THE ISOTOPIC COMPOSITION OF MAN-MADE FRESH WATER: SEAWATER AND BRACKISH GROUNDWATER REVERSE OSMOSIS DESALINATION

The worldwide pressure on fresh water resources, particularly in water-scare areas, has led to a substantial increase in utilization of desalinated water. Given the technological advances of reverse osmosis (RO) desalination and the resulting significant cost reduction, seawater desalination (SWRO) and brackish groundwater desalination (BWRO) are expected to become a major component in water supply in many countries during the next decades. Here we investigate the boron, oxygen, hydrogen, and strontium isotopes, coupled with geochemical variations, of large-scale RO desalination plants in Israel (Elat, Ashkelon, Nitzana) and Cyprus (Larnaca). Our O and H isotopic data from Ashkelon and Elat show that RO produces no measurable diffusive isotopic fractionation on the water molecule so that the product freshwater conserves the original water signature. Likewise, the B isotopic composition of conventional low-pH SWRO desalination (Elat) produces desalted water with seawater signal. In contrast, in a high-pH desalination plant (Ashkelon), where the system is designed to convert B to the charged boron species (B(OH)₄^-) in order to reduce the B content, the desalted water is highly enriched in ¹¹B with an average δ¹¹B of 59 ± 2 ‰ vs. NBS951. The Sr isotope signatures in Elat BWRO-SWRO plant mainly result from mixing of feed waters and from post-treatment processes, but the RO process does not induce isotopic fractionation. These results indicate that fresh waters produced by SWRO have highly specific isotopic characteristics that are different from most natural waters. The O and H isotopic ratios in SWRO desalted water conserve the seawater values at very low salinity. In addition, desalted seawaters from low-pH and high-pH desalination plants would have δ¹¹B of ~ 39‰ and >39‰, respectively, showing a significant enrichment in heavy ¹¹B with respect to natural fresh waters. Product waters from BWRO will show much higher geochemical and isotopic variability depending on the initial characteristics of feed waters. The specific isotopic fingerprints of RO desalinated waters could be used a valuable tracer of man-made fresh water in natural water resources.