog. It was previously posted on June 13, 2011.

Day 3 and 4 – Craters, Very Old Rocks, Fukushima and Extinctions

August 30, 2013

Here is my Goldschmidt summary part 3 comprising both day 3 and day 4. I had to prepare my own talk, that I gave on Thursday (day 4) so I had to put the blog on hold to practice.

Here are a few of the most interesting talks that I went to:

Fred Jourdan hailing all the way from Curtin University in South Australia gave a talk called  – Volcanoes, asteroid impacts and mass extinctions (abstract). In his funny and very interesting talk Dr. Jourdan asked the question what is responsible for mass extinctions in geologic history? There has always been considerable debate in the scientific community about what caused all of the mass extinctions that have taken place over geologic time. Was it the volcanoes or the meteorite impacts? Dr. Jourdan compared the dates mass extinctions and tied these to the dates of volcanic eruptions and meteorite impacts to see if any two or three occurred at the same time. He found that volcanic eruptions coincided with mass extinctions better than meteorite impacts and concluded that volcanoes have played a dominant role in mass extinctions throughout Earth history. However, meteorite impact dating needs to improve since they also play some role.

Dr. Phillippe Van Cappellen  from the University of Waterloo gave a fantastic keynote address (abstract) on the mysterious part of the groundwater world called the hyporheic zone. The hyporheic zone is the magical place in a stream bed where groundwater flows into the stream. Sounds pretty simple right? According to Dr. Van Cappellen, wrong! Very wrong. It turns out that the hyporheic zone is extremely complicated and can have major impacts on the flow of chemicals from groundwater into surface water. Imagine this scenario: a local aquifer is contaminated with PCB’s. This is bad, but they have not made it into the nearby stream yet, so we only have to remediate groundwater. However, these chemicals will get stored or released by the hyporheic zone and could potentially contaminate a larger area than we thought. Dr. Van Cappellen’s work aims to understand how the h-zone functions under different chemical conditions and what sort of environmental factors such as water level, organic content or freeze-thaw cycles can affect it.

John O’Neill a new professor from uOttawa gave a terrific talk called Earth’s Hadean Crust: Insights from the Nuvvuagittuq Greenstone Belt about some really, really, really old rocks (abstract). In fact he has dated some rocks located in Northern Quebec at 4.4 billion years old!!!! The Earth is only 4.6 billion years old so these rocks have been around right since the beginning. John’s talk was very well attended and he presented some very interesting results to prove that these rocks are so old. This is still a very controversial topic and I am sure that discussions will continue for quite a while.

The next talk was very interesting to me. Dr. Yasuyuki Muramatsu, one of the leaders in the field of radio-iodine research, presented his talk right before mine. His talk was called: Reconstruction of the Accident-Derived I-131 Deposition in Fukushima Through the Analysis of I-129 in Soil (abstract). A lot of iodine-131 was released from Fukushima, which as a short half life of 8 days. This meant that it was very difficult for researchers to map its fallout over Japan, which is essential. However, using iodine-129 as a proxy for iodine-131 is possible and Muramatu’s group set out to do just that and they produced some really nice maps showing the fallout pattern of iodine in Japan.

So that is it for Day 3 and 4. Instead of doing a Day 5 summary I am going to try and do an interview with someone and cover their research in a bit more detail. So stay tuned for that!

Cheers,

Matt

Goldschmidt2013