An assessment of seawater desalination impact on salinities in the Arabian / Persian Gulf using a 3D circulation model

The Arabian / Persian Gulf, hereinafter referred to as the Gulf, is one of the major sources of freshwater through desalination in Kuwait, Bahrain, Qatar, the United Arab Emirates, the eastern Kingdom of Saudi Arabia (KSA), as well as in the coastal regions of the Iranian provinces Bushehr and Hormozgan, and the Oman governorate Musandam. Over the past four decades, the seawater desalination capacities in these states have been steadily growing. By 2023 the total production capacity has reached 25.6 Mm³/day, more than double that in 2006 (∼12.1 Mm³/day), and it is expected to add at least 6.2 Mm³/day by 2030. However, not only the Gulf serves as a source of freshwater for domestic, agricultural and industrial use, but also as a sink for the reject brine. Being a semi-enclosed water body characterized by relatively high evaporation rates of 1.5 – 2 m/year on average, small and reportedly declining river inflows, and negligible precipitations, the Gulf may eventually become impacted by anthropogenic changes in salinities, raising concerns about acceptable freshwater production rates and the overall resilience of its ecosystem. In this study, to estimate a combined impact of the desalination facilities on salinities on global and local scales, a 3D high-resolution free-surface baroclinic model of the Gulf's hydrodynamics was established using the open-source unstructured-grid Semi-implicit Cross-scale Hydroscience Integrated System Model. The horizontal resolution of the numerical mesh varied from approximately 4 km in the Sea of Oman to higher than a hundred meters in some coastal areas of the Gulf, which is notably higher than in previous studies, with the vertical resolution of up to 64 local-sigma-coordinate levels. The desalination capacities of 738 coastal plants were considered based on the 35^th Worldwide Desalting Inventory of 2023. Two simulations were carried out over 3 years to assess their impact: one without the effect of the plants (baseline), and the other one with the effect of the plants. The baseline model was validated against field measurements. The volume, near-surface and near-bed averaged salinity impacts in the whole Gulf were found converging to 0.03, 0.04, and 0.05 g/l, respectively. Higher salinity increments were modeled closer to the plants, in some cases at considerable distances. For example, the impact on the median salinities at the seabed was estimated to exceed 0.1 g/l as far as 50 km away from Jubail in KSA, and Al Abu Fontas and Umm Al Houl in Qatar. Even higher increments were modeled in poorly flushed zones, such as ∼0.3 g/l in the whole Gulf of Salwa, and in the range of 0.1 – 0.5 g/l in most of the Abu Dhabi canals. Although such salinity impacts are unlikely to pose environmental threats in the present, the effect of desalination may become more profound in the future, especially in view that salinities in the Gulf have remained quasi-equilibrium over the past century, whilst the freshwater production capacity has already become comparable to ∼2.5% of the annual mean freshwater loss due to evaporation, and it continues to grow.