Our finite groundwater resources
Groundwater, a primary source of vital fresh water, is under increasing stress from a growing population and the accelerating development of infrastructure throughout the world. Our groundwater resources are not only threatened by overuse but also due to contamination. To make sure that we can provide all inhabitants of the Earth with water, now and into the future, for both drinking and irrigation, having a sound knowledge of our groundwater resources is becoming ever more important.
As the word “groundwater” implies, this article focusses on water found within the ground beneath our feet – the subsurface – interlinked with the geology, stored both in sediments and in fractures within hard rocks. And this is, of course, predominantly invisible from above ground.
How do we keep track of groundwater today?
There are many methods of mapping and monitoring groundwater reserves, such as drilling, geophysical survey, and well logging or sampling. Every method has unique advantages and complications, however there are some universal challenges to locating and estimating the extent of this finite and invisible groundwater resource, namely scale, accessibility and depth. So, what do we mean by this?
The search area for locating a reliable source of groundwater can often be quite large (from a few hundred square meters up to nationwide mapping campaigns) and can comprise both urban and rural areas, with open or forested terrain. The areas can potentially be quite inaccessible for vehicles and heavy equipment, due to lack of roads or the terrain conditions. And the groundwater resources can sometimes be found at great depths, with very varying geological conditions. To complicate the picture further, the geological conditions can vary both vertically and horizontally, over relatively small distances compared to the total search area giving rise to uncertainty when relying upon traditional methods such as drilling or surface geological reconnaissance.
However, despite these challenges the most common way to undertake groundwater investigations is by drilling, providing the client with exact information on, for example, the geology, moisture content and groundwater level beneath just a single, very limited point. This way of investigating the ground, quickly becomes both expensive and potentially imprecise when the aim is to map larger areas, especially when great depths are required.
Can we be more efficient when investigating the groundwater resource?
Rather than employ a relatively slow and expensive point-by-point method, such as drilling, it might be better if we can instead look at one of the Earth’s physical parameters that we could measure more widely and quickly from the ground surface. Ideally, it should be something that we can measure to great depths and that will provide us with a more comprehensive model of our groundwater resources in varying geological settings. And, of course, it must be a physical parameter whose values will vary in response to the presence or absence of groundwater. Do we have such parameter? Yes, the resistivity of the subsurface!
What is resistivity? And how is this related to groundwater?
All materials in the subsurface have an electrical resistance, in other words a property defining how difficult it is for an electrical current to pass through that specific material. When a current is applied to a material the material’s resistance will result in a voltage field that can be measured. The higher the resistance, the higher the voltage, it´s all about Ohm´s law. This resistance can be converted to a more useful value, apparent resistivity (measured in ohm-m). This is what’s called the “volumetric” parameter for resistance – a value, which is fixed for a given material type, irrespective of how much of that material is present. The relationship between resistance and resistivity is a bit like the relationship between mass and density: if I have more of a particular material it will weigh more, but the density of the material does not change. We refer to it as “apparent” resistivity at this stage because the raw data have been collected from the surface and thus the measured response is not that of a discrete point in the ground, but of all the material around it too. The final stage of a survey is to process the data, to develop a model of what, we hope, is the true distribution of resistivity beneath the ground.
Typically, water has a low electrical resistivity and, because of that, most materials in the ground in which water is present will display a change in resistivity from its original value. The more saturated with water a material becomes, the more its resistivity will change. For instance, a glass of dry Sahara-sand has a resistivity of say 1000-10 000 ohm-m, but the same glass with a fully saturated sand instead has a resistivity of around 500 ohm-m. A clear and measurable difference.
This means that for groundwater investigations the physical property of resistivity can be used to distinguish water from surrounding geologies, regardless of whether the water is found in unconsolidated materials, such as gravel, sand or silt, in rock fractures, or in permeable rock units. So, you get both information on groundwater and the groundwater container, the geology.
Out in the field it is possible to measure the electrical resistivity in two ways. You can either inject a current into the ground by using physical “galvanic” contact (i.e. steel rods – electrodes) or by creating and collapsing magnetic fields using an inductive method. The first approach is normally referred to as electrical resistivity (ER), or just “resistivity”, investigations, whilst the second method is known as TEM (transient electromagnetic) investigation.
Now, instead of isolated sporadic drilling, we can start to collect more extensive information of the subsurface and the groundwater within it.
What will the resistivity investigations give us?
Resistivity and TEM investigations can be done on anything from a local scale to nationwide mapping of groundwater resources. Electrical resistivity can reach 0-500 m meters below ground surface and TEM even further, around 800 meters. Both tools are non-destructive and give a cost- and time-efficient tool for groundwater investigation.
Electrical resistivity is, in most cases, measured with multiple electrodes and long multi-core cables, ultimately providing a 2D model of the resistivity distribution. During a field day, using four 21-takeout cables, with a five-meter spacing between electrodes, you can collect continuous data for approximately 800-1200 meters in optimal field conditions and reach depths of 70-80 meters. The work can be tough, but doable in even rough dense forest terrain. With larger electrode spacing, both the coverage and depth will be increased.
The groundwater will normally show as a clear decrease in resistivity; in this example, the upper surface is seen as the light green zone, between the yellow and blue, undulating at a level of 10 to 20 m.a.s.l. The total groundwater reservoir is limited by the bedrock below, seen at the level of -10 to -30 m.a.s.l. where the resistivity begins to increase again. This result can be used to estimate the available volume of water or to pin–point the best location for a groundwater well and how deep it should reach.
TEM measurements are done point-by-point (rather than building up a 2D profile from one location) but a TEM measurement is quick to complete, so often several points along a line are subsequently processed to form 2D profiles or even 3D volumes of resistivity data. During a day in the field, 15-20 points could be investigated, giving important information hundreds of meters down. Open ground is best for the loops creating the source signal, but the lightweight cable can be fitted around and within trees without too much trouble. With TEM, again, the low resistivity areas will point at the presence of groundwater.
For the future
We have a problem: our freshwater resources in the form of groundwater; a finite resource, under the threat of overuse and contamination. But fortunately, today, we also have the means we need for investigation, data gathering and large-scale mapping of these resources. We can do this in both a cost- and time efficient way, in almost any type of terrain, non-destructively. And now is the time to identify and secure these precious and invisible resources while there is sufficient time to plan and implement extraction and protection strategies.
Jaana Gustafsson, Applications Specialist, Phd