*3.1. Tracers*

The tracers were analyzed using spectrofluorimetry in the CETRAHE lab at the University of Orléans (Table 5). The assay of each of these tracers was performed using a calibration curve established with the same tracer used for the test. It should be noted that the spectral analysis technique, via the characteristic excitation and emission spectra, makes it possible to confirm the presence of these fluorescent tracers even when the concentration is low and close to the detection limit, to avoid any confusion with the natural fluorescence of water. The maximum net concentrations obtained at each piezometer are summarised in Table 6.


**Table 5.** Analytical methods and detection thresholds for the analyzed tracers.

Legend: ICP-MS = Inductively Coupled Plasma-Mass Spectrometry; IC = Ionic chromatography; DL = Detection Limit).

**Table 6.** Maximum net tracer concentration per piezometer (in µg·L −1 ).


Legend: Distance = Distance from injection well in the downstream direction (positive values) and upstream direction (negative value), DL = Detection Limit, AGA = Amino G Acid, \* background noise.

It thus appears that the PZ1 piezometer (upstream of the injection well) was not reached by the tracer plume and that the PZ2BIS piezometer (5 m downstream) is the only piezometer where the presence of all the tracers was proven. Starting from PZ3, located 10 m downstream of the injection well, some tracers such as the Amino G Acid and the bromide were not detected. Starting from PZ5, located 30 m downstream of the injection well, sulforhodamine B was also no longer detected. At PZ6, the piezometer that is most removed (60 m downstream of the injection well), no tracer was detected in significant concentrations by the end of the monitoring period. Uranine and lithium are the only tracers that were detected in all the piezometers located downstream of the injection point, except at PZ6. These are thus the best-suited tracers for this hydrogeological context, the first one because it is easily detectable in situ (using a field fluorimeter) including at low concentrations (0.1 µg·L −1 ) and the second one because it proved to be more conservative.

For uranine and lithium, the results were also interpreted using TRAC software [18] considering Fried's analytical solution [19] for the brief injection of a mass of tracer into an infinite volume in flow (Equation (1)):

$$\mathcal{C}\_{(\mathbf{x},y,t)} = \frac{m}{4\,\mathrm{b}\,\pi\,\omega\,t\,\sqrt{D\_{L}D\_{T}}} \cdot \exp\left[-\frac{(\mathbf{x}-\boldsymbol{u}\,t)^{2}}{4\,D\_{L}\,t} - \frac{y^{2}}{4\,D\_{T}\,t}\right] \tag{1}$$

where *C(x,y,t)* is the concentration of tracer (kg·m−<sup>3</sup> ) at the point with coordinates (*x*, *y*) (m) and at time *t* (s), *m* is the mass of injected tracer (kg), *b* the thickness of the aquifer (m), ω the cinematic porosity (−), *<sup>D</sup><sup>L</sup>* the longitudinal dispersion (m<sup>2</sup> ·s −1 ), *D<sup>T</sup>* the transversal dispersion (m<sup>2</sup> ·s −1 ), and *u* the real flow speed (m·s −1 ). In the context of this test, the fixed parameters are *m* = 10−<sup>3</sup> kg, *b* = 14 m and *x* which corresponds to the distance of each piezometer from the injection well. Note that the y coordinate has been left free, which makes it possible to check whether the piezometers are properly aligned in the main flow axis of the aquifer with respect to the injection well.

At PZ2BIS, 5 m downstream of the injection well, the concentration peak was achieved on the day of injection itself at 11:44 for lithium and at 15:28 for uranine, that is, 1.15 and 4.83 h respectively after the start of injection (Figure 10). Despite this difference in transit time, the hydrodynamic parameters used for the calibration of the breakthrough curves are the same for the two tracers: only the retardation factor varies, being fixed at 1.0 for lithium and 1.6 for uranine. The cinematic porosity is thus equal to 1.40 <sup>×</sup> <sup>10</sup>−<sup>2</sup> and the permeability 1.10 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>m</sup>·<sup>s</sup> −1 , values in accordance with those previously obtained in test pumping.

*Appl. Sci.* **2020**, *10*, x FOR PEER REVIEW 13 of 18

**Figure 10.** Evolution of uranine (green curve) and lithium (purple curve) concentrations at the nearest downstream piezometer (PZ2BIS). (**a**) Detail of the 3 first days**.** The peak at 148.1 µg·L<sup>−</sup>1 obtained with the lithium is not shown because it could not be simulated. **Figure 10.** Evolution of uranine (green curve) and lithium (purple curve) concentrations at the nearest downstream piezometer (PZ2BIS). (**a**) Detail of the 3 first days. The peak at 148.1 µg·L <sup>−</sup><sup>1</sup> obtained with the lithium is not shown because it could not be simulated.

From the PZ3 piezometer, 10 m downstream of the injection well, it is no longer possible to fit the data on a single curve because the recovery is bimodal. The first peak reflects a rapid arrival of the tracer by a preferential path (fissured zone?) while the second peak corresponds to a slower propagation within the matrix aquifer. In Figure 11, two distinct fits were therefore applied to each first peak (dashed curves) and second peak (solid curves). Table 7 shows that the porosity obtained is fairly uniform around the mean value of 5.97 × 10−2 m.s−1 regardless of the piezometer or tracer studied, but that the permeability varies more strongly around the mean value of 4.11 × 10−3 m.s−<sup>1</sup> depending on the adjustment made. These values are also significantly higher than those obtained at PZ2BIS, which is interpreted as resulting from an environment with multiple porosity, of both matrix and fissure type, once a larger aquifer volume is involved. As before, we observe a faster propagation of the plume at PZ4 (3.9 m.d−1) than at PZ3 (1.6 m.d−1): however, these speeds are 2 to 3 times lower than during the tracing test carried out in 2012 (10 m.d−1 and 3 m.d−1, respectively), which seems to be due to a low groundwater table which started exceptionally early this year. It should also be noted that this speed artificially reached 104 m.d−1 during the injection, at the PZ2BIS which is a piezometer directly influenced by the injection conditions. From the PZ3 piezometer, 10 m downstream of the injection well, it is no longer possible to fit the data on a single curve because the recovery is bimodal. The first peak reflects a rapid arrival of the tracer by a preferential path (fissured zone?) while the second peak corresponds to a slower propagation within the matrix aquifer. In Figure 11, two distinct fits were therefore applied to each first peak (dashed curves) and second peak (solid curves). Table 7 shows that the porosity obtained is fairly uniform around the mean value of 5.97 <sup>×</sup> <sup>10</sup>−<sup>2</sup> <sup>m</sup>·<sup>s</sup> −1 regardless of the piezometer or tracer studied, but that the permeability varies more strongly around the mean value of 4.11 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>m</sup>·<sup>s</sup> −1 depending on the adjustment made. These values are also significantly higher than those obtained at PZ2BIS, which is interpreted as resulting from an environment with multiple porosity, of both matrix and fissure type, once a larger aquifer volume is involved. As before, we observe a faster propagation of the plume at PZ4 (3.9 m·d −1 ) than at PZ3 (1.6 m·d −1 ): however, these speeds are 2 to 3 times lower than during the tracing test carried out in 2012 (10 m·d <sup>−</sup><sup>1</sup> and 3 m·<sup>d</sup> −1 , respectively), which seems to be due to a low groundwater table which started exceptionally early this year. It should also be noted that this speed artificially reached 104 m·d <sup>−</sup><sup>1</sup> during the injection, at the PZ2BIS which is a piezometer directly influenced by the injection conditions.

**Table 7.** Average hydrodynamic characteristics resulting from the calibration of the recovery curves. **Table 7.** Average hydrodynamic characteristics resulting from the calibration of the recovery curves.


*Appl. Sci.* **2020**, *10*, x FOR PEER REVIEW 14 of 18

**Figure 11.** Evolution of the tracer concentrations at the downstream piezometers: (**a**) PZ3; (**b**) PZ4; (**c**) PZ5. The curves correspond to the fit of the first peak for the dashed curve and of the second peak for the solid curve; the purple curve represents lithium and the green curve uranine. **Figure 11.** Evolution of the tracer concentrations at the downstream piezometers: (**a**) PZ3; (**b**) PZ4; (**c**) PZ5. The curves correspond to the fit of the first peak for the dashed curve and of the second peak for the solid curve; the purple curve represents lithium and the green curve uranine.

### *3.2. Dissolved Helium Concentration 3.2. Dissolved Helium Concentration*

The dissolved helium was extracted from the water sampling vessels by partial degassing by mechanical agitation, after which the extracted gaseous mixture was directly analyzed on site using The dissolved helium was extracted from the water sampling vessels by partial degassing by mechanical agitation, after which the extracted gaseous mixture was directly analyzed on site using

an ALCATEL ASM 122D mass spectrometer. The results obtained during baseline measurements at

an ALCATEL ASM 122D mass spectrometer. The results obtained during baseline measurements at the two reference piezometers, located upstream (PZ1) and far downstream (PZ6), indicate that the groundwater does not contain a significant amount of helium. The measured values of dissolved gas are less than the equilibrium concentration with the surface-atmosphere (helium content about 5 ppm). groundwater does not contain a significant amount of helium. The measured values of dissolved gas are less than the equilibrium concentration with the surface-atmosphere (helium content about 5 ppm).

*Appl. Sci.* **2020**, *10*, x FOR PEER REVIEW 15 of 18

During the experiment, the arrival of the helium plume at the PZ2BIS piezometer, 5 m downstream of the injection well, occurred very quickly after injection (Figure 12): the maximum concentration of 1.47 mg·L <sup>−</sup><sup>1</sup> was measured 30 min after injection. After this, the helium concentration in water decreases. It has almost returned to its initial state at this piezometer 40 days after injection. In PZ3 and PZ4 piezometers, located respectively 10 m and 20 m downstream, the dissolved helium concentrations were significantly lower as at PZ2BIS. The arrival of the helium was detected 3 h after the injection at PZ3 piezometer; and a little more than 5 h after at PZ4. At these two piezometers, the maximum helium concentrations were about 3 and 8 µg·L −1 , respectively, and were recorded 9 days after injection. No significant trace of dissolved helium was measured at the other piezometers located further downstream (PZ5 at 30 m and PZ6 at 60 m). During the experiment, the arrival of the helium plume at the PZ2BIS piezometer, 5 m downstream of the injection well, occurred very quickly after injection (Figure 12): the maximum concentration of 1.47 mg·L−1 was measured 30 min after injection. After this, the helium concentration in water decreases. It has almost returned to its initial state at this piezometer 40 days after injection. In PZ3 and PZ4 piezometers, located respectively 10 m and 20 m downstream, the dissolved helium concentrations were significantly lower as at PZ2BIS. The arrival of the helium was detected 3 h after the injection at PZ3 piezometer; and a little more than 5 h after at PZ4. At these two piezometers, the maximum helium concentrations were about 3 and 8 µg·L−1, respectively, and were recorded 9 days after injection. No significant trace of dissolved helium was measured at the other piezometers located further downstream (PZ5 at 30 m and PZ6 at 60 m).

**Figure 12.** Dissolved helium concentrations in the samples taken at the downstream piezometers PZ2BIS, PZ3, and PZ4. **Figure 12.** Dissolved helium concentrations in the samples taken at the downstream piezometers PZ2BIS, PZ3, and PZ4.

### **4. Discussion 4. Discussion**

Following this test, we can see that the experimental protocol for saturating water with gas and then injecting it into the groundwater is operational, as is the way of monitoring the saturated zone. However, the results obtained lead us to propose a certain number of improvements for the hydrogen injection experiment. Following this test, we can see that the experimental protocol for saturating water with gas and then injecting it into the groundwater is operational, as is the way of monitoring the saturated zone. However, the results obtained lead us to propose a certain number of improvements for the hydrogen injection experiment.

Concerning the gas saturation of the water in the 5 m3 tank, it will not be possible—for safety reasons—to allow hydrogen to bubble all night to obtain maximal saturation at the time of injection, as was done with the helium. Hydrogen is an easily flammable gas requiring the establishment of an ATEX zone and suitable control measures. These measures are difficult to ensure overnight. As a result, the hydrogen bubbling will have to be interrupted in the evening to resume the next morning. Concerning the gas saturation of the water in the 5 m<sup>3</sup> tank, it will not be possible—for safety reasons—to allow hydrogen to bubble all night to obtain maximal saturation at the time of injection, as was done with the helium. Hydrogen is an easily flammable gas requiring the establishment of an ATEX zone and suitable control measures. These measures are difficult to ensure overnight. As a result, the hydrogen bubbling will have to be interrupted in the evening to resume the next morning. This can

lead to a delay of several hours in reaching an optimum level of hydrogen saturation in the water in the tank and, consequently, the same delay in all the subsequent operations (injection, monitoring measurements, etc.). To increase saturation kinetics, the number of gas outlets at the bottom of the 5 m<sup>3</sup> tank will be doubled, from 200 to 400.

The first 1 m<sup>3</sup> tank will hold the fluorescent and ionic tracers that have been shown to provide the best performances: uranine and lithium. Considering the weakness of the signal obtained during this test, owing to a strong dilution of the tracers in the groundwater, they will be used at a concentration higher than an order of magnitude: 10 g·L −1 instead of 1 g·L −1 . In addition, the water in this tank will also be saturated with helium, to be used as an inert tracer gas to compare its behavior with that of hydrogen, a potentially reactive gas in this aquifer context.

The second 5 m<sup>3</sup> tank will be saturated with hydrogen by means of bubbling in a gaseous state during the first day of preparing the materials, as well as during the following morning. The injection of the hydrogen-saturated water will therefore take place at the start of the afternoon. In the test conducted with helium, the two tanks were drained successively, but owing to their respective geometries, the injection rate of the second tank was found to be significantly higher (3.2 m<sup>3</sup> ·h −1 ) than that of the first (1.7 m<sup>3</sup> ·h −1 ). Following this, the two plumes probably coalesced, which hampered the interpretation of the tracer breakthrough curves, and probably diluted the plume of dissolved gas. To avoid this, we will apply a latency time of <sup>1</sup> 2 h between the two injections, and the emptying rate of the second tank containing dissolved hydrogen will be retained at less than or equal to that of the first tank containing the tracers.

The PZ2BIS piezometer placed directly downstream of the point of injection provided the best recovery curves and will thus be considered to be the principal monitoring piezometer. As such, given the speed of the response obtained (1.15 h), it must be equipped with a specific monitoring device to provide continuous data acquisition: dissolved hydrogen measurement probe, physicochemical measurement probe, and borehole fluorimeter. This equipment must be available in duplicate to be able to monitor the other piezometers manually. As soon as the tracer signal has disappeared from the PZ2BIS, the continuously recording borehole fluorimeter will be moved to the piezometers located further downstream (PZ3, PZ4, and PZ5).

The piezometers must not all be sampled at the same frequency, but at specific time intervals in function to their distance from the injection well, and the current hydrogeological conditions, by taking particular account of the propagation speed of the fluorescent tracer. During the current test, the duration of monitoring (40 days) did not permit a satisfactory sampling of the most distant piezometers, namely PZ5 (30 m downstream) and PZ6 (60 m downstream). This duration will therefore be significantly increased, but at a rate of only one sample per week from the 5th week of monitoring: the total duration of the monitoring may vary from 60 to 80 days depending on the hydrogeological conditions at the time (high or low water). It is, however, suggested that a period of high water is favored to reduce the monitoring time. In all cases, only the PZ2TER will operate continuously for the measurement of dissolved gases using Raman and IR spectrometers (O2, N2, H2, CO2, and CH4).

Finally, the piezometry of the aquifer will be measured twice a day at all piezometers during the week of injection, and then once a day thereafter, to detect any variation in the speed or flowing direction of the aquifer. An automatic water depth measurement probe will also be placed at the bottom of PZ2 to measure the amplitude of the piezometric dome induced by the injection.
