Determination of the water flow velocity by means of hydrogeochemical techniques applied to a MAR (managed aquifer recharge) site in a salinized aquifer with intergranular porosity (Plana de Castellón-Spain)

Abstract

A significant process of marine intrusion affects the detritic aquifer of Plana de Castellón, located on the Spanish Mediterranean coast. In its southern sector, an artificial recharge scheme with low mineralization water (330 µS/cm) has been implemented. The experience has lasted 175 days, with an average injection flow of 21 L/s and total refilled volume of over 300000 m3. The hydrodynamic effect caused was immediate, with an appreciable rise in piezometry in the first 24 hours around the recharge wells (radius of 80 m). Maximum water-table levels were obtained 79 days after the start of operations, with increase of 3-3.50 m. The decreases are very rapid when the injection ceases and the initial piezometry recovers after 16-17 days. The behaviour of the injected water, as well as its effects on the underground flow, has been studied by applying five hydrochemical parameters: electrical conductivity (EC), chloride concentration (Cl-), nitrate concentration (NO3-), oxygen 18 (δ18O) and deuterium (δ2H). With each of these variables, the rate of expansion of the recharge water has been determined. The velocity of the groundwater flow in each direction, calculated from the five parameters discussed, ranges from 2.4 to 6.3 m/d. The EC parameter is the one that best fits the average speeds, so it is ideal to be applied in this type of studies due to its low cost, immediacy and simplicity in obtaining the data. Consequently, the EC was also selected to characterize the movement of the recharge water through the different permeable formations of the aquifer, from vertical EC profiles made at the control points. Once the lithological columns are known at these points, the velocities of the groundwater flow have been correlated with the lithologies of the differentiated permeable sections. The highest, 6.0 m/d, correspond to gravels and sands with low clay content, while the smallest, 1.0 m/d, are obtained in sands with gravel.