07 November 2024
The build-up of mine water in disused coal mines poses major challenges for engineers. How do you protect people and the environment from the possible consequences of rising water levels? We find answers to the most important questions with Nele Pollmann, water expert at DMT, a subsidiary of the TÜV NORD GROUP.
Ms. Pollmann, the end of 2018 saw the closure of the last two German coal mines, Prosper-Haniel in Bottrop and the Ibbenbüren colliery. But they leave behind a legacy of “eternal” tasks, including dealing with mine water. What exactly is that?
Nele Pollmann: Mine water is any water that is or has been in contact with deep or opencast mining operations. Its origins are rainwater and surface water which seep into mine workings as groundwater, as well as deep water that flows laterally and thermal deep water. These mine workings consist of vertical shafts and drifts and haulageways, the cross-connections underground that used to lead to the workings. To keep the mines dry when they were in active use, the mine water was pumped upwards and channelled into streams and rivers. Without the powerful pumps used in these dewatering systems, the mines would have filled up with water. This would have been dangerous for the miners, as well as making it impossible to mine the hard coal. Since the pumps consume a lot of energy and cost a lot of money, the end of the coal mining industry in Germany will see a lot of pumps shut down in the interests of economy; the idea is to let the mine water rise to an optimal level. The company charged with these measures is RAG Aktiengesellschaft (RAG for short), the last operator of hard coal mines in the Ruhr region, in Ibbenbüren and the Saarland. Since the end of active coal mining, it has overseen everything involved in the eternal tasks, including systematic water management.
What is the aim here?
The aim is to raise the water levels at central locations and discharge the water into medium-sized to large rivers such as the Rhine in the Ruhr region or the Saar in Saarland. The idea is to take the strain away from smaller waterbodies, as has already been done in the project to restore the natural environment of the Emscher in the Ruhr region, for example. DMT was also involved in an expert capacity in this project of the century.
But you can’t turn off the pumps completely in the Ruhr region, can you?
This isn’t the plan. According to the regulations, mine water shouldn’t come into contact with groundwater, which is used for drinking water supply, which means that it must be closely monitored by experts.
How does DMT help exactly?
We scrutinise the measures taken by RAG, for example, as part of its mine water concept, and suggest improvements where we consider it necessary. It was for this purpose that experienced colleagues here at DMT developed the “box model” mine water modelling software many years ago. This model enables us to calculate mine water rises over a large area. The sheer size of a mining area means that the engineering challenges are enormous. Just imagine: The area consists of several disused pits that are connected to each other underground. In North Rhine-Westphalia, for example, this affects half the Ruhr area. With our “box model”, underground regions can be divided into manageable sections (“boxes”). This enables us to calculate mine water rises over a large area.
© Adobe StockThe mine water in disused mines rises - and is discharged into rivers such as the Rhine. This is intended to relieve smaller bodies of water.
What exactly can you read using this “box model”?
There are many ways in which water flows underground between the individual mine workings. We can model the mine water flow, including its temperature and chemical composition. The “box model” enables us to carry out complex analyses to anticipate the consequences of technical interventions in simulated scenarios and to work out suitable measures for a controlled and safe rise in mine water levels. My colleague Julia Sunten and I program the box model, and we’re constantly adapting it as a team at DMT whenever new data becomes available – including the pump concept, for example. The water can now be pumped cyclically or in dependence on the water levels in the rivers, among other things. We also have the effects of climate change on our radar, of course, and are adapting our model accordingly. As well as that, we work independently to determine where the mine water should be treated before it can be discharged into a river.
The mine water model (box model) for the Ruhr area: The colours show the depth of the mines, red is high, blue is very deep.
What quantities of mine water are we actually talking about?
According to RAG, it has to pump 70 million cubic metres of mine water up above ground every year in the Ruhr region alone.
Has your work as an expert also taken you underground?
Yes, several times. I first went underground at the Prosper-Haniel colliery in Bottrop in 2021. By then, the mine had already been stripped. There was nothing down there, not even electricity. At the time, we had to crawl on our hands and knees through the narrow, pitch-black dam pipes before walking through the abandoned mine workings. That made a big impression on us.
How does it feel to be 1,100 metres underground?
As long as you don’t stop to think about all those tons of rock above your head, it’s fine.
Nele Pollmann, water expert at DMT underground in miner's gear.
What exactly do you look at underground?
We look at which measuring points we should measure the mine water; we also consider which substances it will make sense to measure and in which cycles we should measure the water. We look at which substances are in the water and in what concentrations. We also look at the pathways taken by the mine water. There are also maps for this, of course, but it’s something else to see these things in reality. I remember a situation where the issue was the number and size of certain pumps. It was only when I was actually in the mine myself that I realized how little space there was and how complex and difficult it would be to get the pumps there. This isn’t nearly as clear when you’re just sitting at your PC. It’s also important for all this information to be incorporated into the DMT “box model” to allow us to clear up lots of questions, like how long it will take for individual pits to flood and what flow volume and concentrations of water pollutants can be expected. We know where actual mining took place and when, which is important, for example, because certain substances such as hydraulic oils containing PCBs were only used to operate machines at certain times. And we also think outside the box: How and where might mine water be used for geothermal heating? The last thing we do is to generate independent expert reports which RAG then submits to the mining authorities.
About Nele Pollmann:
Dr. Nele Pollmann heads the Hydrogeology and Water Management team at DMT. She works as a volunteer and in the TÜV NORD GROUP to raise the profile of women in STEM professions.
You grew up in the Ruhr region. Does that give you extra motivation?
It’s my home, and coal mining is part of my history as a child of the Ruhr, where I still live with my family. I have a strong personal interest in avoiding the negative impacts of rising mine water levels, which is what drives me to do something for my region. The consequences of mining will keep us at DMT busy for a long time to come, that’s for sure. They aren’t called eternal tasks for nothing.
