GC_2-2013_KB.indd  Geologia Croatica 66/2 151 Zagreb 2013 Geologia CroaticaGeologia Croatica Comment on the paper "Defi nition of the river Gacka springs subcatchment areas on the basis of hydrogeological parameters" by Lukač Reberski et al., published in Geologica Croatica 66/1, 39–53 (2013)  Zvjezdana Roller-Lutz1, Tamara Hunjak1 and Hans O. Lutz1,2 1 Medical Faculty, Rijeka University, Braće Branchetta 20, 51000 Rijeka, Croatia; (roller@medri.uniri.hr) 2 Faculty of Physics, Bielefeld University, Bielefeld, Germany doi: 10.4154/gc.2013.12 This paper by Lukač Reberski et al. (in the following referred to as paper I) presents measurements of hydrological and hy- drochemical parameters as well as results of an isotopic ana- lysis of the spring waters and the precipitation. As the authors of paper I mention, the isotopic analysis has been performed in Graz and in Rijeka. We want to point out that the data for the year 2008 resulted from a cooperation project which was headed by Rijeka University and supported by the Interna- tional Atomic Energy Agency (IAEA Vienna); the correspond- ing isotope results were preliminary and shown only in an un- published and not citable report. In spite of a confi dentiality agreement, the authors of paper I took these data without our consent. This breach of trust and the somewhat sloppy data handling has some unfortunate consequences: The isotope data shown in Fig.7 appear to scatter be- tween the Local and the Global Meteoric Water Lines, LMWL and GMWL. This was used to argue a direct corre- spondence between precipitation and spring recharge. Firstly, note that this LMWL is plotted incorrectly in Fig. 7; the cor- rect LMWL (eq.5 in paper I) had to be lower by about −1‰ (i.e. shifted to more negative δ2H values) if compared to the plotted line. Therefore, the majority of data points are actu- ally located slightly above the correct LMWL. The fi nal anal- ysis of the 2008 isotopic data further corroborates this small but noticeable effect; as a consequence, the d-excess d = δ2H − 8۰δ18O is somewhat higher for the spring waters than that of the precipitation, supporting the conclusion that recharge happens mainly during the cold season (c.f. also MANDIĆ et al. (2008)). Furthermore, the absolute altitude values of the recharge areas (c.f. the end of section 7.3 in paper I) cannot simply be inferred from the measured isotopic ratios; the available data can only provide (approximate) values for the altitude dif- ferences. The reason is that the derivation of absolute values requires some reference data, e.g. the long-term mean δ18O of the precipitation at the spring altitude (more precisely, taking into account the mean residence time of the ground water as well as the infi ltration into the catchment); or a re- liable value of the absolute altitude of at least one of the re- charge areas (assuming that the soil surface and thus the in- filtration is of approximately the same nature for all catchments). Without further discussion neither is really known; for example, depending on the season, the δ18O val- ues of the collected and analyzed rainfall in the Gacka spring region in the years 2006 and 2008 vary between approxi- mately −4‰ and −13‰. The authors do not provide any clue for the reference data they used. The publication of the fi nal analysis of our results, to- gether with an estimate of the mean residence time of the ground water as obtained from tritium model calculation, is presently under preparation. REFERENCES MANDIĆ, M., BOJIĆ, D., ROLLER-LUTZ, Z., LUTZ, H.O., KRAJ- CAR BRONIĆ, I. (2008): Note on the spring region of the Gacka River (Croatia).− Isotop. E nvir. Health Studies, 44/2, 201–208. Available online April 01, 2013 Geologia Croatica 66/2Geologia Croatica 152