WaterUnderground

Water Underground

Humanitarian groundwater projects; notes on motivations from the academic world

Humanitarian groundwater projects; notes on motivations from the academic world

Post by Margaret Shanafield, ARC DECRA Senior Hydrogeology/Hydrology Researcher at Flinders University, in Australia. You can follow Margaret on Twitter at @shanagland.

___________________________________________________________

What led me down the slippery slope into a career in hydrology and then hydrogeology, was a desire to combine my love of traveling with a desire to have a deeper relationship with the places I was going, and be able to contribute something positive while there. I figured everyone needs water, and almost everyone has either too much (flooding) or too little of it.

But, from an academic point of view, aid/humanitarian/philanthropic projects can be frustrating and offer few of the traditional paybacks that universities and academia reward.  Last week, for example, I spent much of my time working on the annual report for an unpaid project, and I am soft money funded. And what’s worse, I couldn’t even get the report finished, because most of the project partners hadn’t given me their updates on time. When I went across the hall to complain to my colleague, he admitted that he, too, was in a similar situation.

So what is the incentive?

Globally, the need for regional hydrologic humanitarian efforts is obvious. Even today, 1,000 children die due to diarrhoeal diseases on a daily basis. Water scarcity affects 40% of global population, with 1.7 billion people dependent on groundwater basins where the water extraction is higher than the recharge.  And, the lack of water availability is only going to get worse into the future.

But being a researcher with pressure to “publish or perish” and find ways to fund myself and my research, what was/is my incentive to address these problems? From an academic point of view, water aid projects are often time-consuming, with expected timelines delayed by language and cultural barriers, difficulties in obtaining background data, expectations on each side of the project not matching up, and activities and communication not happening on the timescales academics are used to. And the results are typically hard to publish.

An online search revealed numerous articles discussing the pros and cons of pursuing a career in development work, including: having a job aligned with one’s morals and values, an exciting lifestyle full of change, motivated co-workers, the opportunity to see the world and different cultures, the opportunity to make a difference, and last but not least, because it is a challenge (in a good way).

As a scientist, I get elements of all these pros in my daily work. But, while much of what academics do fits under the umbrella of “intellectually challenging”, aid projects provide applied problems with real-world implications that can sometimes be lacking in the heavily research-focused academic realm, except for the creative “broader impacts” and outreach sections of grant proposals. They are therefore an opportunity for scientists to have an impact on the world by contributing to the collective understanding of water resources and hydrologic systems. And hey, many of us enjoy travelling and get to visit interesting places for work, too.

Pulling myself out of my philosophical waxings, I focused on these highlights and the benefits of working in an interdisciplinary project to address some of those global problems I mentioned earlier – and got back to report writing.

Training project partners in Vietnam to take shallow geophysical measurements (left). Sweaty days in the field are rewarded by cheap beers, magnificent sunrises, and relaxing evenings at the coast where the river meets the sea (right). Photos by M Shanafield.

___________________________________________________________

Margaret Shanafield‘s research is at the nexus between hydrology and hydrogeology. Her current research interests still focus on surface water-groundwater actions, although she work’s on a diverse set of projects from international development projects to ecohydrology. The use of multiple tracers to understand groundwater recharge patterns in streambeds and understanding the dynamics of intermittent and ephemeral streamflow are her main passions. Since 2015, she has been an ARC DECRA fellow, measuring and modelling what hydrologic factors lead to streamflow in arid regions. You can find out more about Margaret on her website.

Good groundwater management makes for good neighbors

Good groundwater management makes for good neighbors

Post by Samuel Zipper, postdoctoral fellow at both McGill University and the University of Victoria, in Canada. You can follow Sam on Twitter at @ZipperSam.

___________________________________________________________

Dedicated Water Underground readers know that this blog is not just about water science, but also some of the more cultural impacts of groundwater. Keeping in that tradition, today’s post begins with a joke*:

Knock, knock!

Who’s there?

Your neighbor

Your neighbor who?

Your neighbor’s groundwater, here to provide water for your plants!

Figure 1. Typical reaction to joke written by the author.

Ahem.

Perhaps this joke needs a little explanation. As we’ve covered before, groundwater is important not just as a supply of water for humans, rivers, and lakes, but also because it can increase the water available to plants, making ecosystems more drought resistant and productive. However, we also know that groundwater moves from place to place beneath the surface. This means that human actions which affect groundwater in one location, like increasing the amount of paved surface, might have an unexpected impact on ecosystems in nearby areas which depend on that groundwater.

Imagine, for example, two neighboring farmers. Farmer A decides retire and sells his land to a developer to put in a new, concrete-rich shopping center. Farmer B continues farming her land next door. How will the changes next door affect the groundwater beneath Farmer B’s land, and will this help or hurt crop production on her farm?

In a new study, my colleagues and I explored these questions using a series of computer simulations. We converted different percentages of a watershed from corn to concrete to see what would happen. Our results showed that the response of crops to urbanization depended on where the land use change occurred.

Figure 2. Conceptual diagram showing how urbanization might impact crop yield elsewhere in a watershed. From Zipper et al. (2017).

In upland areas where the water table was deep, replacing crops with concrete caused a reduction in groundwater recharge, lowering the water table everywhere in the watershed – not just beneath the places where urbanization occurred. This meant that places where the ecosystems used to be reliant on groundwater could no longer tap into this resources, making them more vulnerable to drought. However, places where the water table used to be too shallow saw boosts in productivity, as the lower water table was closer to the optimum water table depth.

In contrast, urbanization happening in lowland areas had a much more localized effect, with changes to the water table and yield occurring primarily only in the location where land use changed, because the changes in groundwater recharge were accounted for by increased inflows from the stream into the groundwater system.

So, what does this mean for the neighboring farmers we met earlier?

For Farmer A, it means the neighborly thing to do is work with the developers to minimize the effects of the land use change on groundwater recharge. This can include green infrastructure practices such as rain gardens or permeable pavement to try and mimic predevelopment groundwater recharge.

For Farmer B, the impacts depend on the groundwater depth beneath her farm. If the groundwater beneath her farm is shallow enough that her crops tap into that water supply, she should expect changes in the productivity of her crops, especially during dry periods, and plan accordingly.

*Joke written by scientist, rather than actual comedian.

___________________________________________________________

For More Information:

Zipper SC, ME Soylu, CJ Kucharik, SP Loheide II. Indirect groundwater-mediated effects of urbanization on agroecosystem productivity: Introducing MODFLOW-AgroIBIS (MAGI), a complete critical zone model. Ecological Modelling, 359: 201-219. DOI: 10.1016/j.ecolmodel.2017.06.002

___________________________________________________________

 

Sam Zipper is an ecohydrologist. His main research focuses broadly on interactions between vegetation and the water cycle, with a particular interest in unintended or indirect impacts of land use change on ecosystems resulting from altered surface and subsurface hydrological flowpaths. You can find out more about Sam by going to his webpage at: samzipper.weebly.com.

Of Karst! – short episodes about karst

Of Karst! – short episodes about karst

Episode 2: Dissolving rock? (or, how karst evolves).

Post by Andreas Hartmann, Lecturer in Hydrology at the University of Freiburg (Universität Freiburg), in Germany. You can follow Andreas on twitter at @sub_heterogenty.

Didn’t get to read Episode 1? Click this link here to do so!

___________________________________________________________

In the previous episode, I introduced karst by showing how it looks in different regions in the world. This episode will now deal with the processes that create such amazing surface and subsurface landforms. The widely used term “karstification” refers to the chemical weathering of easily soluble rock composed of carbonate rock or gypsum. Most typical is karstification of limestone (consisting of the mineral calcite, CaCO3) or dolostone (consisting of the mineral dolomite, CaMg(CO3)2). If exposed to CO2 rich water these rocks are dissolved to form aqueous calcium (Ca2+) or magnesium (Mg2+) and bicarbonate (HCO3 ) ions. For calcite, karstification is described by the following chemical equilibrium:

The dissolution of carbonate rock depends on various factors. Imagine a solid block of salt, which you pour water on. If completely solid, the water will flow down the salt surface slowly dissolving the block. If fractured, water will eventually enlarge the fractures in the salt block and dissolution will occur much faster. Now imagine smashing the salt block before pouring water on it. In such circumstances the salt will dissolve even faster as the surface area exposed to the water is much larger.

Karst and its evolution (educational video provided by Jennifer Calva on Youtube).

The same is true for karstification. If the carbonate rock is heavily fractured, it will dissolve faster than unfractured carbonate rock. Another factor is the availability of CO2, that depends on the relative amount of CO2 in the air, air temperature and soil microbiotic processes. Other factors are the purity of the carbonate rock, the availability of water, and the supply of CO2 from the surface. As soon as karstification takes place, more water will be able to pass the dissolution enlarged fractures providing more and more CO2, and creating a positive feedback between rock dissolution and water flow:

Positive feedback between carbonate rock dissolution and water flow (Hartmann et al., 2014, modified).

The hydrochemical processes described in this episode of the Of Karst! Series not only create beautiful karst landscapes but they also have a strong and particular impact on water flow paths in the subsurface, which will the topic of episode 4 that can be expected in early 2018. Before, I will present a special feature about karst in the movies as topic of episode 3 in autumn 2017.

Further reading

Hartmann, A., Goldscheider, N., Wagener, T., Lange, J. & Weiler, M. 2014. Karst water resources in a changing world: Review of hydrological modeling approaches. Reviews of Geophysics, 52, 218–242, doi: 10.1002/2013rg000443.

Ford, D.C. & Williams, P.W. 2013. Karst Hydrogeology and Geomorphology. John Wiley & Sons, 576 pages.

___________________________________________________________

 

 

Andreas Hartmann is a lecturer in Hydrology at the University of Freiburg. His primary field of interest is karst hydrology and hydrological modelling. Find out more at his personal webpage www.subsurface-heterogeneity.com.

 

Groundwater & Education – Part One

Groundwater & Education – Part One

Post by Viviana Re, postdoctoral researcher at the University of  Pavia (Università di Pavia), in Italy. You can follow Viviana on Twitter at @biralnas.

Part one of a two part series on groundwater and education by Viviana.

___________________________________________________________

Education /ɛdjʊˈkeɪʃ(ə)n
The process of receiving or giving systematic instruction, especially at a school or university.

  • from Latin educatio(n-), from the verb educare
  • Educare is a combination of the words e (out) and ducare (lead, drawing), or drawing out.

Based on this definition, I should change the title of this post to: Drawing out groundwater (from the well). This is actually the main occupation of groundwater scientists, isn’t it? Not only are we always withdrawing groundwater from a well or a borehole while sampling, but we also often have to “draw it out” when dealing with managers and policy makers, as sometimes they seem to forget about this hidden (but very important) component of the water cycle. Therefore, we are quite used to these forms of “drawing out” – but what about education? Are we really that effective in “drawing out” groundwater in explaining its peculiarities, issues, and connections within the whole water cycle and, more generally, with the environment?

Indeed, the effort of shedding light on something that is not so visible nor easily studied has the side effect of forcing us to focus solely on it, with a resulting tendency of developing sectorial approaches to water management.

In the preface of a UNESCO Technical paper, I found the following excerpt: “Water resources schemes are now increasingly considered as integrated systems and consequently, civil engineers, geologists, agricultural engineers and hydraulic engineers engaged in planning and design no longer work in isolation”. The document is dated 1974 but, still in 2017, we are somehow struggling to fitting groundwater into Integrated Water Resources Management (IWRM) and to connecting mental and structural “silos”. Quoting Daly (2017), the latter is particularly relevant (especially when education is at stake): if on the one hand, specialization can be the driver for a sound knowledge; on the other hand, this can encourage people to get stuck in their own individual disciplines (or said in other words, their “silos”). Indeed, “silos” exist in their structures, but can also exist as a state of mind that can go hand in hand with tunnel vision (Tett, 2015).

Therefore, in my opinion, the new generation of groundwater scientists (and teachers) should have a new mission: to work (and therefore, to teach) coherently with the integrated and complex nature of the water cycle. In fact, the role of hydrogeologists and groundwater scientists in times of increasing freshwater demand, exacerbated by population growth and climate change effects, requires a serious shift towards a more holistic approach targeting sound groundwater assessment and long-term management.

Arguably, if we are still discussing possible ways of practically implementing this integration, we should definitely start asking ourselves if the the “business as usual” way of working and teaching is effective.  If it is not, we must begin investigating how we can go beyond classical approaches to draw groundwater out of the well.

Playing with kids while sampling … can we call it capacity building?!

 

To be continued …

[Read More]

What is the difference between ‘water withdrawal’ and ‘water consumption’, and why do we need to know?

What is the difference between ‘water withdrawal’ and ‘water consumption’, and why do we need to know?

Post by Inge de Graaf, University of Freiburg, Environmental Hydrological Systems group

________________________________________________________________________________________________________________

Last week I had to teach my first class in global hydrology. When I showed the global trend on increasing demands and withdrawals (see Figure) I needed to explain the different terms as sometimes the term “water use” gets, well, misused.

The term “water use” often fails to adequately describe what happens to the water. So I told the students; if you see or hear to term ‘water use’ always ask yourself what’s actually being said. The term is often used for water withdrawals or water consumption, and it’s important to understand the difference.

Water withdrawal describes the total amount of water withdrawn from a surface water or groundwater source. Measurements of this withdrawn water help evaluate demands from domestic, industrial and agricultural users.

Water consumption is the portion of the withdrawn water permanently lost from its source. This water is no longer available because it evaporated, got transpired or used by plants, or was consumed by people or livestock. Irrigation is by far the largest water consumer. Globally irrigated agriculture accounts for 70% of the total water used and almost 50% is lost either by evaporation or transpiration.

Understanding both water withdrawal and consumption is critical to properly evaluate water stress. Measurements of water withdrawal indicate the level of competition and dependence on water resources. Water consumption estimates help to quantify the impact of water withdrawals on downstream availabilities and are essential to evaluate water shortage and scarcity. For example, most water used by households is not consumed and flows back as return flow and can be reused further downstream. However, water is rarely returned to watershed after being used by households or industry without changing the water quality, increasing water stress levels.

Already more than 1.4 billion people live in areas where the withdrawal of water exceeds recharge rates. In the coming decades global population is expected to increase from 7.3 billion now, to 9.7 billion by 2050 (UN estimate). This growth, along with rising incomes in developing countries, is driving up global food demands. With food production estimated to increase by at least 60% (FAO estimate), predicting water withdrawal and consumption is critically important for identifying areas that are at risk of water scarcity and where water use is unsustainable and competition amongst users exist.

Global trend I showed in my class, published in Wada et al (2016).

Ref:

Wada, Y., I. E. M. de Graaf, and L. P. H. van Beek (2016), High-resolution modeling of human and climate impacts on global water resources, J. Adv. Model. Earth Syst., 8, 735–763, doi:10.1002/2015MS000618.

 

How prehistoric water pit stops may have driven human evolution

How prehistoric water pit stops may have driven human evolution

Post by Matthew Robert Bennett, Bournemouth University and Mark O Cuthbert, Cardiff University

Our ancient ancestors seem to have survived some pretty harsh arid spells in East Africa’s Rift Valley over five million years. Quite how they kept going has long been a mystery, given the lack of water to drink. Now, new research shows that they may have been able to survive on a small networks of springs.

The study from our inter-disciplinary research team, published in Nature Communications, illustrates that groundwater springs may have been far more important as a driver of human evolution in Africa than previously thought.

Great rift valley.Redgeographics, CC BY-SA

The study focuses on water in the Rift Valley. This area – a continuous geographic trench that runs from Ethiopia to Mozambique – is also known as the “cradle of humanity”.

Here, our ancestors evolved over a period of about five million years. Throughout this time, rainfall was affected by the African monsoon, which strengthened and weakened on a 23,000-year cycle. During intense periods of aridity, monsoon rains would have been light and drinking water in short supply. So how did our ancestors survive such extremes?

Previously, scientists had assumed that the evolution and dispersal of our ancestors in the region was solely dependent on climate shifts changing patterns of vegetation (food) and water (rivers and lakes). However, the details are blurry – especially when it comes to the role of groundwater (springs).

We decided to find out just how important springs were. Our starting point was to identify springs in the region to map how groundwater distribution varies with climate. We are not talking about small, babbling springs here, but large outflows of groundwater. These are buffered against climate change as their distribution is controlled by geology – the underlying rocks can store rainwater and transfer it slowly to the springs.

The lakes of the African Rift Valley.SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE

We figured that our ancestors could have stayed close to such groundwater in dry times – playing a greater part in their survival than previously thought. When the climate got increasingly wet, groundwater levels would have risen and made springs more plentiful – feeding smaller rivers and leading to lakes becoming less saline. At this point, our ancestors would have roamed across the landscape free of concerns about water.

Life and death decisions

To test this idea, we embarked on a computer experiment. If the springs and water bodies are thought of as the rest stops, or service stations, then the linkages between can be modelled by computers. Our model was based on what decisions individuals would have taken to survive – and what collective behaviours could have emerged from thousands of such decisions.

Individuals were give a simple task: to find a new source of water within three days of travel. Three days is the time that a modern human and, by inference, our ancestors could go without drinking water. The harder and rougher the terrain, the shorter the distance one can travel in those vital three days.

We used the present landscape and existing water springs to map potential routes. The detailed location of springs may have changed over time but the principles hold. If our agent failed to find water within three days, he or she would die. In this way we could map out the migration pathways between different water sources as they varied through 23,000-year climate cycles. The map shows that there were indeed small networks of springs available even during the driest of intervals. These would have been vital for the survival or our ancestors.

The model also reveals movement patterns that are somewhat counter-intuitive. One would assume that the easiest route would be along the north to south axis of the rift valley. In this way, hominins could stay at the bottom of the valley rather than crossing the high rift walls. But the model suggests that in intermediate states between wet and dry, groups of people may have preferred to go from east to west across the rift valley. This is because springs on the rift floor and sides link to large rivers on the rift flanks. This is important as it helps explain how our ancestors spread away from the rift valley. Indeed, what we are beginning to see is a network of walking highways that develop as our ancestors moved across Africa.

Mapping human migration.

Human movement allows the flow of gossip, know-how and genes. Even in modern times, the water-cooler is often the fount of all knowledge and the start of many budding friendships. The same may have been true in ancient Africa and the patterns of mobility and their variability through a climate cycle will have had a profound impact on breeding and technology.

This suggests that population growth, genetics, implications for survival and dispersal of human life across Africa can all potentially be predicted and modelled using water as the key – helping us to uncover human history. The next step will be to compare our model of human movement with real archaeological evidence of how humans actually moved when the climate changed.

So next time you complain about not finding your favourite brand of bottled spring water in the shop, spare a thought for our ancestors who may died in their quest to find a rare, secluded spring in the arid African landscape.

The ConversationThis research was carried out in partnership with our colleagues Tom Gleeson, Sally Reynolds, Adrian Newton, Cormac McCormack and Gail Ashley.

Matthew Robert Bennett, Professor of Environmental and Geographical Sciences, Bournemouth University and Mark O Cuthbert, Research Fellow in Groundwater Science, Cardiff University

This article was originally published on The Conversation. Read the original article.

Is highway de-icing ‘a-salting’ our aquifers?

Is highway de-icing ‘a-salting’ our aquifers?

Post by Mark Cuthbert, Cardiff University, and Michael Rivett, GroundH20 plus Ltd; University of Strathclyde.

If you live in a cold climate, have you ever wondered where all the de-icing salt (or ‘grit’ as we call it in the UK) that gets spread on the roads in winter time ends up, aside from that accumulating salty grime that coats your car? As you might expect, most of the salt gets washed off the highways as the salt has the desired effect of melting the ice, or carried away by rain. This salty ‘runoff’ ends up in streams nearby via pipes which drain the highway. However, that is not the end of the story…

We studied a major highway intersection on the edge of UK’s second largest city, Birmingham (more than a million people), to see how much of the salt spread on the highway ends up in groundwater. Our interest stems from concerns of the national regulator to understand not only how much salt ends up in streams, but also the potential long-term build-up of salt in underlying groundwater resources that are pumped for public and private water supply. Although the origin of salt in streams and lakes is relatively well studied, the pathways by which salt moves from highways to groundwater are poorly understood and quantified.

The various ways salt could be getting in to the groundwater are shown in Figure 1 and a bit of detective work was required to find out what was going on.

Figure 1*

This involved dressing in silly (and warm) clothes, and pulling our equipment around on a sledge, which was a lot of fun (see photos below).

The authors, Mark (left) and Mike (right), enjoying some urban winter fieldwork.

Our main field activities were fortuitously timed to include the severe winters of 2009-10 and 2012-13. We collected information about the amount of salt spread on the roads and measured the salt concentrations in the local streams, as well as in the shallow and deep groundwater wells in the area around the highway network.

7 km of motorway drained to the studied stream. Over the winter of 2012-13 we estimated that around 510 tonnes of de-icing salt was applied to the highway and major road network across the catchment. Most of that washed off the road via drains when it next rained or as the snow and ice melted, and ended up in the stream which flows under the highways. Some of it ended up on the roadside verges – this was not quantified, but would likewise eventually leach into the underlying groundwater over time.

We estimated about 12% of the de-icing salt, that’s around 63 tonnes over the 2012-13 winter, leaked through the bottom of the stream channel into the groundwater in the sandstone aquifer beneath, and may pose a risk to groundwater supplies in the area (see “9” in Figure 1, and results in Figure 2). While increases in groundwater chloride concentrations observed to date have been modest, and fortunately remain far below the drinking water standards, the steady year-upon-year build-up of salt in groundwater remains a concern. This is especially the case as the UK’s de-icing salt applications reached record levels in the recent severe winters and add to a potential de-icing salt legacy in our aquifers that has accumulated now over some 50 plus years.

Figure 2*

So how concerned should we be that so much salt is flowing down these streams and into the groundwater around our highways? There isn’t yet a simple answer to that but, with a global trend towards increasing urbanisation, this is an important area of ongoing and future research.

 

Further details of our published research to date on the Birmingham case study may be found at:

Rivett, M.O.,  Cuthbert, M.O., Gamble R., Connon, L.E., Pearson, A., Shepley, M.G., Davis, J., 2016. Highway deicing salt dynamic runoff to surface water and subsequent infiltration to groundwater during severe UK winters.  Science of the Total Environment 565, 324-338. http://dx.doi.org/10.1016/j.scitotenv.2016.04.095 (*figures reproduced with permission) [Read More]

Fire and groundwater

Fire and groundwater

Post by Andy Baker, University of New South Wales

The effects of fire on the surface environment are clear to see. Landscapes are coated in ash. Intense fires can destroy all vegetation and alter soil properties. Less intense fires destroy just the surface leaf litter, grasses and shrubs.  Grass fires can be fast moving, destroying buildings and threatening lives. Intense fires can even form their own local weather systems.

But what about the effects of fire on water underground? Let’s think about what happens on the surface, and translate that to what is likely to happen to the subsurface.

Firstly, ash is burnt vegetation. It is rich in carbon, nutrients and trace metals, and can be mobilised after heavy rainfall. Heavy rainfall events are most likely to cause groundwater recharge as some of the rainfall makes its way down to the replenish the water table. As this happens we should also expect nutrients and carbon to be moved downwards towards the water table, as well as horizontally to rivers.

Secondly, where a fire destroys tree cover, then the canopy shading will be lost. We would predict hotter surface temperatures. This could alter the water balance, changing evaporation and also the amount or timing of groundwater recharge.

Finally, we would expect increased sediment movement. This could block flow routes to the subsurface, such as fractures and sinkholes. This would change the routes by which water moves from the surface to the water table.

Of course, the problem with water underground is that it is hard to see and measure. So how can we observe the effects of fire in the subsurface? We have been using caves in karstified limestone as a way to sample the water as it moves from the surface to the groundwater.

In caves, you can monitor the water chemistry and hydrology after a wildfire. If you know when a fire will occur, you can make measurements before and after. And you can investigate the chemical record of past fires preserved in cave stalagmites.

What do we see from the cave? We see increased evaporation and decreased recharge in the years immediately after  an intense wildfire. We see nutrient flushes from the surface to the subsurface, moderated by vegetation uptake as regrowth occurs. And we have started to compare the stalagmite geochemical signals of fire, cyclones and global warming.

We are starting to understand a little bit about the effects of fire on water as it moves through the unsaturated zone of limestone. What about other rock types? What about the groundwater aquifer itself? We need to know more about the effect of wildfires on our subsurface water resource.  The fire season is getting longer. A greater extent of the earth’s surface is being burnt. Both will continue to increase with global warming, but how will this affect the water underground?

image source

Squeezed by gravity: how tides affect the groundwater under our feet

Squeezed by gravity: how tides affect the groundwater under our feet

Post from the Conversation, by Gabriel C Rau, Ian Acworth, Landon J.S. Halloran, Mark O Cuthbert

When returning from a swim in the ocean, sometimes it seems as though your towel has moved. Of course, it’s just that the water line has shifted.

The natural rise and fall of the ocean at the beach is an excellent demonstration of gravitational forces exerted by the Sun and the Moon. Although the tidal force is small, it is strong enough to pull regularly on the ocean, making an enormous volume of water rise and fall.

What you might not know is that tidal forces from the Sun and Moon also influence the air we breathe and the solid ground we stand on. These effects are referred to as atmospheric and Earth tides.

While we don’t tend to notice Earth and atmospheric tides, they do affect both the land and the world’s largest freshwater resource located underneath our feet: groundwater. This occupies the pores that exist in geological materials such as sand or soil, much like water in a kitchen sponge.

We have developed a method that incorporates tidal influences to monitor our precious groundwater resources without the need for pumping, drilling or coring.

Water beneath our feet

It has been estimated that groundwater makes up 99% of the usable freshwater on Earth. If all of Earth’s groundwater were extracted and pooled across the world’s land surface, it would be enough to create a lake 180 metres deep.

While this sounds like a lot of water, it is important to remember that not all groundwater is available for use. In fact, groundwater is currently mined on a global scale, especially in drier parts of the world, where groundwater underpins human activities during times of drought.

Groundwater extraction can lead to a downward shift in the land surface level (known as “subsidence”), particularly if groundwater is removed from underground zones that contain soft clays. This is a significant global problem, especially in coastal areas, due to urbanisation and associated water demand.

Alternatively, a long wet period with excess rainfall can cause the groundwater to rise up and cause flooding.

Effect of tides on groundwater

Deeper groundwater buried underneath layers of different types of sediments is under great pressure (in groundwater terminology this is called “confined”). The gravity change from Earth tides squeezes the sediment, and therefore changes the pressure of the water in the pores.

The atmospheric tides add to the weight that is sitting on top of the groundwater and cause a change in stress that results in a downward squeezing.

Groundwater at that depth responds to these stress changes, which can be measured as tiny water level fluctuations inside a groundwater borehole.

We have developed a new approach that exploits these tidal influences to calculate important subsurface properties. For example, this can predict how the pressure is lowered when groundwater is pumped, and by how much the land surface would sink as a result of shrinking subsurface material (just like squeezing a kitchen sponge).

The method basically allows accurate calculation of the compressible subsurface properties from the groundwater response to Earth and atmospheric tides.

This development is significant because it will allow analyses of a subsurface water reservoir (called an aquifer) without human-induced stresses such as pumping or taking physical samples of the material through drilling or coring in addition to constructing a borehole.

All that’s needed for this analysis is a roughly 16-day period of continuous measurements of groundwater levels and atmospheric pressure at hourly intervals.

Groundwater levels are routinely recorded as part of water monitoring programs around the world and in Australia, as funded by the Federal Government groundwater NCRIS scheme. Atmospheric pressure is a standard parameter measured by weather stations, such as operated by the Bureau of Meteorology.

The effects of tidal forces on groundwater might be less apparent to us than their effects on the ocean, but they’re just as important. Our new method of understanding the influence of tides on groundwater significantly reduces the effort to predict the response to groundwater pumping and the potential for land subsidence.

This technique can make passive use of existing boreholes and could be applied to the global archive of groundwater levels to inform more sustainable groundwater resource development in the future.

Groundwater Speed Dating! Can you find a match?

Groundwater Speed Dating! Can you find a match?

Post by Matt Herod

Welcome to the first edition of groundwater speed dating. In today’s post I introduce you to a motley crew of isotopes and chemicals that hydrogeologists and geochemists use to date the age of groundwater. After meeting all of the contestants it will be up to you to pick your favourite and perhaps propose a second date. On your groundwater samples that is.

Starting to find some answers on water chemistry of baseflow samples from the Yukon. The first step in groundwater dating…picnic style. (Photo: Matt Herod)

Before I introduce you to our contestants I should briefly make it clear why groundwater dating is important. Understanding how old groundwater is may be one of the most, if not the most important aspect of protecting groundwater as a resource and preventing depletion of groundwater reserves from overpumping. For example, pumping an aquifer with a groundwater age of 10 years can be done semi-sustainably as any water extracted will take ~10 years to replace. However, pumping water with an age of 100,000 years is exploiting a nearly non-renewable resource. There may be lots of it, but the aquifer could take a long time to recover. Think of it like this: the water being pumped has to come from somewhere. Pumping could draw more water into the aquifer from recharge (not always an option) to replace what is lost, the water pumped could be from groundwater already stored in the aquifer, or it could be groundwater that was leaving the aquifer via discharge into a river or lake that is now diverted to your well.

Another great reason to know the groundwater age is to assess the vulnerability of an aquifer to contamination. If groundwater is young it is likely that the host aquifer is more vulnerable to contamination. Furthermore, knowing the age of groundwater throughout an aquifer will also allow a hydrogeologist to assess how quickly contamination will spread and if it can be contained. There are other reasons that it is beneficial to know the age of groundwater and if you’re interested I refer you to some of the references below.

I should also mention that the clock on a groundwater age starts once it becomes groundwater. That means that once my rain drop infiltrates into the ground and reaches the water table. The time it takes for the water to infiltrate through the soil layer is not included in the date which can add several months. Therefore, when I say groundwater is one year old, this means that it was likely rain from last year that has now reached the well, but it may be slightly older when you factor in the vadose zone travel time.

Are you ready to meet your speed dating contestants!? Pick the one that you’d like to date (the best isotope for your particular groundwater sample).

Note: You’ll see reference to cosmic rays a lot below. For reference see this primer in a previous blog post.

Name: Carbon-14
Nickname: 14C, The Cool One
Personality: Bada^s, Awesome
Half-life: 5730 years
Groundwater age range: 100 -100,000 years
A little about me:
14C, nicknamed radiocarbon, is the isotope that everyone wants to meet. Used by hydrogeologists in a vast range of dating applications for almost any organic material (organic = has carbon in it) it can also be used to date groundwater. 14C is produced by cosmic ray interactions with nitrogen in the atmosphere. 14C was also produced in significant quantities by atomic weapons testing and created a “bomb pulse” like our contestant tritium below which is also used to date groundwater. The great thing about radiocarbon is that since we know exactly how much is produced we can always estimate an age. However, dater beware. The age obtained from 14C and many other groundwater dating tools is the apparent age, which means it is inexact and vulnerable to aggregation errors when mixing young and old water mix, and requires the dater to consider other sources of inorganic carbon that contain no 14C such as ancient limestone. There are ways to correct for these issues, but great care must be taken so don’t be deterred from choosing 14C…maybe a bit of that coolness will rub off on you!? (Of course, as hydrogeologists we don’t need any extra coolness).

Name: Krypton 85 and Krypton 81
Nickname: 85Kr and 81Kr, The Twins
Personality: Different, One short tempered, the other slow to anger
Half-life: 81Kr: 10.75 years, 85Kr: 230,000
Groundwater age range: 10 – 100 and 10,000 -1,000,000 years (using different Kr isotopes)
A little about me:
These two twins could not be more different. Both are isotopes of krypton but with hugely different applications. You won’t see them in Twins magazine (twinsmagazine.com). The source of 85Kr is low level emissions from the nuclear industry, mainly fuel reprocessing. It has a short half-life meaning it can only be detected in groundwater a few decades old.

On the opposite end of the spectrum 81Kr is used for dating extremely ancient groundwater and is a relatively new dating tool for hydrologists. See me previous post on atom trap trace analysis for the details on this method that has made 81Kr dating possible. 81Kr is produced by cosmic ray interactions with gases in the atmosphere that become incorporated into rain that can recharge groundwater. 81Kr is the newest tool in the isotope hydrologists kit and has been used to date waters over 100,000 years old! It isn’t often you see twins so different than this pair. Nevertheless, they may be worth a longer look in your future?

Name: Tritium
Nickname: 3H, The Friendly One
Personality: Popular, Nice
Half-life: 12.3 years
Groundwater age range: 10-100 years
A little about me:
Tritium is the popular isotope in groundwater dating. Of all the isotopes in this competition, 3H is picked more often than the rest combined. Tritium has a short half life making it an ideal tracer and dating tool of young groundwater. Before the 1950’s all tritium in groundwater was natural and produced by cosmic ray interactions in the atmosphere. However, following the atomic weapons testing in the 1950’s and 60’s the tritium concentration in the atmosphere, rain and groundwater increased drastically. This made it possible to date groundwater using what hydrogeologists know as the “bomb peak”. This method has been enhanced using the decay product of tritium, 3He, as well to overcome the loss of tritium by decay over the time since weapon’s testing ended. While 3H may become less useful in the northern hemisphere as the bomb peak decays, the natural variability of 3H production in the southern hemisphere with fewer anthropogenic sources suggests 3H may become ever more useful! http://www.hydrol-earth-syst-sci-discuss.net/hess-2016-532/. Don’t be deterred by it’s popularity and the crowds, tritium is the real deal!

Name: Chlorine-36
Nickname: 36Cl, The Hard to Get
Personality: Evasive, Sends Mixed Signals
Half-life: 301,000 years
Groundwater age range: 10,000 – 1,000,000
A little about me:
36Cl is produced in the atmosphere by cosmic rays and has been used widely for dating ancient groundwater that is tens to hundreds of thousand years old. However, chlorine-36, while popular, is also “high-maintenance”: 36Cl requires more than just a run of the mill accelerator mass spectrometer (AMS) to measure. Indeed, in order to measure 36Cl on an AMS it needs to have the ability to remove the isobar sulphur-36 that interferes with the measurement of the much rarer (and thus sought-after by hydrogeologists) atoms of 36Cl in a sample. Only the highest energy AMS instruments or those with special capabilities, such as an isobar separator, can perform the measurement of 36Cl accurately. Nevertheless, sometimes it is worth putting in the extra effort for the reward. Will you?

Name: Uranium 234-238 Disequilibrium
Nickname: U-Disequilibrium (234U/238U), The Complicated One
Personality: Confusing, Game Player
Half-life: N/A
Groundwater age range:
A little about me:
You don’t come across U-disequilibrium dating that often, but when you do have a beer. U-disequilibrium is a bit mind-bending and requires a very thorough understanding the of the nuclear and geochemical processes at work in your sample location. Basically, it works because as 238U decays in rocks it shoots out alpha radiation, aka. 4He nuclei, at high energies. These bullets of helium break the crystal lattice of the minerals around the 238U atom allowing groundwater the get in. The groundwater then preferentially dissolves some of the 238U grand-daughter, 234U. This means there is more 234U than 238U dissolved in the water (by activity). The (activity) ratio of 234U/238U is greater than one and is predictable over time if you know the geochemical and hydrogeological characteristics of the system. This means you can correlate the ratio you measure to an age. This is an oversimplification of the method, but at it’s core that’s how it works. If you’re persistent, and work hard to understand U-disequilibrium the date is worth it. Will you put in the hard work?

Name: Iodine-129
Nickname: 129I, The Forgotten One
Personality: Quiet, Interesting
Half-life: 15,700,000 years
Groundwater age range: < 80,000,000 years
A little about me:
My personal fave but so forgotten it didn’t merit the figure above! See my PhD thesis for why. The short of it is that I think 129I has a wide variety of applications but we don’t yet fully understand its transport in groundwater and thus applying it is difficult. Therefore, people often overlook 129I for groundwater dating. It is similar to 36Cl in that it requires an AMS to measure, and they are both halogens. 129I has a very long half life and it is interesting because it is produced in three ways: cosmic rays, 238U fission in rocks, and nuclear fuel reprocessing. This makes it a tracer of modern groundwater and allows it to constrain the age of water that is less than 80 million years old as well! If you’re willing to take a chance and explore, perhaps 129I could be the one for you!?

Pick the isotope you’d like to date and leave a comment below!

Feature image from warsaw social.

Follow

Get every new post on this blog delivered to your Inbox.

Join other followers: