RECHARGING INJECTION WELLS WITH FRESHWATER IN FORMING HYDRAULIC BARRIER TO HALT SALTWATER LANDWARDS

Abstract. The study was conducted in a coastal barangay in Pasuquin, Ilocos Norte, Philippines with pre determined high saltwater content (> 1000.0 ppm) of the groundwater system. A matrix of 5 deepwells and 25 shallow tubewells were constructed in a 7.5 ha agricultural production area. The 5 deepwells were used for the injection and addition of freshwater, while, the 25 wells were the observation wells. Electrical conductivity (EC) and depth to water table were measured and recorded daily for 6 months. Results of three-batches: single well; three-well and five- well of freshwater recharging process showed that the hydraulic barrier created during the single- well injection process has its sphere of influence only at the point of injection. This would mean that the hydraulic barrier formed had push more saltwater landwards, thus increased in groundwater salinity in the study area. The outcome of the second freshwater addition resulted to the formation of a hydraulic barrier in three locations resulted in an increased pressure that served as a driving force pushing salt ions to the edges of the groundwater system. Migration of salts through the effects of diffusion and dispersion cause the increase in saline concentration. The five-well process demonstrated the best freshwater addition process. Results of the addition of freshwater simultaneously to five injection wells resulted to decreased salinity was maintained considerably at low levels in the whole study area.