Data on Seasonal variations in the Interaction Between Surface Water, Groundwater, and Thermal water at Čateško Polje, SE Slovenia

Herein, we present a dataset of geochemical data (pH, temperature, conductivity, dissolved oxygen) from two sampling seasons (winter - February 2021 and summer - June 2021) to evaluate geochemical processes in the thermal water-groundwater-river water system before the construction of a hydrothermal power plant at Čateško polje, SE Slovenia.
The samples were taken from surface water (a Krka tributary) on the right bank of the river. Deep groundwater samples (V-2/07 and V-1/07) were collected using a Solinst discrete interval sampler, while shallow groundwater samples (M-29/84, PM-3/07) were obtained using a Model 12.04 SA-250 ml bailer sampler set. Thermal water samples (V-16/97, V-3/69, V-15/88, and K-1/69) were collected by opening the pumps two hours before sampling in cooperation with the company Terme Čatež d.d. from the pipe, and the Terme Čatež fountain was sampled directly from the pipe.
Field parameters were measured with a WTW MultiLine 3510 IDS, alkalinity by Gran titration, δ13CDIC by CF-IRMS, elemental concentrations by ICP-MS, and 87Sr/86Sr by MC-ICP-MS, while TOC and anions were analyzed by infrared spectrometry and ion chromatography. PHREEQC modeling showed groundwater is generally oversaturated with CO₂ (except V-2/07) and with calcite/dolomite, except in some thermal waters.
Groundwater is dominated by HCO₃⁻, Ca²⁺, Mg²⁺, Na⁺, and K⁺, with trace elements following the order B > Li > Mn > Zn > Al > Rb > Mo > Fe > Ni > Cu > V > Cr; δ13CDIC ranges from −13.8‰ to −5.7‰ and 87Sr/86Sr from 0.70858 to 0.70934. Soil CO₂ dominates shallow wells and river water, carbonates prevail in deep and thermal waters, and only well V-2/07 shows a higher silicate contribution.
Data from field and laboratory work were used to calculate saturation indices and CO₂ sources/sinks (SIcalcite, SIdolomite, pCO₂), followed by statistical analysis in R (PCA, t-tests, correlations), which identified four clusters. The Water Quality Index (WQI), calculated according to Meng (2016), ranked the sites as: V-1/07 > Terme Čatež wells > M-29/84 > PM-3/07 > V-2/7 > Terme Čatež fountain > River Sava after the Krka confluence.
The data provides valuable background information on aquatic geochemistry and biogeochemical processes, which will be crucial for the future management of hydropower plant construction at Čateško polje, SE Slovenia, as well as for the broader Sava River system.