Estonia's largest, unique lake system dries up under shale mining pressures

Kurtna lake system.
Kurtna lake system. Source: RMK

Mining at Eesti Energia's Narva quarry will soon move south and the northwest area closes permanently. However, both past and future mining will affect Estonia's largest and most environmentally valuable Kurtna lake system. The lake closest to the quarry dried up fully – "sacrificed" to Narva quarries, Tallinn University geologist, Marko Vainu, said

Marko Vainu, a geo-ecologist at the Tallinn University School of Natural Sciences, told the program "Osoon" that the magnificent Kurtna lake system has been irreparably harmed by human activity.

Kurtna lake system. Source: ERR

A few hundred meters to the west of the oil shale Narva Quarry is a vast, awe-inspiring panorama of the eastern border of Estonia's greatest lake system, Kurtna.

It is said to be "the most densely populated lake basin" in Estonia as it contains nearly 40 lakes within 30 square kilometers.

Many of these lakes have exceptional conservation value and the entire area is under protection.

The Kurtna Landscape Reserve, situated between several oil shale quarries, was established in 1987, and since 2018, it has been part of Alutaguse National Park.

"Their uniqueness is precisely what makes them so special. We stand now along the shore of Lake Valgejärv, one of the few lakes in Estonia that still harbors protected species of nature conservation category 2, including Lobelia dortmanna, Dortmann's cardinalflower or water lobelia, and Isoetes lacustris, a lake quillwort or Merlin's grass. Historically, there were even more lakes with transparent, shallow water in the central part of this lake system," Vainu said.

Marko Vainu. Source: ERR

"It is also one of the few iron-rich lakes in Estonia, the water is exceptionally ironic. There are here in the system both open (groundwater flow) and closed lakes. Their scientific interest stems precisely from the fact that their biota and hydrological regime are so specific and distinct," he continued.

To the east is the Narva Quarry, which has lowered the water level in the eastern part of the lake, to the west is the Estonia mine, which has also significantly lowered the water level. But the proximity of the lake system to these mining activities is not the only reason why the lakes are not doing well, the scientist said. "It is an unfortunate situation that our most important body of water is surrounded on both sides, but also in the middle of it there are various manifestations of human activity."

"In the central part of the lake system there are boreholes for drinking water, which have also lowered the water levels and to the east is the Oru peat farm, where peat was extracted in the past," Vainu said.

"Moreover, the Pannjärve sand quarry is also located in the middle of the lake system. And in addition, the water pumped from the Estonia mine is diverted through the southern lakes, which has changed the water regime and chemistry of these lakes – so these lakes are indeed subject to a wide range of human pressures and impacts."

Human pressures on Kurtna lake system. Yellow areas: various mining activities. Source: ERR

The lake system has been already studied extensively by scientists in the past. A thorough study was conducted in the late 1980s, and the Tallinn Ecology Center began an in-depth investigation in 2012.

"As an initial step water level gauges were installed on some lakes. One of the Valgejärv Lake measures was adjusted to its position in May 2012 where its apex was parallel to the lake. In spring 2014, the situation remained essentially unchanged," Vaino said.

"By autumn 2015, water levels had fallen to 80 centimeters. In 2018, it rose to almost its original level, but in recent years it has fluctuated around 30-60 centimeters below that level," he said.

Measuring point by Lake Valgjärvi. Source: ERR

There are theoretically many reasons for the decline in water levels. It was a very dry year, and the year before that was also dry, which added greatly to the decline in 2015. "However, other than the impact of the quarry, which has moved closer and closer to the lake [Valgejärv}, it is difficult to explain why the level has continued to fluctuate around a lower level than before," he said.

Several hundred meters east of the lake, a groundwater well has measured water levels since the 1970s. It now also shows that the aquifer's water level has dropped a few meters, although the swamp on the other side may be holding back some of the groundwater before it reaches the lake.

"That's actually what these measurements of the lake show – the water here has become yellower and the transparency has decreased. In the 1950s, for example, the transparency was about four meters, but a few years ago it was only one and a half meters, so the nutrient regime here has definitively changed," Vaino said.

Vaino said the lake Kihljärv, an extinct lake, was the closest to the Narva Quarry – 500 meters from its border. In the late 1980s, this lake's water transparency was measured at two meters, even despite its high vegetation.

Lake Kihljärv in Kurtna lake system. Source: ERR

Until 2013, orthophotographs showed the lake as visually appealing. 2014 aerial photographs reveal only dirt and a few raised areas. Since then, no permanent lake has been found here, even though water occasionally pools in a few patchy areas during the spring. The lake has basically drained away.

The soil, however, is still saturated. A closer examination reveals that an aquatic plant, the mare's tail (Hippuris vulgaris), grows here, even though it usually grows in lakes only. It has still managed to flourish here for a time being.

Hippuris vulgaris, known as mare's-tail or common mare's-tail, is a common aquatic plant. Source: ERR

In the past, miners attempted to lessen the impact of their operations. In the late 1980s, there was much debate surrounding this region and dire predictions that every lake was going to dry up. Following that, specific new tech approaches have been implemented to maintain the water level raised, Vaino explained the history of the problem.

"By stacking clay-like materials around the perimeter of the quarry, a higher groundwater level has been preserved on this side. Also, in order to maintain water level regulation, water is consistently drained from the quarry through an infiltration basin located at its perimeter," he said.

"Clearly, these measures have been useful, because at the end of the 1980s, when the quarry was two kilometers further away than it is now, it was predicted that Valgejärv, for example, would have sunk by now. However, as can be seen in the case of Kihljärv, these measures have not been absolute."

Narva quarry edge. Source: ERR

Mining in the area is currently being phased out. Even once the ongoing human activity here ends, latest groundwater models show that the water level in the quarry will not return to its prior levels.

"This is due to the fact that mining continues to the south; otherwise, the miners will be swamped. It is 30 meters above sea level, but we are 40 meters above sea level. So the water level in the quarry will be 10 meters lower than it could be in this lake," Vaino explained.

Even a tenth of a meter of water would change the topography of Kihljärv again, converting it to something else entirely, it could resemble something like a swamp that has been inundated or an unusual formation of some kind, but not a lake.

Lake part of the lake system of Kurtna. Source: ERR

Mining activities in the Narva Quarry will not cease immediately, it will continue for the time being, and what will happen next to the Kurtna lake system is dependent on specific measures that should be implemented now.

The excavation will eventually move south, and the northern portion will be flooded, and as the excavation progresses southward, it will approach Valgejärv and other bodies of water further south.

"If no action is taken, such as building an infiltration barrier, our groundwater model developed a few years ago predicts that the water level in Valgejärv could drop by around a meter, with the southern lakes experiencing an even bigger drop in water levels," Vaino said.

"In theory, the lake system should be well protected if precautions are adopted because the distance between them and the mine is larger than it was between extinct lake Kihljärv and the mine," he said.

Lake part of the lake system of Kurtna. Source: ERR

"In sum, Kurtna lake system is such a contradictory place – it has always been understood that this is a place of exceptional natural value, but at the same time all this human impact seemed unavoidable," the scientist said.

"This shale has to be extracted somewhere, because we need it for our economy. It is the same with other pressures: groundwater is pumped here, because drinking water is needed for Jõhvi and Kohtla-Järve. It has been and will continue to be such a difficult region in this sense. But somehow we have to balance the fact that we still want to preserve the natural values as well.

"Perhaps in the case of Kihljärv you could say that it was a kind of inevitable loss or sacrifice, so that we would have electricity. It is a difficult problem.

"But we are still trying to protect this ecosystem, because nowhere else in Estonia is there such a magnificent network of lakes and ponds of such distinct characters," he said.


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Editor: Marit Valk, Kristina Kersa

Source: "Osoon"

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