*4.2. Groundwater Chemistry from Irrigation Wells*

Chemical comparison between the groundwater samples from the studied localities indicate that shallow groundwater is more depleted in Cl<sup>−</sup> and Na<sup>+</sup> compared to the deep one. Cl<sup>−</sup> is a relatively mobile element that does not incorporate into secondary minerals after being released from the dissolution of the basaltic protolith [51]. The different Cl− contents between the analysed borehole groundwater samples are attributed to their distance from seawater [73], origin, and circulation. In particular, the sampling site of the deep groundwater is located closer to the Aegean Sea compared to the shallow one. The aforementioned difference is attributed to the mixture between the deep groundwater from Microthives and seawater and confirmed by the ionic ratios (Table 3), coupled with the Langelier–Ludwig [74] and Piper plots [46] (Figure 6a,b). The shallow groundwater presents similar Na–Cl contents compared to those of the groundwater from Iceland (Figure 6a). This suggests that both water samples were not affected by mixture processes with seawater.


**Table 3.** Ionic ratios of water samples from the region of Microthives (mg/L) [28].

**Figure 6.** (**a**) Ludwig–Langelier [74] diagram for the water sample of Microthives and the seawater sample from the Aegean Sea. (**b**) Schoeller diagram [75] for the Microthives water sample and the Aegean seawater [28,51].

The groundwater composition is also affected by the water–rock interaction during the circulation of rainwater through basalts [51].

#### *4.3. Indications of Enhanced Heat*

Volcanic rocks in the Porphyrio and Microthives localities were developed in an extensional back-arc geotectonic setting affected by the activity of the Northern Anatolian Fault [19–21]. This back-arc extension was evolved with respect to the active volcanic arc of the South Aegean [21] and gave rise to the generation of Late Pleistocene basalts. The age of the magmatism is very crucial to the determination of the heat source [76]. In particular, the active magmatism is indicative of elevated heat sources, compared to the inactive or extinct magmatism that are associated with heat remnants and/or additional radioactive-heat [76,77].

The back-arc extension developed in Porphyrio and Microthives localities indicates a recent-inactive enhanced heat, characterised by the development of relatively shallow and young magma chambers [76]. These systems are mainly developed in divergent plate margins [76], usually including two distinct zones of different *T* and pH conditions [76,78–80]. In particular, the outflow zone has a lower *T* and neutral-to-alkaline pH groundwaters [80] compared to the upflow, which is more acidic [81]. In the cases of inactive magmatic sources, the produced heat is strongly associated with crystallised, but still-cooling, magmatic bodies [76]. According to this model, the main heat source is provided by the Pleistocene magmatic melts, whereas the presence of faults further enhances the recharge of meteoric waters [76]. A similar heating source was developed in Hungary as a result of a Miocene extension that caused a high thermal attenuation of the lithosphere [82,83]. In the current study, the elevated water temperatures were mainly observed close to the basaltic rock occurrences (*T* = 30.2 and 23.0 ◦C for GTES-038 and GTES-040 groundwater irrigations wells, respectively). Enhanced water temperatures are also recorded in the adjacent regions of Kamena Vourla (Central Greece; East Thessaly) and Lichades islands (Central Greece; North Evoikos Gulf), corresponding to 25–41.3 ◦C and 41 ◦C, respectively [84]. These regions are related to scattered volcanic centers, which were active during the Late Pleistocene–Quaternary period, similarly to those of the Porphyrio and Microthives localities. The above data suggest that this activity is associated with the extensional back-arc tectonic setting. Based upon the geological mapping of the Porphyrio and Microthives localities, coupled with the elevated temperatures of the groundwater samples (irrigation wells GTES-038 and GTES-040), the elevated temperatures in the studied region are strongly associated with the basalt occurrence underneath the Neogene alluvial sediments. The water pH in the Microthives locality (pH: 7.20–7.30; [28]) and the adjacent regions of Kamena Vourla and Aidipsos (pH: 6.28 and 6.80 respectively; [85]) indicate that these waters are derived from the outflow zone, which is characterised by a neutral-to-alkaline pH [81].

## *4.4. A Case Scenario for Mineral Carbonation in the Micothives Basalts*

The Microthives and Porphyrio basaltic occurrences are potential sites for CO2 storage [86]. The research area is located 10 km away from the industrial zone of Volos, a significant source of CO2 emissions. The case study scenario presented in this study is based on the results of the CarbFix project [50,55]. Carbon storage through injection of water dissolved CO2, is a potential applicable CCS scenario for the volcanic rocks of Microthives and Porphyrio localities.

The CarbFix method does not require the presence of a cap rock, since the dissolved CO2 is not buoyant [55]. The process of CO2 dissolution during the injection into basaltic rocks [55] of the Microthives and Porphyrio localities, can be enhanced due to the higher porosity that these rocks present (average porosity: 18%). There is a strong association between the porosity and permeability of the basaltic rocks and their alteration grade [55]. Thus, the younger and less-altered basalts are more appropriate for CO2 storage compared to the older types. Basaltic rocks of the current study belong to the relatively young extensional Pleistocene volcanic activity, and, hence, they were not affected by a high alteration grade. The pH value in the groundwater from the Microthives locality is

7.3, which is similar to that of the target zone prior to the injection of CO2 in the CarbFix project [10,50]. After the initial pH decrease, due to the mixing of the groundwater fluids with the hydrous injected CO2, the reaction paths of basaltic glass at 25 ◦C [51] indicate that pH becomes more alkaline due to the PCO2 decrease during the water–rock interaction.

Regarding the diffusivity of water-dissolved CO2 in basalts, we provide preliminary calculations with Equation (12) [87]:

$$D = D\_0 \cdot \varphi^{\rm m} \tag{12}$$

where *D* is diffusion coefficient; *D*<sup>0</sup> is diffusion of the water dissolved CO2, (1.92·10−<sup>5</sup> cm2/s [88]; ϕ is porosity of basalt (0.18–0.23 for our studied basalts); and m is Archie's coefficient, (m: 2.3 [89]). By applying the aforementioned equation, it is estimated the diffusion coefficient ranges from <sup>38</sup> ×·10−<sup>8</sup> cm2/s to 65 <sup>×</sup> <sup>10</sup>−<sup>8</sup> cm2/s, respectively.

One of the major parameters in the CarbFix project is the substantial quantities of water for the dissolution of CO2 during injection [50]. Basaltic outcrops of Microthives–Porphyrio localities are in proximity with the Aegean Sea, giving the potential for high storage capacities, due to the unlimited seawater supply [6,50,90,91].

We provide preliminary calculations that estimate the CO2 that could be stored in the frames of pilot projects for the two basalt locations of Microthives and Porphyrio. For this purpose, we apply the function below:

$$\text{Storage Capacity} = \sum (V \times \varphi \times \rho \times \varepsilon) \tag{13}$$

where *V* is the volume of the basaltic outcrop; ϕ is the average porosity = 18%; ρ is the specific gravity of the sCO2; and ε is the sCO2 storage ratio.

The Microthives basaltic outcrop has a surface of ~8 km2; therefore, the potential pilot project can be realised at an estimated volume of 300 m (length) <sup>×</sup> 200 m (width) <sup>×</sup> 300 m (depth) = 18 <sup>×</sup> 106 m3. Taking into consideration the average porosity of basalts from our studied site (18%), the specific gravity of the scCO2 (400 kg/m3; at 10 MPa and 50 ◦C [92,93]), and the scCO2 storage ratio of basalts (5% [94]), the Microthives basaltic outcrop could store an amount of 64,800 tons of CO2. The Porphyrio basaltic formation is smaller, and, therefore, by assuming an estimated volume of 200 m (length) × 100 m (width) <sup>×</sup> 300 m (depth) <sup>=</sup> <sup>6</sup> <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3, it could store a calculated amount of 21,600 tons of CO2. The maximum capability of CO2 storage, considering the highest porosity of the studied suite (23%), corresponds to 82,800 tons and 27,600 tons for the Microthives and Porphyrio basalts, respectively. The size of these outcrops could serve for storage of much larger amounts of CO2 after deployment of pilot tests.

The charged water can significantly increase the energy consumed for the CO2 injection. From the CarbFix experience, it is evident that the cost of storage and transport corresponds to \$17/ton of dissolved CO2 injected [50,95], which doubles the cost compared to the classic CO2 injection in sedimentary basins [50,96]. This cost is balanced by the lower monitoring after the injection period, due to the non-buoyant nature of the mineralised CO2 [50]. The development of a cost-effective scenario is further enhanced by the relatively short distance of the basaltic dominated areas (~10 km) from the industrial area of Volos, reducing the cost of transport.

#### **5. Conclusions**

Pleistocene volcanic rocks are present in the region of Volos (Central Greece) and in the specific localities of Microthives and Porphyrio. They are classified as basaltic and trachyandesitic lavas and were formed due to back-arc extension of the Aegean Sea. Their geochemical affinities suggest that these are alkaline basalts of OIB affinity. Results from detailed petrographic examination show that their porosity ranges between 5% and 40% with vesicles, which, in a few rock samples, partly host calcite. The vast majority of the studied samples exhibit porosity that ranges between 15% and 23%.

A recent-inactive magmatic heating source present in the Microthives basaltic vicinity, affected the groundwater temperature regime. Enhanced groundwater temperatures are also recorded in adjacent regions with basalts of similar composition and age, suggesting that this activity is associated with the extensional back-arc tectonic setting. Deep and shallow groundwater samples are classified as Ca-Mg-Na-HCO3-Cl and the Mg-HCO3 hydrochemical types respectively. Measured groundwater temperatures from irrigation wells, at depths between 170 and 250 m, reach up to ~30 ◦C.

Basalts from the region of Volos have the necessary appropriate physicochemical features to be considered as potential sites for implementing carbon capture and storage (CCS) technologies due to (i) low alteration grade, (ii) silica-undersaturated alkaline composition, (iii) presence of Ca-bearing minerals, (iv) high porosity, and (v) indications of enhanced heat. The proximity of the basaltic rocks to the sea gives the opportunity for exploitation of the unlimited water sources during the CO2 injection. Furthermore, these outcrops are in close distance to the industrial area of Volos, providing the potential for the development of a financially feasible scenario. Preliminary calculations suggest that potential pilot projects at the Microthives and Porphyrio basaltic formations can store 82,800 and 27,600 tons of maximum CO2, respectively, although their size could serve for storage of much larger amounts of CO2 after deployment of pilot tests. Further and detailed petrological, petrophysical, geochemical, hydrochemical, geothermal, and financial research studies are needed prior to deployment of pilot tests in the region of Volos.

**Author Contributions:** All authors actively participated in a balanced manner at all stages of the research presented in this paper. This involved participation of all authors in sample collection in the field, performing laboratory work and manuscript writing.

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

**Acknowledgments:** We would like to express our sincerest thanks to the Reviewers and the Editor for their constructive comments and useful suggestions that have substantially helped to improve this paper.

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