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 Nabala- Rakvere 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 input� output 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 tions vary and maximum linear flow velocities range between \~225 and 800 m/h. A threshold-controlled throughput capacity and interaction with the adjacent aquifer cause imbalances between the input and output discharges of the karst system. Tracer tests coupled with surface and groundwater level relation curve analysis allowed the specification of flow threshold conditions for the Witch's Well spring group and the overflow threshold for the Witch's Well, among other key hydrological events. The hydraulic regime of the karst system was divided into three stages, each with differing threshold controls and hydrodynamic characteristics. Key words: karst system, shallow carbonate aquifer, monitoring, tracer test, water level relation curve, threshold, overflow.  Dolo\vanje mejnih vrednosti tristopenjskega hidravlji\vnega obna\vanja pokritega plitvega karbonatnega vodonosnika (Tuhalski kras, severna Estonija) Kra\vki vodonosniki se pogosto obna\vajo nelinearno, zato lahko z ugotavljanjem mejnih vrednosti v hidravli\vnem re\vimu zagotovimo bistvene informacije o delovanju vodonosnika. To velja tudi za silurijsko-ordovicijski vodonosni sistem v Estoniji, ki ga sestavljajo razli\vne karbonatne kamnine in je pomemben vir pitne vode na severu dr\vave. Vklju\vuje tudi vodonosnik Nabala- Rakvere, ki je lokalno pomemben vodni vir. Na tem obmo\vju se nahaja Tuhalski kras, ki je znan po ob\vasno bruhajo\vem kra\vkem izviru Witch's Well (\varovni\vin 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 izbolj\vati razumevanje hidravli\vnega obna\vanja in hidrodinami\vnih lastnosti kra\vkega sistema. Od oktobra 2014 do decembra 2016 smo opravili obse\vno terensko delo, ki je obsegalo spremljanje vodostajev in fizikalno-kemijskih parametrov v 23 povr\vinskih in podzemnih to\vkah ter izvedbo \vtirih kvantitativnih sledilnih poskusov. Pridobljene podatke smo ovrednotili na podlagi krivulje odvisnosti vhodnih in izhodnih vodnih nivojev. Vzpostavili smo konceptualni model sistema. Rezultati ka\vejo na obstoj mo\vne hidravli\vne povezave predvsem z dobro razvitimi kanali med obmo\vji napajanja in praznjenja kra\vkega sistema. Odvisno od hidrolo\vkih razmer se spreminjajo smeri in hitrosti toka podzemne vode, ki zna\vajo od \~225 do 800 m/h. Kapaciteta prepustnosti, ki jo uravnavajo dolo\vene mejne vrednosti, in posledi\vne in terakcije s sosednjimi vodonosniki povzro\vajo neravnovesja med vhodnimi in izhodnimi preto\vnimi vrednostmi kra\vkega sistema. Zdru\vevanje rezultatov sledilnih poskusov z analizo krivulje odvisnosti povr\vinskih in podzemnih vodostajev nam je omogo\vilo, da smo dolo\vili mejne preto\vne vrednosti izvira Witch's Well in druge klju\vne hidrolo\vke dogodke. Hidravli\vni re\vim kra\vkega sistema smo razdelili na tri faze, od katerih ima vsaka dolo\vene mejne vrednosti in hidrodinami\vne lastnosti. Klju\vne besede: kra\vki sistem, plitvi karbonatni vodonosnik, monitoring, sledilni poskus, krivulja odvisnosti vodnih nivojev, prag, preliv