Not far from the coast of Izola, Slovenia, are twelve funnel-shaped depressions on the seabed containing springs of warm, sulphurous water. These submarine depressions with springs are divided into three groups: from east to west, Izola, Bele skale, and Ronek. Water with similar thermal and sulphuric properties also occurs in the terrestrial spring in Izola. Some of these submarine springs (M03 - Izola, M05 - Bele skale and M10, M11 - Ronek) and the terrestrial spring (K04) were sampled for hydrogeochemical study. Three sampling campaigns took place in June and July 2020, October 2020, and April 2021. In addition, samples of seawater (SW01, SW02, SW03), groundwater from a well (VK01) and local non-sulphuric springs (K01, K02, K03) were taken for comparison. The submarine spring water was sampled using 100-ml syringes. In April 2021, an 8-litre Niskin water sampler (OceanTest Equipment, Inc.) was used in addition to the syringes. Samples collected in April 2021 using two different methods are labeled with an additional letter: /B for syringes and /N for Niskin. In-situ physicochemical parameters - pH, temperature, electrical conductivity, oxidation-reduction potential, and dissolved oxygen - were measured in the samples from the seawater and the submarine springs using a digital Multi 3430 meter (WTW GmbH, Weilheim, Germany) as soon as they were brought aboard the boat. Measurements of the terrestrial waters were performed on site using the same digital meter. A total of 26 water samples were collected for further analysis. All water samples were collected in HDPE bottles, except those for isotopic composition of dissolved inorganic carbon, which was sampled directly to Labco glass ampoules after 0.45 µm pore-sized membrane filtration. The samples were then chemically treated where required and stored in a refrigerator until analysis. The samples were then prepared for further laboratory analysis: geochemical composition (cations, anions), isotopic composition of oxygen (δ¹⁸O) and hydrogen (δ²H), total alkalinity (TA), isotopic composition of dissolved inorganic carbon (δ¹³CDIC), tritium activity (³H), isotopic ratio of strontium (⁸⁷Sr/⁸⁶Sr), and isotopic composition of sulphur (δ³⁴Sₛ₀₄ ‰) and oxygen (δ¹⁸Oₛ₀₄ ‰) in sulphate. This database contains information on the sampling dates and basic characteristics of the sampling sites, as well as the results from field measurements and laboratory analyses.
