When water is scarce, it must be conserved, for example, in aquifers, which are subterranean soil and rock layers that can absorb, channel and provide water again. The Israeli scientist Alex Furman, Associate Professor for Soil Physics and Hydrogeophysics at the Technion in Haifa and the German geophysicists Klaus Haaken, a doctoral candidate in the group of Andreas Kemna, head of the department of applied geophysics at the University of Bonn, examine the way in which water seeps down to these subterranean layers. They presented their first results at the Status Conference 2015 “German-Israeli Co-operation in Water Technology Research” held in Berlin on 23 March 2015. A conversation about their joint project follows.
Both of you are intrigued by water. What fascinates you about this?
Alex Furman: Owing to its scarcity, water is an extremely important topic in Israel. One has to constantly create new strategies to conserve, process and store it. Biologically treated wastewater, for instance, has already been used in agriculture for many years. There are natural subterranean aquifers on the Mediterranean coast, i.e. soil layers which are useful for interim water storage. We do research on how to optimise them.
Klaus Haaken: For my doctoral thesis I focus on these kinds of storage processes. It is fascinating to study the soil’s water balance with geophysical methods. For example, I take ERT (Electrical Resistive Tomography) measurements, with which I determine the soil’s electrical resistance to indicate its water content.
How did your cooperation start?
Furman: The German-Israeli Foundation for Scientific Research and Development allocated me a small budget, because I am a young scientist. This, among others, enabled me to travel to Germany and meet people working in the same field. When I visited the Jülich research centre, I met Andreas Kemna, currently Klaus Haaken’s thesis supervisor at the University of Bonn. We have always stayed in casual contact since then, but this is our first joint project.
Haaken: Some years ago, I worked on another project in Israel where Alex Furman lent me measuring equipment. The idea for this project was born at that time.
On what exactly are you focusing at present?
Furman: The Mekorot Water Company operates the Shafdan wastewater treatment plant south of Tel Aviv. It is one of the largest Soil Aquifer Treatment (SAT) plants in the world. Pre-treated wastewater is channelled onto large fields where it seeps through a soil layer, approximately thirty meters thick, into the aquifer. This filters and further purifies the water. Owing to the continuous increase in wastewater volumes, Mekorot needs larger infiltration areas, which the government is not prepared to provide. This is where we earn our keep as scientists: Our goal is to enable the water to seep through soil layers quicker.
How do you want to reach this goal?
Furman: Our idea is to control the water supply better. We therefore really need to understand the system. We are interested in the air in the soil, because this may hinder water movement.
How can the water supply be better controlled?
Furman: Currently, when possible, the area is flooded. Subsequently, there is a two-day waiting period, as these dry phases are considered beneficial to the soil. This has not been systematically researched to date. We want to take a closer look at the soil to find the optimal rhythm between the flooding and dry phases in order to increase the infiltration rate and improve the soil’s biochemistry, which is important for the purification processes.
Haaken: This is where our ERT measurements become relevant. ERT is a long-established method to obtain data on the deeper soil layers. In contrast to a drill sample, which only represents a limited part of the soil layer, a surface network of sensors allows three-dimensional moisture monitoring of entire subterranean areas. Our new approach is now to examine the dynamic behaviour; in other words, showing the movement of water masses through the soil layers. This provides us with information on the inhomogeneous nature of the soil due to its sandy and clayey zones. We have to be careful with the interpretation of the measurement data, as we use mathematical models with inherent uncertainties which become increasingly unreliable the further the place is from the sensor.
Mr Haaken, were you actually on site in Israel for your research?
Haaken: Over the last three years, I visited Israel five times, each time for several weeks. I installed the measuring systems with my colleagues. We had to move the sensors beneath the surface, as the area is ploughed from time to time. Initially, I took the measurements myself, but later Israeli students took over this task.
What have you learned from the measurements to date?
Haaken: We can follow water movement through the soil with our methods. We can also confirm that the currently practised rhythm of wet and dry phases is not optimal. This was indeed confirmed by additional simulations. Other rhythms will probably lead to higher infiltration rates. We want to test this in an on site experiment.
You mentioned that air plays an important role in the seeping process. In what way?
Furman: Soil also contains air, which makes space for water by escaping upwards. If a small volume of water penetrates the soil, for example, during a rain shower, there is an equilibrium between water and air. If we flood the area, we obstruct the air’s upward escape route. As the water is heavier, it presses the air downwards. This eventually creates a barrier that blocks the water passage. Only if the pressure is high enough, the air will escape intermittently. Currently, we examine at what depth this air barrier forms, how the pressure builds up, and the way in which this depends on the irrigation practices.
Do you have ideas on how to break the air barrier?
Furman: We have established that undulated soil favours air ventilation. Until now, this has been applied inconsistently, because fields had to be ploughed every now and then for operational reasons. We hope that our experiments will give us clues on how to deliberately create soil ventilation and, thus, accelerate water absorption. In addition, it could be useful to drill air ventilation passages.
What will be your next steps?
Furman: We must complete our measurements and their analysis. Then we must examine how the soil’s biochemistry changes with ventilation. A changed biochemical environment may trigger biofilm growth on the surface, which could possibly obstruct the water passages. It could also favour processes resulting in the release of certain minerals, which may have positive or negative consequences for the water quality. We also like to use our geophysical methods to determine this and continue our co-operation. German-Israeli co-operation always results in very innovative projects with perfect project management.