## 5 Application to a Real Aquifer
These results do not signify the existence of a regional groundwater source at 124.5 m from P-4. The regional flow of this aquifer comes mainly from the Campo de Elche region, an agricultural area in the province of Alicante (SE of Spain) located about 15 km from the piezometers. What rather confirms the estimation is that the water temperature of the regional flow is 20.0 \({}^{\circ}\)C when reaching the horizontal location of 124.5 m upstream P-4. It is from this location that the temperature profiles that have been used as data to estimate the groundwater velocity start to develop. The above results satisfactorily approximate the empirical estimates deduced using Darcy's law and hydraulic potential isoline profiles [29]. To determine the hydraulic conductivity, field tests were carried out in the P-3 and P-4 piezometers. The results of these tests are as follows:  
\[\text{P-4~{}Gilg-Gavard test K }=5.1\times 10^{-3}~{}\text{m/s}\]  
\[\text{P-3~{}Lefrance test K }=2.5\times 10^{-3}~{}\text{m/s}\]  
Undoubtedly, this is a heterogeneous aquifer with hydraulic conductivity values closer to more permeable soils (clean sands) in the region under study. With the piezometric data, \(\text{h}_{\text{P-4}}=-2.15~{}\text{m}\) and \(\text{h}_{\text{P-3}}=-4.00~{}\text{m}\), and the distance between wells, \(\Delta\text{x}=300~{}\text{m}\), the resulting hydraulic gradient is \(\frac{\text{h}_{\text{P-4}}-\text{h}_{\text{P-3}}}{\Delta\text{x}}=0.0062\). Furthermore, the range of Darcy velocities is \(\
u_{\text{Darcy}}\) [\(1.76\times 10^{-7}\), \(2.81\times 10^{-6}\)] m/s. Since this is a coastal aquifer, it is necessary to investigate whether density-driven flows caused by concentration changes due to salinity changes produce fluid velocities comparable to the estimated velocity. The expression for these velocities is \(\
u_{\text{dd}}=\frac{\kappa\text{g}\left(\Delta\rho_{\text{e,w}}\right)}{ \epsilon\mu}\), with \(\Delta\rho_{\text{e,w}}\) indicating the saltwater density changes, \(\kappa\) the intrinsic permeability, \(\mu\) the dynamic viscosity and \(\epsilon\) the porosity of the aquifer (\(\epsilon=0.6\)) [31]. For calculations, Figure 6 shows the salinity-depth profiles of wells P-3 and P-4, measured in March 2019. Although this is a complex coastal aquifer, the salt concentrations in the aquifer caused by marine intrusion are negligible compared to the remaining concentrations caused by former salt mining, as shown in these curves. Regarding vertical density gradients, in the worst case, it only occurs in a vertical region near the surface in P-4. These density changes induce fluxes of the value \(\
u_{\text{dd}}=\frac{\kappa\text{g}\left(\Delta\rho_{\text{e,w}}\right)}{ \epsilon\mu}\), with the values \(\kappa=4.6\times 10^{-11}~{}\text{m}^{2}\)[32], \(\text{g}=9.8~{}\text{m/s}^{2}\), \(\Delta\rho_{\text{e,w}}=6.40~{}\text{kg/m}^{3}\), and \(\mu=10^{-3}~{}\text{kg/(m\cdot s)}\). \(\
u_{\text{dd}}=4.81\times 10^{-6}~{}\text{m/s}\), which is 6.0% of the estimated veloc  
Figure 6: Vertical profiles of electrical conductivity recorded in March 2019 in boreholes P-3 and P-4. ity. These calculations allow the effects of density-driven flows in the problem to be neglected overall.