In contrast to rivers and streams, groundwater is not visible to the naked eye, so its study and characterization are more complex. There are exceptions, of course, such as when we see them ‘coming to the surface’ in a spring or well, or when we take them out of the aquifer with the help of a well.
So, if groundwater cannot be seen, can it be studied? The answer is emphatically yes. If not, we hydrogeologists would not exist, would we?
There are many methods for analyzing groundwater. The simplest is to take a water sample from a spring or a well, take it to the laboratory and analyze it to find out its composition. But there are many others: for example, to make a balance of the water that enters the aquifer by infiltration of rain and compare it with what comes out of the springs and wells. And things can get much more complicated if we use mathematical models, electrical currents (geophysical methods) or even isotopes!
How about tracer tests?
In this article we will talk about one of the most used methods to know where groundwater flows and at what speed it moves. We are talking about tracer tests. They consist in introducing a substance into an aquifer (for example, through an existing well) and seeing how it moves through it. Logically, it must be a substance that is non-toxic and that we can measure elsewhere in the aquifer. An example? Cooking salt is a great tracer: cheap, readily available, dissolves very well in water, and we can track its passage using equipment that measures the salinity of the water.
In studies with larger budgets and more ambitious objectives, artificial tracers, usually colorants, such as the one in the image below, are generally used.
Its application in LIFE Nirvana project
Tracer tests can be classified into two categories: qualitative and quantitative. The first ones are used to demonstrate the connection between two points (a well and a spring, for example). The quantitative ones also allow us to determine when the injected substance passes through, with what concentration and for how long. In other words, they give us much more information.
Within the framework of the LIFE Nirvana project, two tracer tests have been carried out before the injection of Fe (iron) nanoparticles, one at each aquifer level (upper and intermediate).
Image: Schematic of the project pilot. OBS: observation well. INY: injection well. PBO: pumping well. The blue arrows show the direction of subsurface flow.
The tests were carried out with common salt because, as said before, it is a harmless substance, easy to handle, very soluble in water and easily detectable with the salinity measurement equipment we already have installed in the pilot plant.
We injected 16 kg of salt in the intermediate level and 50 kg in the upper level, previously dissolved in 500 liters of water. The result was real brines (salt-laden water), one with the same salinity as seawater and the other with twice as much!
And the results…
As expected, the passage of salt through the different control points resulted in increases in water salinity (red and blue lines in the figure below).
The results of these salt tests have allowed us to know the speed of water displacement in the aquifer, the mixing produced inside the aquifer and the duration of the injection effect. All these parameters are very important and necessary before starting an aquifer treatment process such as the one being carried out in the LIFE Nirvana project.