*Statistical Analysis*

Application of factor analysis [11–13] in all the chemical analyses of the period 2014–2016 showed that three factors, explaining 71% of the total variance, could describe the main hydrogeological and hydrochemical processes which take place in the Mygdonia basin. The first factor accounts for 32.6% of the total variance of the data and has high loadings on Mg, Na, EC, HCO3, and Cl. This factor correlates to the mixing activities of water.

The second factor accounts for 21.5% of the total variance and exhibits high loadings with respect to calcium (dissolution of carbonates), and the third factor (16.8% of the total variance) shows high loadings on sulfate indicating an origin from fertilizers and/or geothermal fluids [14].

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

Hydrochemical analysis was carried out to draw conclusions about the groundwater quality of the Mygdonia basin. From the analysis, a possible connection with geothermal fluids emerged. More specifically, regarding temperature values in some places, they were slightly increased (values up to 21.8 ◦C), which is probably connected to the existence of a geothermal field, while SO4<sup>2</sup>− appears elevated along the northern coastline of Lake Koronia, which may also be due to their mixing with geothermal fluids. Alongside, nitrate ions show high concentration values in the lowland part of the Mygdonia basin, probably due to the intense agricultural activity and use of fertilizers.

Based on Piper and Durov diagrams, it follows that the groundwater of the area has calcium and sodium as its main cations and bicarbonate as its main anion. That is, the dominant hydrochemical type of groundwater is Ca-HCO3 and mixing water types. Based on SAR values, the water is suitable for irrigation. Large numbers of samples exceeded the permissible upper limit of 50 mg/L (EU Council) for drinking water.

A set of measures, including monitoring (water quality, groundwater level, level of lakes, climatic data, water abstractions, etc.), should be applied for the protection of water quality and the sustainability of aquifers alongside the socio-economic development of the wider area. The application of rational fertilization waste-water treatment plants, as well as the delineation of protection zones around the boreholes and springs for water supply, will contribute to water quality protection.

Further investigation into the role of the geothermal field on groundwater quality should be implemented in the area of the boreholes in which high-temperature values were recorded. The continuous monitoring of groundwater quality characteristics is essential for the rational and sustainable managemen<sup>t</sup> of groundwater in the Mygdonia basin.

**Author Contributions:** Conceptualization, K.V.; methodology, K.D. and K.V.; data curation, K.D., T.K. and C.M.; writing—original draft preparation, K.D. and C.M.; writing—review and editing, K.D., C.M. and T.K.; supervision, K.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** A part of this research was carried out in the framework of the Thesis of Kyriaki Devlioti, Laboratory of Engineering Geology & Hydrogeology, Department of Geology, Aristotle University of Thessaloniki, Greece (Supervisor: K. Voudouris).

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