From source-water monitoring to SWRO operating envelopes: Seasonal variability, long-term drift, and carbonate-control baselines in the Arabian Gulf

Seawater reverse osmosis (SWRO) systems are highly sensitive to source-water variability, particularly in regions where seawater quality is affected by strong seasonal forcing, high salinity, and long-term environmental changes. This study develops an integrated monitoring-to-SWRO framework using a multi-year Arabian Gulf source-water quality dataset to connect seasonal variability, percentile-based operating envelopes, long-term drift detection, and carbonate-control baselines within a single process-oriented assessment. The analysis combined monthly distributional assessment, percentile thresholding, Mann–Kendall trend testing, Sen's slope estimation, Welch-test monthly comparison, and carbonate-control calculations using pH, alkalinity, calcium, carbonate speciation, Langelier Saturation Index (LSI), and calcite saturation state. The results showed a strong seasonal thermal pattern, high-salinity source-water conditions, and episodic turbidity behavior that was better captured by upper-percentile thresholds than by average values alone. Temperature ranged from 11.25 to 36.10 °C, while turbidity showed a moderate median value of 2.54 NTU but reached 8.32 NTU during upper-tail events. Long-term trend analysis revealed that several parameters were not stationary, with significant decreasing trends in pH, conductivity, turbidity, chloride, and alkalinity. The decline in pH and alkalinity was particularly important because it shifted the carbonate-control baseline, reducing alkalinity from 136.63 mg/L as CaCO 3 in the earlier measured baseline to 117.23 mg/L as CaCO 3 in the 2026 drift scenario, while LSI decreased from 1.21 to 1.09 over the same comparison. Although the seawater remained supersaturated with respect to calcium carbonate, the declining saturation margin indicates that carbonate chemistry should be treated as a dynamic SWRO design input. These findings highlight the value of long-term source-water monitoring for developing adaptive SWRO operating envelopes that support RO design, pretreatment control, chemical dosing, scaling management, and post-treatment stabilization.