From black past to green future

Project

Why monitoring groundwater is vital for the municipality of Heerlen.

Client

Municipality of Heerlen

Country

The Netherlands

Background to installing the monitoring network

‘It’s become steadily busier here underground. It’s essential to plan well. And that’s what occupies us daily in Heerlen municipality’s Spatial and Economic Development department. We’re here to set up the available space in the best possible way for all the residents and businesses.’

Jean Weijers: ‘I work in the department as a Subsurface & Mining Specialist, and I focus mainly on the environment and sustainability. Groundwater plays a significant role in this. We have the cleanest groundwater in the Netherlands, and it has to stay that way. And certainly here in Heerlen, where we keep having to deal with the consequences of previous mining.’

The delayed effects of mining

‘Heerlen’s last mine, the Oranje Nassau 1, closed its gates in 1974. Mine water was being pumped out until 1994, to prevent neighbouring mines in Germany from being flooded. That’s because the mines were interconnected, to serve as escape routes for each other. A million cubic metres of mine water was pumped out annually. There was monitoring at the time, and you could see that the water table stayed at the same level until 1994, but after 1994 when the German mines also closed and pumping was halted, the water began to rise.’

‘Our ground is becoming increasingly important and monitoring is needed’
Jean Weijers – Subsurface & Mining Specialist

‘So we’re talking about mine water here. This is indeed a type of groundwater, but we call it mine water because it has a different composition. It comes from carboniferous rock and is a lot saltier than groundwater. It also contains minute quantities of heavy metals. That’s why you can’t discharge it into the groundwater. In that context you could say that it’s contaminated water, but it is a natural composition.’

‘Here in Heerlen the mine water has risen from the deepest point, at 700 metres, to where it is now. Initially it went very fast, but the higher it rose, the slower it went. Right now the mine water is rising at nearly a centimetre a day. The rising mine water also exercises an effect on the groundwater. You could say that the salty mine water is forcing the sweet groundwater upwards in a way. The mine water is held in the carboniferous rock. Above that you have the overburden, around 100 metres thick. This overburden contains all sorts of materials like clay, sand, gravel and limestone. This is also where the groundwater resides. That rising mine water particularly affects the groundwater level, not so much the groundwater quality.’

‘Climate change is associated with a different precipitation pattern.
We want an insight into the consequences of this for the groundwater’

Monitoring essential

‘In closed mining operations, as we have here in Heerlen, roughly speaking two types of material were extracted. One was coal, which was transported out immediately and was then used. The other is the material produced in creating the shafts and galleries. This is carboniferous rock, shale and sandstone, also containing high concentrations of metals like nickel. All this material ended up in the mining slagheaps or dumps. When you used to arrive in Heerlen, you would see these slagheaps everywhere. Now they’re almost unrecognisable, because after 1974 they were completely dug up or levelled.’

‘The Oranje Nassau 1 slagheap was largely levelled, and a residential suburb and a park were then built on it. But the carboniferous material still lies below it. There was a lot of concern about this issue in the 1970s and 1980s. People wanted to know what happened to it, and what its influence was on public health. So then you need to know what’s where, and where it goes. And where it goes is again related to the groundwater. At the time the importance of monitoring was also realised for this and other types of contamination, and a groundwater testing network was set up for both the groundwater levels and the quality.’

‘We have to monitor changes to rising mine water and groundwater. That’s because these also influence the discharge of water into the rivers. The municipality also wanted to be able to track any contamination that might be present. Another reason for monitoring is climate change. Climate change is associated with a different precipitation pattern. In the future we expect showers with a higher intensity. We want an insight into the consequences of this for the groundwater. Drainage systems also play a role. To maintain drainage systems, you need to know the dimensions and where the groundwater lies, to determine in advance whether you need to drain or not. A fourth reason, and one that is important to Heerlen, concerns energy. That’s because mine water has been used since 2009 to heat and cool buildings, homes and offices. As a municipality, we also have a duty when it comes to groundwater.’

How did Eijkelkamp Soil & Water make the difference?

‘A couple of years ago there was critical consideration of the groundwater testing network. It turned out that a large number of the loggers were no longer working. We decided to refurbish the testing network. A tendering process followed, in which Eijkelkamp Soil & Water turned out to be the best in terms of both product and price. The original plan was only to replace the defective loggers. But the water-level logger offered, Diver, turned out to be cheaper than thought, so that we decided instead to install it in all the locations.’

‘We also signed a five-year maintenance contract with Eijkelkamp Soil & Water. That’s because we wanted to outsource the maintenance. It was in the tender, but it was a problem for some suppliers. They don’t in fact perform it themselves. At Eijkelkamp Soil & Water we had everything in one: the right material, the installation, the maintenance and management, and also access to the data. We were also charmed by the way you validated the data. We’re not yet accessing the data through telemetry. We’re aware of its benefits, that you don’t have to do the rounds as frequently and if something’s wrong, you spot it immediately. But there is of course a cost consideration involved. Our choices have been made for the coming years, but we do certainly believe that telemetry will ultimately happen.’

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