How does mining affect water quality

The Cottbus Baltic Sea is filling according to plan

Summer 2020: Too little rainfall and weeks of low water in the Spree draw the attention of many to the Cottbus Baltic Sea. For the second summer in a row there is no flood water for him from the Spree. But the lake fills up according to plan, because the plan provides many scenarios. Drying times are also taken into account. At LEAG, the head of geotechnical engineering, Franziska Uhlig-May, is responsible for flooding the Cottbus Baltic Sea. We talk to her about the question of whether the Cottbus Baltic Sea is running out of water.

Franziska Uhlig-May will answer questions about the planning around the Cottbus Baltic Sea, photo: LEAG

Ms. Uhlig-May, the Baltic Sea is already a popular destination for locals and tourists in Lusatia. Watching the lake fill up - the possibilities have so far been limited to that. However, many now also have concerns about when and whether the lake can be used for tourism at all. Is the flooding still on schedule?

The Flooding of the Cottbus Baltic Sea is still on schedule. Of course, the current low water level in the Spree has an impact on flood management for the Baltic Sea. The transnational management concept for the Spree lay one Ranking of water use firmly. The Drinking water supply, the Spreewald, inland fishing, the required fill levels of the dams and many other things basically before the flooding of open pit lakes. That is why we have not been able to flood since the end of March. Only when it rains a lot in the region around Bautzen, i.e. in the catchment area of ​​the Spree, and the water needs of the higher-ranking users mentioned above can be met, the flooding will be possible again. But we expected such dry seasons to occur. The Flooding plan lie different weather situations with different amounts of precipitation for a forecast period from 2003 to 2102 underlying. These are based on long-term water management modelswho the complex relationships and Interactions within the catchment area of ​​the Spree. The water authorities also use these tools to carry out their management planning.

The Cottbus Baltic Sea is set to become the largest man-made lake in Germany, photo: LEAG

Has climate change with higher average temperatures and longer drying times been taken into account in the calculations?

In our Scenarios and management concepts do we have Temperature increases as a result of climate change observed. This then resulted in optimistic and pessimistic scenarios - that is Flooding periods between four years in the optimistic case and six years in the pessimistic case. We are currently in the medium to dry scenario. So that means the lake would have flooded until the mid-2020s.

The filling level of the Cottbuser Ostsee is communicated on the website every week, graphic: LEAG

The rate of evaporation increases with higher temperatures. Could it happen that the water level drops sharply in the future if no water can be discharged?

This year is despite the flooding stop so far no drop in water level occurred. There are several reasons for this: On the one hand, we don't just fill the lake with Spree waterbut only to about 80 percent. That means the the remaining 20 percent are groundwater. On the one hand, this is groundwater of good quality, which we will pump in the future bank areas of the lake via filter wells in order to maintain the stability of the embankments of the former opencast mine. This is the only way to prevent the groundwater from rising too quickly at the lake edges, which would lead to instability of the embankments. We use this water to flood the lake. So it will be initiated again. In addition, when the pumps are switched off, the groundwater rises again automatically after the end of coal extraction and also fills the lake.

Still would be a Fall in the water level during a flooding interruption not excluded. Part of the Seewater seeps away during the flooding in the inner dump of the former opencast mine, the lake floor, and fills the dry pore spaces back on. This is Part of the floodg just like Landslide events on the dump side of the lake their water displacement as a result Water rise in the lake contributes.

The water level of the future lake should be + 62.5 meters above sea level plus / minus 0.5 meters, photo: LEAG

But what about after the flooding stops, when the lake is full and the summers are still hot and dry? Could the water level then drop sharply?

The Sea water level becomes a matter of course as with all natural waters fluctuate due to the weather. Here it is important to know that the Cottbus Baltic Sea is geographically in the Baruther glacial valley below the Lusatian border wall. In this way, the groundwater flows from the Grenzwall on its way towards the Spreewald to the Cottbus Baltic Sea, depending on the gradient. If the lake is flooded, the inflow from its own catchment area is enough to cover one Water level around + 62.5 meters above sea level plus / minus 0.5 meters without additional water discharges to keep. According to our calculations, this inflow of groundwater as well as precipitation will help to balance out evaporation on an annual average.

In the planning were also Scenarios which take into account, among other things, increased evaporation losses and additional periods of lack of precipitation. A The water level has dropped by several meters also then not to be expected.

What can be said about water quality? Does it already suffer from the lack of a flow of the Spree?

We also check the water quality regular measurementsn that show us that the lake areas flooded with Spree water currently with pH values ​​between 7 and 7.3 and very low iron contents of less than 2 milligrams per liter lie fully in the condition prognosis. Groundwater of good quality flows into the lake from the south and southwest. In addition, in comparison to other opencast mining lakes, more calcareous overburden substrates in the Cottbus-Nord opencast mine favor the water quality in the future lake. Fish, water birds and other animals will also feel comfortable in and around the water.

Is there a risk of increased sulphate pollution for the Spree as a result of the Baltic Sea project?

Detailed reports show that both the flooding of the Cottbus Baltic Sea and the later discharge of lake water did not lead to an increase in the sulphate content of the Spree to lead. Those initiated into the Spree Sulfate loads become even declining be. With the gradual Cessation of groundwater lifting will the The proportion of the sulphate load from the former Cottbus-Nord opencast mine will decline and only with that Start of water drainage from the Cottbuser See increase again slightly. The planning approval decision for the flooding of the Cottbuser Ostsee specifies how much sulphate may be discharged from the lake.

The Cottbus Baltic Sea at a glance, graphic: LEAG

But does the current low water level in the Spree lead to higher sulphate levels? The majority of mining water currently flows in it.

The Management of the Spree area regarding the Guide value for sulfate is by Spremberg-Wilhelmsthal gauge due to the current low water situation of the Spree through the WG river basin management of the water authorities of the states of Brandenburg and Saxony suspended until the end of the year in order to be able to use the water primarily to secure the necessary volume outflows.

At the Spree gauge in Neubrück may after the enacted in April last year by the state of Brandenburg Management decree for sulphate does not exceed the guideline value of 280 milligrams per liter on 328 days a year become. That could be in the Summer months observed become, although the proportion of open pit water in the Spree especially higher in this dry period was than in the winter months. Thus, no direct conclusion can be drawn according to which the open pit water discharged into the Spree by active mining and the old mining of the LMBV is the cause of increased sulphate concentrations below the Spreewald.

In some mineral waters, the sulphate content is considerably higher than the limit of 250 milligrams per liter set for drinking water, Photo: LEAG

Is the sulphate in the Spree a threat to the drinking water supply in Berlin and Frankfurt an der Oder?

In order to maintain the function of the Spree as a drinking water resource, we operate as LEAG, together with the LMBV, developed a complex water management system based on water quantity and quality. We work closely with the relevant water suppliers and the water authorities.

But you have to know that Sulphate neither toxic is it is still one of the substances hazardous to water. That's why there is also for Surface and groundwater have no limit value according to the EU Water Framework Directive. For the Drinking water, on the other hand, has a limit of 250 milligrams per literwhile in Mineral and medicinal waters sulphate concentrations of 1,000 milligrams per liter and more are to be found. Ultimately, it can also be stated that, despite the sulphate input, water discharge from mining cannot be dispensed with in the near future. Only with the introduced mining water can at all a sufficient water supply and thus an ecological and benefit-oriented management of the Spree as far as Berlin can be guaranteed.

More about the Cottbus Baltic Sea project

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