Brackish water vs. brine outfall: impact of desalination plant discharge in vulnerable coastal sites

The potential of coastal springs has recently been explored as an alternative water source for desalination plants. Unlike conventional seawater-fed facilities, this approach produces a brackish effluent. Our study aims to investigate the environmental implications of the discharge of such brackish water from a planned desalination facility in a vulnerable coastal area of Southern Italy. We employ numerical modeling of hydrodynamics and plume dispersion to compare two scenarios: (1) discharge of hyposaline brackish water, as envisaged by the proposed approach; and (2) discharge of hypersaline brine, as in traditional plants. Year-long simulations reveal key differences between the two scenarios. We introduce and discuss two relevant metrics for assessing plume impacts: the eddy diffusivity, which quantifies mixing efficiency, and the characteristic time scale, which links plume length scale to ambient current speed. Our main findings highlight (i) how differences in plume density alter local hydrodynamics; (ii) the extent to which plume spreading time and length scales differ, with consequent variation in potential ecosystem impacts. Results suggest that brackish effluent management could represent a more sustainable alternative for coastal desalination, minimizing environmental risks. In contrast, hypersaline plumes may require offshore release to reduce ecological impact. Although this study is region-specific, the proposed framework is applicable to other coastal areas facing similar desalination challenges.