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Abstract-The well-preserved Kardla impact crater, on Hiiumaa Island, Estonia, is a 4 Ian diameter structure formed in a shallow Ordovician sea -455 Ma ago into a target composed of thin (-150 m) unconsolid ated sedimen tary layer above a crystalline basement composed of migmatite granites, amphibolites and gne isses. The fractured and crushed amphibolites in the crater area are strongly altered and repl aced with secondary chloritic minerals. The most intensive chloritization is found in permeable bre ccias and heavil y shattered basement around and above the central uplift. Alteration is believ ed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater. Chlo ritic min eral associations suggest formation temperatures of 100-300°C, in agreement with the most frequ ent quartz fluid inclusion homogenization temperatures of ISO-300°C in allochthonous bre ccia. The rath er low salinity of fluids in Kardla crater «13 wt% NaCl eq ) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the crater rim walls rais ed abo ve sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fra ctur ed crys talline bas ement. The well-developed hydrothermal system in Kardl a crater show s that the thermal history of the shock-heated and uplifted rocks in the central crater area, rather than coo ling of impact melt or suevite sheets, controlled the distribution and intensity of the imp act-induced hydrothermal processes.

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The DRASTIC method is one of the most widely used groundwater vulnerability assessment techniques. In areas where the main useful aquifers are covered with an extra layer of diverse sediments, a further modification of the DRASTIC method is required for a more precise vulnerability estimation. In this article, the DRASTIC method was improved in areas characterized by a layer of diverse Quaternary deposits remarkably influencing the infiltration conditions. Three parameters of the DRASTIC method were modified: (1) the D-parameter was adjusted to consider the overlying Quaternary deposits that, in some cases, make the aquifer confined, (2) the S-parameter was replaced by the Quaternary sediment-type parameter to assess the hydraulic characteristics of the highly variable deposits, and (3) the I-parameter was replaced by the thickness of the Quaternary deposits parameter to describe the distance from the ground surface to the main useful aquifer. The original and modified DRASTIC methodology was applied in an area with glacial sediments in Central Estonia. Comparing the results using the original and the modified DRASTIC method to a former Estonian groundwater vulnerability method showed that the DRASTIC method was significantly improved and could, thus, be successfully applied in other areas characterized by a heterogenous Quaternary sediment cover.

<p>European water policy requires to carry out nine tests for characterisation of groundwater bodies status, including the development of joint transboundary groundwater management principles. </p><p>Gauja/Koiva and Salaca/Salatsi Rivers have a joint, Estonian/Latvian transboundary water cycle, including the groundwater recharge and discharge cycling. Despite the fact that groundwater is the only drinking water source in Gauja/Koiva and Salaca/Salatsi River basins and ensures the existence of many groundwater dependent ecosystems, the overall awareness of integrated cross-border management practice is still poorly understood and poorly linked in implementation of concrete groundwater protection actions. </p><p>Taking into account the above-mentioned aspects, Latvian and Estonian groundwater and groundwater dependent ecosystems specialists from research, nature protection and groundwater resources management institutions have joined forces in the project funded by the Interreg Estonia-Latvia program: "Joint actions for more efficient management of common groundwater resources in Estonia and Latvia "(WaterAct).</p><p>The ongoing Est-Lat project “WaterAct” (2020–2022) of joint transboundary groundwater management project organised into the three activity blocks: (1) The capacity building of the joint groundwater transboundary management through exchange of knowledge and best management practices between project partners and key experts in other European Union countries; (2) Assessment of groundwater resources in transboundary River basins to improve groundwater management in accordance with valid international directives; (3) Dissemination and outreach activities to increase the overall awareness of ecosystems friendly groundwater management and protection of key actors working with groundwater assessments and locals. </p><p>Firstly, the joint cross-boundary principles of identification and status assessment of shared groundwater bodies will be developed. Adaptation of existing knowledge, cross-boundary harmonization and development of needed methodologies will be used. </p><p>Secondly, the joint cross-boundary assessment principles will be implemented into groundwater resources management. The status assessment of shared groundwater bodies will be carried out in close cooperation between project partners to create materials necessary for the development of last River Basin Management Plans (2022-2027), required by the Water Framework Directive and Groundwater Directive.</p><p>Thirdly, the dissemination of project results will be carried out (1) by compilation of Guidelines of Groundwater Dependent Ecosystems for different levels and fields of decision making and experts, (2) by compilation of Spring Water Monitoring Guide for Volunteers and starting volunteer monitoring. For volunteer monitoring, a special web-based map application will be developed (allikad.info). </p><p> </p><p>The project of “Joint actions for more efficient management of common groundwater resources” (WaterAct, Est-Lat155) funded by ERDF Interreg Estonia-Latvia cooperation programme.</p>