Treshold-controlled three-stage hydraulic behaviour of a mantled shallow carbonate aquifer (Tuhala karst area, North Estonia)

Abstract

UDC 556.34:551.44(474.2) Oliver Koit, Nataša Ravbar, Andres Marandi & Jaanus Terasmaa: Treshold-controlled three-stage hydraulic behaviour of a mantled shallow carbonate aquifer (Tuhala karst area, North Estonia) As karst aquifers are often characterised by non-linear behaviour, ascertaining the turning points in their hydraulic regime may provide essential information on the functioning of the aquifer. These characteristics also apply to the Silurian-Ordovician aquifer system, composed of diverse carbonate rocks, in Estonia. The aquifer system is an important source of drinking water in northern Estonia. It also comprises the NabalaRakvere aquifer, a locally important groundwater resource that underlies the Tuhala karst area famous for an intermittently overflowing karst spring known as the Witch's Well. The water rich in humic substances of the Tuhala River recharges the Tuhala karst system, which is drained by two spring groups. In order to develop measures for the sustainable management of the aquifer in the future, the aim of this study was to enhance understanding of the hydraulic behaviour and hydrodynamic properties of the Tuhala karst system. From October 2014 to December 2016 an extensive field campaign was carried out comprising the observation of 22 surface-and groundwater monitoring points for water level and physico-chemical parameters and the performance of four quantitative tracer tests. The data obtained were evaluated in accordance with the inputoutput water level relation curves and a conceptual model of the system was set up. The results show that a strong hydraulic link exists, primarily through well-developed conduits between the recharge and discharge area of the karst system. Depending on the hydrological conditions, groundwater flow direc- IzvlečekUDK 556.34:551.44(474.2) Oliver Koit, Nataša Ravbar, Andres Marandi & Jaanus Terasmaa: Določanje mejnih vrednosti tristopenjskega hidravljičnega obnašanja pokritega plitvega karbonatnega vodonosnika (Tuhalski kras, severna Estonija) Kraški vodonosniki se pogosto obnašajo nelinearno, zato lahko z ugotavljanjem mejnih vrednosti v hidravličnem režimu zagotovimo bistvene informacije o delovanju vodonosnika. To velja tudi za silurijsko-ordovicijski vodonosni sistem v Estoniji, ki ga sestavljajo različne karbonatne kamnine in je pomemben vir pitne vode na severu države. Vključuje tudi vodonosnik Nabala-Rakvere, ki je lokalno pomemben vodni vir. Na tem območju se nahaja Tuhalski kras, ki je znan po občasno bruhajočem kraškem izviru Witch's Well (čarovničin vodnjak). Tuhalski vodonosnik napaja reka Tuhala, bogata z huminskimi snovmi, prazni pa se skozi dve skupini izvirov. Da bi v prihodnje razvili ukrepe za trajnostno upravljanje vodonosnika, je bil namen te raziskave izboljšati razumevanje hidravličnega obnašanja in hidrodinamičnih lastnosti kraškega sistema. Od oktobra 2014 do decembra 2016 smo opravili obsežno terensko delo, ki je obsegalo spremljanje vodostajev in fizikalno-kemijskih parametrov v 23 površinskih in podzemnih točkah ter izve...

Understanding the temporal variation of flow direction in a complex karst system (Planinska Jama, Slovenia)

“…Flow diversions along overflow passages can influence both quality and quantity of the water resources available, making the management of the aquifer reserves more challenging. While overflow processes are commonly observed in karst aquifers (Herman et al 2008;Ravbar et al 2012;Birk et al 2014;Mayaud et al 2014;Mayaud et al 2016;Gabrovšek et al 2018;Koit et al 2017), they have been up to now rarely investigated as their own (Mayaud et al 2014;Koit et al 2017). Therefore, it is crucial to deepen our understanding of their functioning, in order to better assess the hydraulic response of the aquifer system.…”

“…Common methods used in characterisation of karst aquifers include continuous monitoring of physico-chemical parameters of water and dye-tracing techniques, which are able to assess the flow directions and velocities during the conditions of injection. These methods were used to investigate karst aquifers where an overflow was also present (Herman et al 2008;Ravbar et al 2012;Koit et al 2017). While these techniques are very efficient to prove connections between the aquifer sinks and sources, they are solely applicable at accessible points such as springs, ponors or water-active caves.…”

A multiparameter analysis of environmental gradients related to hydrological conditions in a binary karst system (underground course of the Pivka River, Slovenia)

“…At the contact with karst or a short distance beyond it, they sink into the ground, travel through conduits in the aquifer and eventually discharge downstream through caves and springs. As a result, groundwater and surface water constitute a single hydrodynamic system (Katz 2002;Koit et al 2017). The defining characteristics of karst aquifers are rapid throughput times, localization of flow along essentially one-dimensional flow paths within the conduit system, and the presence of deposits of clastic sediments in many of the conduits.…”