Investigations on the saline groundwater pumping method and its impact on active saltwater intrusion on a layered aquifer system.

Active saltwater intrusion (ASWI) accelerates and intensifies salinization due to buoyancy force-induced density differences and concurrent inland fresh groundwater flow. This study investigates saline groundwater (SGW) pumping as a remediation technique for ASWI through experimental and field-scale analyses in a layered aquifer system characterised by diminishing permeability with depth. Experiments demonstrated that higher permeability layers reduced length of intrusion (Ltoe) whereas lower permeability layers restricted vertical displacement. The SGW pumping would be effective for gentler gradient and could be achieved by positioning the SGW wells at a distance of approximately one-third distance between the freshwater well and the coastal boundary. This hypothesis was tested by applying it to a layered case study with real aquifer settings with comparable conductivity anisotropy. Field study results confirmed the hypothesis, but cumulative impact of pumping rates and SGW well positioning showed smaller wedge penetration than experimentation due to higher field scale dispersion, resulting in optimal location within one-fourth of the distance. Diverging saddle points explain this phenomenon, which is necessary to provide a pulling force greater than freshwater abstraction. In contrast, an aquifer with passive saltwater intrusion location of wells should be near the shoreline to direct the hydraulic gradient towards the seawater boundary. Simulation results suggested that SGW pumping rates should be three to four times freshwater pumping rates to maximise Ltoe reduction. The findings from this study will aid in determining the optimal locations for placing subsurface intake wells for desalination operations in an aquifer that is impacted by ASWI.