Development and test of a functional relationship on groundwater salinity reduction

Abstract

The study was undertaken to develop and test a functional relationship that would determine the effect of spatial change and time on salinity concentration using the data obtained from the freshwater recharging process. Thirty wells — 5 deep wells (injection wells) and 25 shallow tubewells (observation wells) were installed in the study site, 7.5 ha, at Brgy Sulongan, Pasuquin, Ilocos Norte. In site measurement of salinity in terms of electrical conductivity (EC) was measured daily from April to August 2011. Groundwater table depth was monitored simultaneously with EC. Freshwater injection was conducted in three batches: single well, three-well, and five-well processes at 50 m³ per well. EC and groundwater table depth were monitored before and after the freshwater recharging activities using the Salinity-Conductivity-Temperature (SCT) meter. Functional relationships between the parameters monitored were developed using linear and non-linear models.

Results showed that during the single-well injection process, its sphere of influence was only at the point of injection. This meant that the hydraulic barrier formed had pushed more saltwater landwards, thus an increase in groundwater salinity in the study area was observed. The second freshwater addition was similar to the first. The third process, that is, five-well freshwater addition of a total of 250 m³ to the injection wells, demonstrated the best results. The hydraulic barrier formed a wider sphere of influence to the saline concentration of the experimental area that halted the movement of saltwater landwards. The salinity concentration decreased and was considerably maintained at low levels in the whole area. Equation 3 described that during weeks without freshwater injection, groundwater withdrawal is higher; groundwater salinity decreased with time as influenced by groundwater withdrawal. Equation 4 illustrated the condition of the study area during weeks with freshwater injection and depicted that when there is simultaneous injection of freshwater to the injection wells, a hydraulic barrier is built up, preventing saltwater intrusion landwards, thereby decreasing salinity level (< 10000.0 mg/L) in the study area. Further, the relative location (L) of the installation of the injections wells to the sea was significant in the formation of the hydraulic barrier that prevented saltwater migration inlands.

It was concluded that salinity concentration of groundwater in the area is above the 1000 mg/Li threshold limit for freshwater. The use of a five-well simultaneous recharging process at a rate of 50 m³ per day per well demonstrated the best method of mitigating saltwater intrusion inland. The procedure formed a hydraulic barrier at the widest sphere of influence. Groundwater salinity changes with time during weeks without freshwater injection. During times with freshwater injection, groundwater salinity was affected by time and vertical grid in the study area.