**2. Methodology**

The sampling of the groundwater was caried out at three piezometer wells in the Bokanjac–Poliˇcnik basin near the city of Zadar in Croatia. Samples was taken four times in one year in order to over all the seasons, i.e., to obtain samples from one hydrological year. For the sampling, sterile glass bottles were used, and they were refrigerated during transport and up to the time when sample preparation began. The three sites where the samples were taken are marked as J, B, and B4. Due to the complexity of the groundwater samples, which were relatively clear and of high quality according to the regulation parameters for human consumption, the small concentrations of the expected organic pollution could not

be directly measured, and there was a need to concentrate the samples for the analysis. The analyte prepared for analysis was concentrated using the Solid Phase Extraction (SPE) method to increase the selectivity of the method. The SPE method was used to eluate the concentrated analyte in order to simplify the identification and quantification of the expected compounds in extremely low concentrations. The cartridges used for SPE were Bond elut plexa C18, and the samples were eluted with methanol (MeOH) and filtered through a KX syringe filter (PET 25 mm, 0.22). The filtered samples were analyzed using an Agilent 6530 LC/MS QToF device that detects the ion mass (M/z) and relative abundance of isotopes. With this device, we determined which ions were in the samples based on the masses that were detected [14].

LC/MS QToF analysis was performed with an InfinityLab Poroshell 120 EC–C18 (3.0 × 100 mm, 2.7 μm, Agilent Technologies, Inc.) column. We injected 0,4 mL/min of analyte into the following mobile phases: (A) deionized water with 0.1 % formic acid, (B) MeOH with 0.1 % formic acid, and (C) acetonitrile (ACN) with 0.1 % formic acid. These mobile phases were used in 20 min runs in the positive ionization mode, with a column temperature of 35 ◦C.

### **3. Results**

The OPEs that were detected with the above-mentioned methodology were identified with the Agilent Water Screening Personal Compound Database and Library (PCDL), and the results are shown in Figures 1–6.

**Figure 1.** *Cont*.

**Figure 1.** OPEs in sample J: (**a**) TCP; (**b**) TCEP.

**Figure 2.** TDCPP in sample J.

**Figure 3.** OPEs in sample B4: (**a**) DEP; (**b**) TCPP.

**Figure 4.** TBEP in sample B4.

**Figure 5.** *Cont*.

**Figure 5.** OPEs in sample B: (**a**) TPPA; (**b**) TBP.

**Figure 6.** DEHA in sample B.

### **4. Conclusions and Discussion**

OPEs are potential pollutants of groundwaters that are not under regulations, and there are no defined maximum available concentrations for such emerging pollutants. OPEs can be found in nature in relatively small concentrations, and it is not easy to identify their presence in groundwater. In this research, nine OPEs were detected in groundwater samples from a karst basin in Croatia, which were taken during each season for one year. In the areas where the samples were taken, the water used for the water supply system is withdrawn from the karst aquifer, used here as a case study. Consequently, there is a need to raise public awareness of the usage and release of OPEs into the environment. Additionally, some of the OPEs found in the samples are toxic, i.e., dangerous for human health, due to their persistence in the water environment. This is why research on their identification, quantification, and behavior in groundwater is important.

**Author Contributions:** Conceptualization, L.P. and A.P.S.; methodology, L.P.; validation, I.G., A.P.S. and R.B.; formal analysis, L.P.; investigation, L.P.; writing—original draft preparation, L.P.; writing—review and editing, I.G., R.B. and A.P.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by EU funded projects, gran<sup>t</sup> numbers KK.01.1.1.04.0006 and KK.05.1.1.02.0022.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
