Oliver Koit

The escalating impacts of global climate change significantly affect regional hydrological systems, particularly in northern areas such as Estonia. This study investigates the hydrological sensitivity of Estonian catchments to climatic variability, focusing on the interplay between surface water and groundwater. Using data from 42 river catchments, it employs various statistical methods in hydrology, emphasizing the autocorrelation function, cross-correlation function, baseflow index, and flow duration curve. The analysis spans the years 2012–2022, integrating hydrological, spatial, and water quality parameters. The research identifies four distinct hydrological behavior clusters: plateau, sandstone upland, carbonate upland, and lowland. Key findings include diverse catchment sensitivities to groundwater recharge, the role of baseflow in streamflow stabilization, the memory effect in catchment responses, and insights from the flow duration curve on flow variability and extremes. The LightGBM model, predicting focus parameters, highlights the critical influence of air temperature and snowpack on streamflow characteristics. This study underscores the diverse hydrological sensitivities of Estonian catchments to hydroclimatic changes, emphasizing the importance of considering catchment-specific characteristics in water resource management and policy-making. Contributing to the broader understanding of hydrological processes, it provides valuable insights for future research and environmental planning in the face of climate variability and change.

As groundwater does not follow human-drawn boundaries such as country borders, groundwater pollution in one country can adversely affect groundwater quality and availability in a neighboring country. It is vital to develop a conceptual understanding of shared groundwater resources not only to ensure their protection, but also to avoid future conflicts, especially in a changing climate. Both the Water Convention and EU Water Framework Directive emphasize the need for joint assessment and management of transboundary groundwater resources (commonly referred to as “transboundary aquifers” or “groundwater bodies''), and it is crucial to establish a representative transboundary groundwater monitoring network to gather the necessary data. Often, the coverage of monitoring points in the existing groundwater monitoring networks is scarce in the peripheral areas and the installation of new wells would be economically unreasonable. Springs are natural groundwater outflows that can fill gaps in monitoring networks. Monitoring springs can be cost-effective, make water sampling easier, moreover, their water can provide information on a significantly larger catchment area than monitoring wells. The spring site cannot be selected like monitoring wells, so selecting the best springs requires a thorough preliminary assessment. A good conceptual understanding of the recharge area is a prerequisite for the selection of suitable monitoring springs. In this study, 46 springs in the transboundary area of Estonia and Latvia (NE Europe) were screened in 2021-2022 for a variety of geochemical parameters (field parameters, major ions, biogenic and trace elements, water stable isotopes). Some springs were sampled more than once to assess seasonal variability. Then springs were clustered based on their geochemical characteristics using multivariate statistics. This study is the first step in the procedure established on how to select representative springs for transboundary aquifer monitoring.This study is financed by the Interreg Estonia-Latvia cooperation program project “WaterAct”, the EEA and Norway Grants Fund for Regional Cooperation project “EU-WATERRES”, and by performance-based funding of University of Latvia Nr.AAP2016/B041 within the “Climate change and sustainable use of natural resources” program.

Groundwater pollution by agrochemicals such as nitrogen fertilizers can cause complex biogeochemical transformations to take place in groundwater-dependent ecosystems. To explore the interaction between nitrogen load and groundwater-dependent, spring-fed ecosystems, a study was conducted in Latvia in an area of suspected high nitrate (NO₃⁻) vulnerability due to its geological settings. A map of NO₃⁻vulnerability along the margins of the carbonate aquifer in Latvia is presented. The map is based on a conceptual model that was developed during an extensive case study involving hydrological, hydrochemical, and habitat investigation of springs discharging from a karst aquifer and spring-fed ecosystems. Areas that should be prime targets for restricting fertilizer application are highlighted on the map. Although the case study revealed increased nitrogen pollution (up to 51 mg L⁻¹, standard deviation of 9 mg L⁻¹, in the springs discharging from the karst aquifer), no clear evidence of adverse effects due to NO₃⁻pollution on the groundwater-dependent ecosystems using biotic indicators was found, highlighting the resilience of spring-fed ecosystems against high nitrogen inputs. In the case study, downstream groundwater-dependent ecosystems retained 70% of the reactive nitrogen during the vegetation season, but only a small proportion during the cold season. Thus, NO₃⁻pollution can be partly mitigated by restoring wetlands along valley slopes where natural groundwater discharge takes place. The conceptual model developed for groundwater NO₃⁻vulnerability is applicable to other areas in the Baltic region and other places with similar climatic and geological conditions.

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. 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 izvedbo štirih kvantitativnih sledilnih poskusov. Pridobljene podatke smo ovrednotili na podlagi krivulje odvisnosti vhodnih in izhodnih vodnih nivojev. Vzpostavili smo konceptualni model sistema. Rezultati kažejo na obstoj močne hidravlične povezave predvsem z dobro razvitimi kanali med območji napajanja in praznjenja kraškega sistema. Odvisno od hidroloških razmer se spreminjajo smeri in hitrosti toka podzemne vode, ki znašajo od ~225 do 800 m/h. Kapaciteta prepustnosti, ki jo uravnavajo določene mejne vrednosti, in posledične in terakcije s sosednjimi vodonosniki povzročajo neravnovesja med vhodnimi in izhodnimi pretočnimi vrednostmi kraškega sistema. Združevanje rezultatov sledilnih poskusov z analizo krivulje odvisnosti površinskih in podzemnih vodostajev nam je omogočilo, da smo določili mejne pretočne vrednosti izvira Witch's Well in druge ključne hidrološke dogodke. Hidravlični režim kraškega sistema smo razdelili na tri faze, od katerih ima vsaka določene mejne vrednosti in hidrodinamične lastnosti. Ključne besede: kraški sistem, plitvi karbonatni vodonosnik, monitoring, sledilni poskus, krivulja odvisnosti vodnih nivojev, prag, preliv.