Experimental and numerical investigations of desalination brine injection into the saline portion of coastal aquifers

Seawater desalination, specifically reverse osmosis (RO) is extensively used globally to supply potable water and minimize coastal aquifer salinization. However, RO produces brine, typically discharged into the ocean, potentially harming nearshore ecosystems. To explore sustainable alternatives, the feasibility and potential environmental consequences of injecting desalination brine solution into the saline portion of a coastal aquifer as a means to mitigate the nearshore environmental impact is investigated. Acknowledging the scarcity of comprehensive studies on the impact of brine injection on the fresh–saline water interface (FSI), this study aims to bridge the knowledge gap by employing experimental (sand tank) and numerical modeling approaches. To accomplish this, five distinct scenarios were examined: (1) brine injection only; (2) saline groundwater (SGW) pumping only; (3) freshwater pumping only; (4) simultaneous brine injection and SGW pumping; and (5) simultaneous brine injection and fresh groundwater pumping. Results show that brine injection creates a high salinity plume that pushes the FSI landward and salinizes the aquifer. Pumping SGW simultaneously with brine injection negates the injection effect on the FSI and results in a more stable interface. Conversely, pumping freshwater simultaneously with brine injection exacerbates seawater intrusion and causes faster salinization of the freshwater aquifer. In all injection scenarios, the brine plume propagation velocity decreases as injection continues. The analysis shows, for the first time, the hydrological implications of brine injection into the saline part of the coastal aquifer using controlled sand tank experiments and numerical modeling, and demonstrates its potential use for desalination plants in protecting the environment.