In Estonia, about one third of the water intended for human consumption comes from karst aquifers. These aquifers are prone to a number of water management problems, such as poor self-cleaning capacity, vulnerability to pollution, as well as drought sensitivity. Due to the dissolution of carbonate rocks, karst porosity forms in the aquifer rocks to make the drainage of surface- and groundwater more efficient. Oliver Koit, a doctoral student at Tallinn University, studied how the karst systems in the karst aquifers of Lower Estonia function.
Although the availability and quality of groundwater in Estonia is generally considered to be good, a recent survey showed that 8 out of Estonia’s 31 groundwater bodies are in poor condition and 11 are potentially endangered . The groundwater bodies consisting of karst aquifers are usually in the worst state. This means that a significant part of the Estonian population living in rural areas, where drinking water is abstracted from private wells, is vulnerable to groundwater quality problems. In the recent past, there have been many heated debates on the issue of sustainable management of karst aquifers in the Estonian media and scientific communities. Aside from the nitrate vulnerable zone of the Pandivere Upland, the Tuhala and Salajõgi karst areas have received the most attention.
In his doctoral thesis, Oliver Koit studied three lowland karst areas in more detail: Salajõgi, Tuhala and Kadaka. All of these are, in essence, karst systems to drain humic-rich rivers flowing from mire watersheds. For karst to develop there needs to be a sufficient hydraulic gradient. In the relatively flat conditions of Lower Estonia, crucial gradient has developed due to the combined effect of post-glacial uplift, mire development and the lowering of the erosional base level.
The rivers sinking into the karst systems through ponors come to direct hydraulic connection with the surrounding (hosting) aquifer. The water then flows down the preferred flow path towards the karst spring. “The water in the Tuhala and Kadaka karst areas flows through well-developed conduits,” Koit describes. “In Salajõgi, it rather flows through a network of smaller fractures. The water in the karst system flows at a significantly higher speed (at least up to 800 m/h) compared to the hosting aquifer. However, this causes suspended solids or particles to be transported through the karst system. These can degrade the quality of the groundwater.” He adds that such a problem occurs, for example, in the shallow wells of Salajõe village.
An important new finding was that under normal conditions, groundwater drains from the hosting aquifer into the conduits of the karst system. However, during flood events the water exchange between the karst area and the surrounding aquifer may be inverted due to over pressurization, forcing water from the karst system into the surrounding aquifer. “The brown colour of dissolved organic matter in the groundwater surrounding the karst system helps us to detect the reversal of the water exchange," Koit explains.
For example, most of the water flowing out of the springs of the Tuhala karst area comes from the river sinking in the upstream ponor. The water supply from the surrounding aquifer and from direct infiltration made up less than a third of the water. In the Kadaka karst area, however, the outflow of the karst spring was formed entirely by the baseflow from the hosting aquifer in the second half of the summer.
The drought of summer 2018 devastated the whole of Europe. During this drought, the outflow from the mire basin in Tuhala seized and the groundwater levels dropped to a critically low level. This caused the Tuhala River, the karst system and the section of the river flowing from its karst spring to dry up. After the drought, the water flowing in the river upstream of the karst system formed entirely basis of the groundwater originating from the hosting aquifer for some time. The outflow of mire water did not recover until much later. This highlighted the significance of groundwater baseflow in supporting small rivers and ecosystems in the lowland watersheds. In addition to its devastating effects, the drought clearly showed the connection of the studied karst systems with the hosting aquifers.
According to the World Meteorological Organization, climate change will lead to an increase in extreme weather events. This will also affect Estonia's surface and groundwater resources.
The supervisors of the doctoral thesis are Professor Jaanus Terasmaa of the University of Tallinn and Andres Marandi from the Geological Survey of Estonia. The opponents are Professor Laurence Gill of Trinity College Dublin and Argo Jõeleht, Associate Professor at the University of Tartu.
The doctoral thesis is available in the ETERA digital environment of the Tallinn University Academic Library.
On 13 April 2022, Oliver Koit from Tallinn University’s School of Natural Sciences and Health defended his doctoral thesis “Surface Water and Groundwater Interaction in Shallow Karst Aquifers of Lower Estonia’’.
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